feat: add class info for headers concept

concepts/headers/introduction.md

@@ -13,33 +13,79 @@ Some projects use `.hpp` or skip the extension completely.
 The definitions are located in a separate `.cpp` file. 
 To reunite both parts, this source file starts by including the respective header file.
 
-If you want to write a library called "myLib", that offers a function "my_func" that returns your favorite number, the files would look like this:
+If you want to write a library called "quick_math", that offers a function "super_root" that you want to use often, the files would look like this:
 
 ```cpp
-// A file named myLib.h
+// A file named quick_math.h
 #pragma once
-namespace magic {
-    int my_func(int a, int b);
+namespace quick_math {
+    double super_root(double x, int n);
 }
 ```
 
 ```cpp
-// A file named myLib.cpp
-#include "myLib.h"
-int magic::my_func(int a, int b) {
-    return 47 * a + 3 * b;
+// A file named quick_math.cpp
+#include "quick_math.h"
+#include <cmath>
+double quick_math::super_root(double x, int n) {
+    while(n) { x = sqrt(x), --n;}
+    return x;
 }
 ```
 
+If you need to include another header, that is only needed by the implementation, the respective `#include` line is only needed in the source file.
+Everything that is included in the header, is also available in the `.cpp` file, like the `string` library in the example below.
 Attention: the `;` is needed after the declaration in the header file, but not after the definition in the source file.
 
 ~~~~exercism/note
 Many C++ exercises on Exercism start with two almost empty files: header and source.
 You have to check the `*_test.cpp` file to see the names and namespaces of the expected functions to solve the exercise.
 ~~~~
 
+## Classes and Headers
+
+Classes can become very complex.
+Many developers separate the public interface from the inner workings.
+Often, all declarations are decoupled from the implementation via header and source files, but there are some exceptions.
+The split between those two might seem arbitrary and the following example can give some guidance.
+
+```cpp
+// A file named robot_flower.h
+#if !defined(ROBOT_FLOWER_H)
+#define ROBOT_FLOWER_H
+#include <string>
+namespace robots {
+    class Flower {
+        private:
+            bool needs_water{};
+            int size{};
+            std::string name{};
+        public:
+            Flower(std::string name, int size = 0);
+            void give_water() {needs_water = false;}
+            std::string get_name() {return name;}
+            int get_size() {return size;}
+            void start_next_day();
+    };
+}
+#endif
+```
+
+```cpp
+// A file named robot_flower.cpp
+#include "robot_flower.h"
+robots::Flower::Flower(std::string name, int size) {this->name = "Robotica " + name; this->size = size;}
+void robots::Flower::start_next_day() {if (!needs_water) ++size; needs_water = true;}
+```
+
+Member variables are usually kept in the header together with the implementation of trivial member functions.
+The simple `give_water`, `get_name` and `get_size` functions are so small, that one would not move them into the `.cpp` file.
+The constructor and the `start_next_day` functions shall be considered more _complex_ in this example and have thus been moved.
+The definitions are prefixed with the namespace `robots` and the class type `Flower`.
+
 ## Include Guards
 
 You may have noticed the `#pragma once` line in the example header file above.
 This include guard ensures that the content of the file is included only once during the compilation to avoid errors.
-There is another, more complex variation that starts with `#ifndef` which serves the same purpose.
+There is another, more complex variation that starts with `#if !defined` and ends with `#endif`.
+It serves the same purpose and its usage is shown in the `Flower` class example above.

concepts/headers/links.json

@@ -2,5 +2,9 @@
     {
         "url": "https://learn.microsoft.com/en-us/cpp/cpp/header-files-cpp?view=msvc-170",
         "description": "Examples and information on header files and include guards"
+    },
+    {
+        "url": "https://www.learncpp.com/cpp-tutorial/classes-and-header-files/",
+        "description": "More information about classes in header files"
     }
 ]