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<!DOCTYPE html>
<html>
<head>
<meta content="text/html; charset=UTF-8" http-equiv="content-type">
<title>ch03-03</title>
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<div class="os1">3.3 使用 QString</div>
<br>
本节介绍 QString 的常见使用，包含 C++ 基本类型和 QString 的互相转换、QString 涉及的运算符、QString
子串查询和操作、利用 QTextStream 对 QString 做输入输出操作等，最后通过一个示例集成测试函数，展示 QString
用法。本节内容较多，可分几次尝试代码，凡是原理性质的内容需要理解，而罗列性质的内容不用死记的，可以到用的时候查看文档。本节示例代码下载地址在最后的小节
3.3.6 里，可以先下载示例对照学习。<br>
<br>
<div class="os2">3.3.1 QString和QChar简介</div>
<br>
QString 是由一系列 16 bit 字符 QChar 组成的字符串，以 NULL 字符结尾（末尾的 NULL 不计入字符串长度）。QChar
是一个 Unicode 4.0 标准的字符，对于超过 16bit 范围的国际码字符，QString 里采用相邻的一对 QChar 来表示。QString
使用的其实是 UTF-16 的双字节编码，tr 函数就是将 UTF-8 变长编码的字符串转成 QString 运行时的内码。UTF-8
编码是属于通用的存储交换格式，但这种编码的缺点就是一个字符的长度不固定，这对字符串操作效率是有影响的，因为得先确定每个字符的长度。因此 QString
采用固定长度字符单元的 UTF-16 编码，这对程序运行时字符串比较、查询操作效率更高。上一节 3.2.4 表格中 utf16() 和
unicode() 函数都没有用 to 前缀，因为这两个函数没有做转换，它们返回的就是 QString 运行时的内码，同 data()
函数。tr 函数不仅可以用于支持国际化翻译，并且能自动将字符串的存储交换格式 UTF-8 转换成运行时的 UTF-16 内码，返回转换过后得到的
QString 对象。<br>
<br>
字符串之间经常有手动复制或者通过函数参数、函数返回值等复制操作，QString 为了优化内存使用效率，避免出现大量相同内容的字符串副本，QString
对复制过程采用隐式共享机制（implicit sharing），比如执行字符串对象 str1 = str2
时，如果这两个对象字符串内容都没有后续改变，那么它们会指向同一块字符串数据，而如果其中之一发生改变，字符串数据块的复制过程才会发生，这样能最大程度地节省内存，而
且在传 QString 类型参数或返回值时，避免了大量数据块的复制过程，优化了程序运行效率。<br>
<br>
QString 内码是 UTF-16，而标准 C++ 的字符串是 UTF-8 编码的，Qt 针对标准 C++ 字符串也提供了 QByteArray
类，用于操作 UTF-8 编码以及其他本地化字符串（如 GBK、Big5）、字节数组（不以 NULL 结尾的纯数据）等，QByteArray
类下一节讲解。<br>
<br>
<div class="os2">3.3.2 基本类型与字符串互相转换</div>
<br>
在编程时经常会出现把数值如 800 转成字符串 "800"，或者反过来把字符串转成数值等情况，本小节罗列 C++ 基本的数值类型和 Qt
对这些类型的别称，然后展示这些基本类型和 QString 对象的互相转换，并编写一些测试函数来示范效果。<br>
<br>
<table class="tabel">
<tbody>
<tr class="d1">
<td style="width: 180px;" align="center"><b>基本类型</b></td>
<td style="width: 180px;" align="center"><b>Qt别称</b></td>
<td style="width: 120px;" align="center"><b>转入函数</b></td>
<td style="width: 120px;" align="center"><b>转出函数</b></td>
<td align="center"><b>描述</b></td>
</tr>
<tr>
<td style="width: 180px;"><b>short</b></td>
<td style="width: 180px;"><b>qint16</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toShort</b></td>
<td> 2 字节长度，有符号短整型。 </td>
</tr>
<tr class="d1">
<td style="width: 180px;"><b>unsigned short</b></td>
<td style="width: 180px;"><b>ushort、quint16</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toUShort</b></td>
<td> 2 字节长度，无符号短整型。 </td>
</tr>
<tr>
<td style="width: 180px;"><b>int</b></td>
<td style="width: 180px;"><b>qint32</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toInt</b></td>
<td> 4 字节长度，有符号整型。 </td>
</tr>
<tr class="d1">
<td style="width: 180px;"><b>unsigned int</b></td>
<td style="width: 180px;"><b>uint、quint32</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toUInt</b></td>
<td> 4 字节长度，无符号整型。 </td>
</tr>
<tr>
<td style="width: 180px;"><b>long</b></td>
<td style="width: 180px;"><b>无</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toLong</b></td>
<td> 有符号长整型，对于 32 位编程 long 是 4 字节长度，对于 64 位编程是 8 字节长度。 </td>
</tr>
<tr class="d1">
<td style="width: 180px;"><b>unsigned long</b></td>
<td style="width: 180px;"><b>ulong</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toULong</b></td>
<td> 无符号长整型，对于 32 位编程 unsigned long 是 4 字节长度，对于 64 位编程是 8 字节长度。 </td>
</tr>
<tr>
<td style="width: 180px;"><b>long long</b></td>
<td style="width: 180px;"><b>qlonglong、qint64</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toLongLong</b></td>
<td> 8 字节长度，有符号长长整型。 </td>
</tr>
<tr class="d1">
<td style="width: 180px;"><b>unsigned long long</b></td>
<td style="width: 180px;"><b>qulonglong、quint64</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toULongLong</b></td>
<td> 8 字节长度，无符号长长整型。 </td>
</tr>
<tr>
<td style="width: 180px;"><b>float</b></td>
<td style="width: 180px;"><b>默认情况下无</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toFloat</b></td>
<td> 4 字节长度，单精度浮点数。 </td>
</tr>
<tr class="d1">
<td style="width: 180px;"><b>double</b></td>
<td style="width: 180px;"><b>默认情况对应 qreal</b></td>
<td style="width: 120px;"><b>arg或setNum</b></td>
<td style="width: 120px;"><b>toDouble</b></td>
<td> 8 字节长度，双精度浮点数。 </td>
</tr>
</tbody>
</table>
<br>
这些基本的数值类型转为 QString 对象都是使用重载的 arg 或 setNum 函数，而 QString
对象转出为其他类型使用单独命名的函数。Qt 对这些类型的别称都定义在头文件 <QtGlobal> 里面，由于其他绝大多数 Qt
头文件都包含了该全局头文件，所以通常不需要自己手动去包含它的。对于上表需要说明的两点：一是 long 和 ulong
长度是根据操作系统和编译器确定的，32 位编程就是 32 位，64 位编程就是 64 位；二是实数 qreal 默认情况下都是对应 double
，例外情况是在编译 Qt 类库本身时配置了 -qreal float 选项参数，这种例外情况极少，通常都不用管的。<br>
首先来介绍一下转入函数，对于整数类型，setNum 函数声明是完全类似的，以 int 为例：<br>
<div class="code"> QString & setNum(int n, int base = 10)</div>
第一个参数就是需要转换的整数，第二个参数是转换之后的目标字符串进制基数，比如转成十六进制字符串、八进制字符串等，默认是转成十进制的字符串。setNum
函数设置好字符串内容后返回 QString 对象自身的引用。<br>
<br>
对于浮点数类型，setNum 函数声明有些区别，以 double 为例：<br>
<div class="code"> QString & QString::setNum(double n, char format =
'g', int precision = 6)</div>
第一个参数是需要转换的浮点数，第二个是转换之后的目标字符串格式（'e', 'E', 'f', 'g' ,
'G'），第三个是目标字符串显示的浮点数精度，默认是 6 。浮点数的格式是与 C 语言类似的，如下所述：<br>
<ul>
<li>'e'：科学计数法，小写 e，如 [-]9.9e[±]999。</li>
<li>'E'：科学计数法，大写 E，如 [-]9.9E[±]999。</li>
<li>'f'：定点数显示，[-]9.9。</li>
<li> 'g'：自动选择用科学计数法或定点数显示，哪种方式最简洁就用哪个，科学计数法的 e 小写。</li>
<li> 'G'：自动选择用科学计数法或定点数显示，哪种方式最简洁就用哪个，科学计数法的 E 大写。</li>
</ul>
setNum 函数示范代码：<br>
<div class="code"> <span style=" color:#808000;">void</span><span style=" color:#c0c0c0;">
</span><span style=" color:#000000;">Test_setNum</span><span style=" color:#000000;">()</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//to</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Hex</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">string</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">short</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numHex</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">127</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setNum</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">numHex</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">16</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Hex:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strTest</span><span style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//to</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Oct</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">string</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numOct</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">63</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setNum</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">numOct</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">8</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Oct:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strTest</span><span style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//to</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">normal</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Dec</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">string</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">long</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numDec</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">800</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setNum</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">numDec</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Normal:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strTest</span><span style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//to</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">float</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">string</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">float</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numFixed</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">123.78999</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setNum</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">numFixed</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'f'</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">3</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Fixed:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strTest</span><span style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//to</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">scientific</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">string</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numScientific</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">456.78999</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strTest</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setNum</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">numScientific</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'e'</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">6</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Scientific:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strTest</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
这个测试函数运行结果就不贴出来了，读者自己手动去试试看。<br>
<br>
接下来重点介绍 arg 函数，这是最常用也是最具特色的。arg
函数无所不包，它的参数可以是数值类型、字符串类型，并且可以串联，格式化参数里还可以指定顺序、重复使用参数等等。对于数值类型，它的声明与 setNum
比较类似，以 int 和 double 为例：<br>
<div class="code"> QString arg(int a, int fieldWidth = 0, int base = 10,
QChar fillChar = QLatin1Char( ' ' )) const </div>
<div class="code">QString arg(double a, int fieldWidth = 0, char format =
'g', int precision = -1, QChar fillChar = QLatin1Char( ' ' )) const</div>
注意 arg 函数声明末尾的 const，这个函数不会改变字符串对象本身的内容，而是会返回一个全新的 QString
对象，所以使用这个函数时，必须用它的返回值。<br>
对于整数类型，它的声明多出来两个：fieldWidth 是指生成的目标字符串宽度，0 表示自动设置长度，最后的 fillChar
是填充字符，如果设置的域宽比较大，多余的空位就会使用这个填充字符填满。<br>
对于浮点数类型，多出来的 fieldWidth 也是生成的目标字符串宽度，fillChar 也是填充字符。默认的填充字符是空格，QLatin1Char
代表一个字节长度的拉丁字符，与 ASCII 码字符差不多。QLatin1Char 有对应的类
QLatin1String，因为仅支持单字节拉丁字符，不支持国际化，它应用的比较少。 <br>
arg 函数比 setNum 函数功能更强大，可以设置目标字符串宽度和填充字符。arg
函数还可以用字符串作为参数，可以将一个字符串填充到另一个里面，比如下面这个函数声明：<br>
<div class="code">QString arg(const QString & a, int fieldWidth = 0,
QChar fillChar = QLatin1Char( ' ' )) const</div>
这个声明和数值类型声明差不多，也可以设置目标字符串宽度和填充字符。<br>
函数声明介绍到这，下面看看这个函数该怎么用。arg 函数的使用方式很特别，它的串联方式也很灵活，来看看示例代码：<br>
<div class="code"> <span style=" color:#808000;"></span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">Test_arg</span><span
style=" color:#000000;">()</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//使用</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">存储</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">arg</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">返回的新对象</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//Dec</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">long</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numDec</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">800</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strMod</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Normal:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1"</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strMod</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numDec</span><span
style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//%1是占位符，第一个arg函数参数变量转后的字符串填充到</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">位置</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Mod:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strMod</span><span style=" color:#000000;"><<</span><span
style=" color:#008000;">"</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">\t</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Result:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strResult</span><span style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//Oct</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numOct</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">63</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Oct:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1"</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numOct</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">4</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">8</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QChar</span><span
style=" color:#000000;">(</span><span style=" color:#008000;">'0'</span><span style=" color:#000000;">));</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//numOct转换后为4字符域宽，8进制，填充0</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//Hex</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">short</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numHex</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">127</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strPrefix</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"0x"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//占位符里可填充数值转的字符串，也可以直接填充原有的字符串</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Hex:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1%2"</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">strPrefix</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numHex</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">16</span><span
style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//串联：第一个arg函数参数填充到%1，第二个arg填充到%2</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//double</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numReal</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">123.78999</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Fixed:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">\t</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Scientific:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%2"</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numReal</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'f'</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">numReal</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'e'</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">3</span><span style=" color:#000000;">);</span><span
style=" color:#000000;"></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//占位符可重复，也可乱序</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">one</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">1</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">two</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">2</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">three</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">3</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"%1</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">小于</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">%2，%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">小于</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%3，%3</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">大于</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">%2</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">。"</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">one</span><span style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">two</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">three</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
上面都是通过 tr 函数封装了一个临时的 QString 对象，然后调用该临时对象的 arg
函数实现数值类型转成格式化字符串，填充到占位符里面。这个工作原理与 sprintf 等 C 语言函数类似，sprintf 函数使用 %n 、%s
之类的格式占位符，QString 的实现方式不一样，它使用 % 加数字的占位方式，%1 对应后面串联的第一个 arg 函数，%2 对应后面串联的第二个
arg 函数，以此类推。具体的 %1 或 %2 等替换后的格式，由对应的 arg 函数来决定，QString 里有非常多的重载 arg 函数，每个
arg 函数对应一个类型，因此 %1 既可以填充数值类型转化后的格式化字符串，也可以填充其他原有的字符串。下面逐个解释一下各个 arg 函数意义：<br>
<div class="code"><span style=" color:#c0c0c0;"><span style=" color:#808000;"></span></span><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">
long</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numDec</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">800</span><span
style=" color:#000000;">;</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strMod</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Normal:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1"</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strMod</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numDec</span><span
style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//%1是占位符，第一个arg函数参数变量转后的字符串填充到</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">位置</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"Mod:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strMod</span><span style=" color:#000000;"><<</span><span
style=" color:#008000;">"</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">\t</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Result:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">strResult</span><span style=" color:#000000;">;</span></pre>
<span style=" color:#000000;"></span><span style=" color:#c0c0c0;"></span></div>
这是最简单的形式，tr函数生成的 strMod 对象里面只有一个占位符 %1 ,arg 函数会将整数 numDec 转成十进制数字符串，然后根据
strMod 构造一个新的字符串对象，并将十进制数字符串填充到占位符 %1 位置。原本的 strMod 不会改变，arg
函数会返回全新的字符串对象，然后复制给了 strResult。qDebug 打印的结果就是：<br>
<span style=" font-family:'Courier'; font-size:10pt; color:#aa00aa;">Mod:
"Normal: %1" Result:
"Normal: 800"</span><br>
<div class="code"><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">
int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numOct</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">63</span><span
style=" color:#000000;">;</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Oct:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1"</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numOct</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">4</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">8</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QChar</span><span
style=" color:#000000;">(</span><span style=" color:#008000;">'0'</span><span style=" color:#000000;">));</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//numOct转换后为4字符域宽，8进制，填充0</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
</div>
这里 arg 函数是将普通数字 63 用八进制数来显示，要转换的数值是 numOct，设置 numOct 转换后的子串至少 4
字符宽度，用八进制显示，空位用字符 '0' 填充。qDebug 打印的结果就是：<br>
<span style=" font-family:'Courier'; font-size:10pt; color:#aa00aa;">"Oct:
0077"</span><br>
<div class="code"><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">
short</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numHex</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">127</span><span
style=" color:#000000;">;</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strPrefix</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"0x"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//占位符里可填充数值转的字符串，也可以直接填充原有的字符串</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Hex:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1%2"</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">strPrefix</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numHex</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">16</span><span
style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//串联：第一个arg函数参数填充到%1，第二个arg填充到%2</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
</div>
这里使用了串联的两个 arg 函数，第一个 arg 函数是填充原有字符串 strPrefix 到 %1 位置，第二个 arg 函数填充 numHex
转换后的十六进制字符串到 %2 位置。第二个 arg 函数参数里的 0 是指不限制域宽，转换后的十六进制字符串该多长就多长，参数 16
是十六进制的意思。占位符本身是没有格式信息的，填充的具体内容由后面串联的 arg
函数决定，想填充原有字符串就填充原有的字符串，想填充转换后的数字字符串，那就填充数字字符串，非常方便。qDebug 打印的结果为：<br>
<span style=" font-family:'Courier'; font-size:10pt; color:#aa00aa;">"Hex:
0x7f"</span><br>
<div class="code"><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">
double</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">numReal</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">123.78999</span><span
style=" color:#000000;">;</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"Fixed:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">\t</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Scientific:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%2"</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">numReal</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'f'</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">numReal</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">'e'</span><span style=" color:#000000;">,</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">3</span><span style=" color:#000000;">);</span><span
style=" color:#000000;"></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
</div>
这里展示的是浮点数转成字符串，第一个 arg 函数将 numReal 以定点数形式（'f'）转成字符串，0 代表不限制宽度，并填充到 %1 位置，没有设
置显示精度（默认为 6 位）。第二个 arg 函数将 numReal 以科学计数法形式（'e'）转成字符串，0 代表不限制宽度，3 代表显示精度为 3
位。qDebug 打印的结果为：<br>
<span style=" font-family:'Courier'; font-size:10pt; color:#aa00aa;">"Fixed:
123.789990 Scientific: 1.238e+02"</span><br>
<div class="code"><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">
int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">one</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">1</span><span
style=" color:#000000;">;</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">two</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">2</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">three</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">3</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"%1</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">小于</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">%2，%1</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">小于</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">%3，%3</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">大于</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">%2</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">。"</span><span
style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span
style=" color:#000000;">one</span><span style=" color:#000000;">).</span><span style=" color:#000000;">arg</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">two</span><span style=" color:#000000;">).</span><span
style=" color:#000000;">arg</span><span style=" color:#000000;">(</span><span style=" color:#000000;">three</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strResult</span><span
style=" color:#000000;">;</span></pre>
</div>
最后一段示例比较有意思，如果是 C 语言的 sprintf 要填充 6 个整型数，那必须用 6 个 %n ，不管有没有重复的。这里仅仅用了
%1、%2、%3，后面对应三个 arg 函数，每个 arg
函数都将参数里的变量转成数字字符串，并填充到正确的位置，而且可以重复填充。占位符的顺序也可以是乱的，规律就是第一个 arg 函数填充所有的 %1
，第二个 arg 函数填充所有的 %2 ，第三个 arg 函数填充所有的 %3 ，以此类推。因此 qDebug 打印的结果就是：<br>
<span style=" font-family:'Courier'; font-size:10pt; color:#aa00aa;">"1 小于
2，1 小于 3，3 大于 2 。"</span><br>
<br>
这正是我们希望看到的结果，可见 arg 函数的灵活性是传统 C 语言 sprintf 等无法比拟的，而且也更安全。学会 arg
函数用法，可应对各种复杂的格式化字符串转换。<br>
<br>
接下来简单看看 QString 的转出函数，可以将数字字符串转成各种类型的数值变量。对于整数类型，它们的函数声明都是类似的，以 int 为例：<br>
<div class="code">int QString::toInt(bool * ok = 0, int base = 10) const</div>
toInt 函数第一个参数 ok 接收一个 bool 变量的指针，用于反馈转换过程是否成功，第二个参数 base 是字符串对象里数字的进制基数，默认的
10 代表十进制，也可以设置二进制、八进制和十六进制等等。如果将 base 设置为 0，那么 toInt 函数将自动识别字符串对象里面的进制标识，对于
"0" 打头的自动按八进制转换，对于 "0x" 打头的自动按十六进制转换，其他情况都按十进制转换。<br>
如果转换出错，ok 指向的变量会设置为 false，返回值为 0 。<br>
<br>
对于浮点数字符串的转换，函数声明有些差异：<br>
<div class="code">double QString::toDouble(bool * ok = 0) const</div>
这个不能指定进制基数，都是十进制的，支持定点数字符串和浮点数字符串转成数值。参数 ok 接收一个 bool 变量的指针，用于反馈转换过程是否成功。<br>
如果转换失败，ok 指向的变量会设置为 false，返回值为 0。<br>
下面示范 QString 对象的转出函数：<br>
<div class="code"><span style=" color:#808000;">void</span><span style=" color:#c0c0c0;">
</span><span style=" color:#000000;">Test_toValue</span><span style=" color:#000000;">()</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">bool</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">bok</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">false</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//dec</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strDec</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"800"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nDec</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strDec</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">toInt</span><span
style=" color:#000000;">(&</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">10</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nDec</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">//成功</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//Hex</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strHex</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"FFFF"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nDec</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strHex</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toInt</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">10</span><span style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">//基数错误，转换失败</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nDec</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">short</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nHexShort</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strHex</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toShort</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">16</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nHexShort</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//FFFF正整数太大，超出范围，转换失败，没有负号</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">-</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">的都算正数。</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">ushort</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nHexUShort</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strHex</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toUShort</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">16</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nHexUShort</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span><span style=" color:#008000;">//成功</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//自动转换</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strOct</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"0077"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nOct</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strOct</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">toInt</span><span
style=" color:#000000;">(&</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nOct</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//字符</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">0</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">打头自动按八进制转</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strHexWithPre</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"0xFFFF"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nHexWithPre</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strHexWithPre</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toInt</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">0</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nHexWithPre</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span><span style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//字符</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">0x</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">打头自动按十六进制转</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">nDecAuto</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strDec</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toInt</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">0</span><span style=" color:#000000;">);</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">//"800"</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">，自动按十进制</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">nDecAuto</span><span
style=" color:#000000;"><<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">bok</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//浮点数转换</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strFixed</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"123.78999"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblFixed</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strFixed</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toDouble</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span>fixed<span style=" color:#000000;"><<</span><span
style=" color:#000000;">dblFixed</span><span style=" color:#000000;"><<</span><span
style=" color:#008000;">"\t"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//科学计数法</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strScientific</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"1.238e-5"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblScientific</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strScientific</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">toDouble</span><span style=" color:#000000;">(&</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span>scientific<span style=" color:#000000;"><<</span><span
style=" color:#000000;">dblScientific</span><span style=" color:#000000;"><<</span><span
style=" color:#008000;">"\t"</span><span style=" color:#000000;"><<</span><span
style=" color:#000000;">bok</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
上面代码的运行结果这里不贴出来了，读者自己动手去试试。对于两个浮点数打印的行，里面带有流操作子，类似标准 C++ 控制台输出对象 cout
的操作子，fixed 是指按定点数显示，scientific 是指按科学计数法显示。<br>
<br>
<div class="os2">3.3.3 字符串运算符</div>
<br>
QString 重载了多个对字符串有清晰意义的运算符，之前见过赋值运算符，可以将一个 QString 对象赋值给另一个 QString
对象。还有其他的比 较运算符和中括号运算符，先将其罗列如下：<br>
<br>
<table class="tabel">
<tbody>
<tr class="d1">
<td style="width: 120px;" align="center"><b> operator </b></td>
<td align="center"><b> 描述 </b></td>
</tr>
<tr>
<td style="width: 120px;"><b> = </b></td>
<td> 赋值运算符，遵循隐式共享规则，在赋值的两个对象有变化时才真正复制数据块。 </td>
</tr>
<tr class="d1">
<td style="width: 120px;"><b> += </b></td>
<td> 追加。将运算符左边和右边字符串拼接后，赋值给左边对象。 </td>
</tr>
<tr>
<td style="width: 120px;"><b> < </b></td>
<td> 小于号。左边字符串字典序比右边的靠前时，表达式为真。 </td>
</tr>
<tr class="d1">
<td style="width: 120px;"><b> <= </b></td>
<td> 小于等于。左边字符串字典序比右边的靠前或相同时，表达式为真。 </td>
</tr>
<tr>
<td style="width: 120px;"><b> == </b></td>
<td> 等于。二者字典序是一致的时候为真。 </td>
</tr>
<tr class="d1">
<td style="width: 120px;"><b> != </b></td>
<td> 不等于。二者字典序不一样的时候为真。 </td>
</tr>
<tr>
<td style="width: 120px;"><b> > </b></td>
<td> 大于。左边字符串字典序比右边的靠后时，表达式为真。 </td>
</tr>
<tr class="d1">
<td style="width: 120px;"><b> >= </b> </td>
<td> 大于等于。左边字符串字典序比右边的靠后或相同时，表达式为真。 </td>
</tr>
<tr>
<td style="width: 120px;"><b> [] </b></td>
<td> 类似数组取数的中括号，从指定位置取出 QChar 字符，另外还可以修改指定位置的 QChar 字符。 </td>
</tr>
<tr class="d1">
<td style="width: 120px;"><b> + </b> </td>
<td> 拼接。这是个友元函数，将两个字符串拼接后返回全新的字符串对象。 </td>
</tr>
</tbody>
</table>
<br>
上面运算符的意义是一目了然的，主要解释一下赋值运算符 = 的隐式共享（Implicit
Sharing），在执行赋值时，真正的字符串数据拷贝没有发生，这是为了优化运行效率，避免大量数据的拷贝。隐式共享实现方式就是对数据块做引用计数，多一个对象赋值或
参数、返回值拷贝时，引用次数加 1，这个赋值过程只需要设置一下数据指针和增加引用计数，不会真的拷贝大量数据，这种拷贝称为浅拷贝（shallow
copy）。<br>
在赋值的一个字符串发生变化，要做写入修改时，这个要发生变化的字符串会重新分配一块内存，将旧的数据拷贝到新的内存空间，并对其做相应的写入修改，这个过程叫深
拷贝（deep copy），也可称为 copy-on-write（写时拷贝）。深拷贝会将旧的数据块引用计数减
1，然后将变化的字符串数据指向新空间，新空间引用计数加 1。<br>
如果发生字符串超出生命期销毁或清空，那么对应的数据引用计数减 1，当引用计数减到 0 时，数据块空间才会真的被释放。<br>
Qt 对象能够赋值或传参数、返回值的，一般都是采用隐式共享机制，所以 Qt
的参数和返回值传递运行效率是很高的。这也将信号和槽机制传递参数的效率大大提升了。面向对象的高级编程语言一般都支持类似的功能，比如 Java 和
Python 的垃圾回收机制，也是类似的。<br>
<br>
下面通过简单示例展示运算符的使用：<br>
<div class="code">
<style type="text/css">
p, li { white-space: pre-wrap; }
</style> <pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">void</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">Test_operator</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">=</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QString</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE1</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE2</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE3</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE1</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">QObject</span><span style=" color:#000000;">::</span><span
style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span style=" color:#008000;">"abcd"</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE2</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strE1</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE3</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">strE2</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//打印数据指针</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strE1</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">strE2</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">strE3</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">();</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//改变字符串，追加</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">strE2</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">append</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">QObject</span><span
style=" color:#000000;">::</span><span style=" color:#000000;">tr</span><span style=" color:#000000;">(</span><span
style=" color:#008000;">"1234"</span><span style=" color:#000000;">)</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//再次打印数据指针，谁修改了数据，谁的数据指针就变</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">qDebug</span><span
style=" color:#000000;">()<<</span><span style=" color:#000000;">strE1</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">strE2</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">()<<</span><span style=" color:#008000;">"\t"</span><span
style=" color:#000000;"><<</span><span style=" color:#000000;">strE3</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">data_ptr</span><span
style=" color:#000000;">();</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>