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docs: QUANTITY_SPEC macro tip added

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Mateusz Pusz
2023-08-26 19:47:10 +02:00
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docs/users_guide/framework_basics/systems_of_quantities.md
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@@ -119,6 +119,17 @@ In the **mp-units** library all the information about the quantity is provided w
class template. In order to define a specific quantity a user should inherit a strong type class template. In order to define a specific quantity a user should inherit a strong type
from such an instantiation. from such an instantiation.
!!! tip
Quantity specification definitions benefit from an
[explicit object parameter](https://en.cppreference.com/w/cpp/language/member_functions#Explicit_object_parameter)
added in C++23 to remove the need for CRTP idiom, which significantly simplifies the code.
However, as C++23 is far from being mainstream today, a portability macro `QUANTITY_SPEC()`
is provided and used consistently through the library to allow the code to compile with C++20
compilers, thanks to the CRTP usage under the hood.
*[CRTP]: Curiously Recurring Template Parameter
For example, here is how the above quantity kind tree can be modeled in the library: For example, here is how the above quantity kind tree can be modeled in the library:
=== "C++23" === "C++23"