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+What's new in mp-units 2.0?
+After a year of hard work, we've just released mp-units 2.0.0. It can be obtained from
+GitHub and
+Conan.
+In this post, we will describe all the changes introduced by the new version.
+
+
+Why 2.0 if 1.0 was never released?
+Version 2 of the mp-units project is a huge change and a new quality for the users.
+We did not want to pretend that 2.0 is an evolutionary upgrade of the previous version of the
+project. It feels like a different product.
+We could start a new repo named "mp-units-v2" similarly to range-v3
+but we decided not to go this path. We kept the same repo and made the scope of the changes and
+potential breakage explicit with a drastic bump in the project version.
+What has changed?
+The answer is "nearly everything". The whole library and its documentation were rewritten nearly
+from scratch.
+Here are the significant changes that the users can observe:
+
+-
+
Repository name
+If you didn't notice, the repository name was changed from "mpusz/units" to "mpusz/mp-units".
+
+-
+
Header files content and layout
+Previously, all the header files resided in the include/units directory, and now they can be
+found in include/mp-units. The project file tree was significantly changed as well. Many
+files were moved to different subdirectories or renamed.
+
+-
+
Namespace
+Previously, all the definitions were provided in the units namespace, and now they are in the
+mp_units one.
+
+-
+
Abstractions, interfaces, definitions
+The interfaces of all of the types were refactored, and we have a new way to construct a quantity
+and quantity_point, we got unit symbols, and nearly all of the template arguments are now passed
+by values thanks to class NTTP extensions in C++20. Also, systems definitions are much terser now
+thanks to a new design approach.
+
+-
+
Conan 2.0
+Also, now we support Conan 2.0, which provides an updated way of handling dependencies.
+
+
+What is gone?
+Some cornerstones of the initial design did not prove in practice and were removed while
+we moved to version 2.
+The downcasting facility
+The first and the most important of such features was removing the downcasting facility.
+This feature is still a powerful metaprogramming technique that allows users to map long class template
+instantiations to nicely named, short, and easy-to-understand user's strong types.
+Such mapping works perfectly fine for 1-to-1 relationships. However, we often deal with N-to-1 connections in the quantities and units domain. Here are only a few such examples:
+
+- work and torque have the same dimension \(L^2MT^{-2}\),
+- becquerel and hertz have the same definition of \(s^{-1}\),
+- litre and cubic decimetre have the same factor.
+
+In the above examples, multiple entities "wanted" to register different names for identical class
+template instantiations, resulting in compile-time errors. We had to invent some hacks and
+workarounds to make it work, but we were never satisfied with the outcome.
+Additionally, this facility could easily lead to ODR violations or provide different results
+depending on which header files were included in the translation units. This was too vulnerable
+to be considered a good practice here.
+No UDLs anymore
+Over the years, we have learned that UDLs are not a good solution either. More information on this
+subject can be found in the
+Why don't we use UDLs to create quantities?
+chapter.
+New look and feel
+Here is a concise example showing you the new look and feel of the library:
+#include <mp-units/format.h>
+#include <mp-units/systems/isq/isq.h>
+#include <mp-units/systems/si/si.h>
+#include <format>
+
+using namespace mp_units;
+using namespace mp_units::si::unit_symbols;
+
+quantity<isq::speed[m / s]> avg_speed(quantity<si::metre> d,
+ quantity<si::second> t)
+{ return d / t; }
+
+int main()
+{
+ auto speed = avg_speed(220 * km, 2 * h);
+ std::println("{}", speed); // 30.5556 m/s
+}
+
All of the changes we provided, although breaking ones, resulted in much better,
+easier, and safer abstractions. These offer a new quantity on the market and hopefully will be
+appreciated by our users.
+Please check our new documentation to learn about the latest version of the project and find out
+how to benefit from all the new cool stuff we have here.
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