Wide Compatibility¶

The mp-units allows us to implement nice and terse code targeting a specific C++ version and configuration. Such code is easy to write and understand but might not be portable to some older environments.

However, sometimes, we want to develop code that can be compiled on a wide range of various compilers and configurations. This is why the library also exposes and uses special preprocessor macros that can be used to ensure the wide compatibility of our code.

Note

Those macros are used in our short example applications as those are meant to be built on all of the supported compilers. Some still do not support std::format, C++ modules, or C++ versions newer than C++20.

Various compatibility options¶

Depending on your compiler's conformance, you can choose to use any of the below styles to write your code using mp-units:

C++23C++20C++20 with header filesC++20 with header files + libfmtWide Compatibility

#include <format>
#include <iostream>
import mp_units;

// ...

inline constexpr struct horizontal_length : quantity_spec<isq::length> {} horizontal_length;

// ...

std::cout << std::format(...) << "\n";

#include <format>
#include <iostream>
import mp_units;

// ...

inline constexpr struct horizontal_length : quantity_spec<horizontal_length, isq::length> {} horizontal_length;

// ...

std::cout << std::format(...) << "\n";

#include <mp-units/format.h>
#include <mp-units/ostream.h>
#include <mp-units/systems/international.h>
#include <mp-units/systems/isq.h>
#include <mp-units/systems/si.h>
#include <format>
#include <iostream>

// ...

inline constexpr struct horizontal_length : quantity_spec<horizontal_length, isq::length> {} horizontal_length;

// ...

std::cout << std::format(...) << "\n";

#include <mp-units/format.h>
#include <mp-units/ostream.h>
#include <mp-units/systems/international.h>
#include <mp-units/systems/isq.h>
#include <mp-units/systems/si.h>
#include <fmt/format.h>
#include <iostream>

// ...

inline constexpr struct horizontal_length : quantity_spec<horizontal_length, isq::length> {} horizontal_length;

// ...

std::cout << fmt::format(...) << "\n";

#include <iostream>
#ifdef MP_UNITS_MODULES
#include <mp-units/compat_macros.h>
import mp_units;
#else
#include <mp-units/format.h>
#include <mp-units/ostream.h>
#include <mp-units/systems/international.h>
#include <mp-units/systems/isq.h>
#include <mp-units/systems/si.h>
#endif

// ...

QUANTITY_SPEC(horizontal_length, isq::length);

// ...

std::cout << MP_UNITS_STD_FMT::format(...) << "\n";

Tip

Depending on your preferences, you can either write:

terse code directly targeting your specific compiler's abilities,
verbose code using preprocessor branches and macros that provide the widest compatibility across various compilers.

Compatibility macros¶

This chapter describes only the most essential tools the mp-units users need. All the compatibility macros can be found in the mp-units/compat_macros.h header file.

Tip

The mp-units/compat_macros.h header file is implicitly included when we use "legacy" headers in our translation units. However, it has to be explicitly included when we use C++20 modules, as those do not propagate preprocessor macros.

`QUANTITY_SPEC(name, ...)`¶

Quantity specification definitions benefit from an explicit object parameter added in C++23 to remove the need for CRTP idiom, which significantly simplifies the code.

This macro benefits from the new C++23 feature if available. Otherwise, it uses the CRTP idiom under the hood.

`MP_UNITS_STD_FMT`¶

Some of the supported compilers do not support std::format and related tools. Also, despite using a conformant compiler, some projects still choose to use fmtlib as their primary formatting facility (e.g., to benefit from additional features provided with the library).

This macro resolves to either the std or fmt namespace, depending on the value of MP_UNITS_API_STD_FORMAT CMake option.