Installation and Usage¶

This chapter explains how to obtain mp-units and build your code with it.

Obtaining dependencies¶

Most dependencies are provided via the Conan Package Manager. If you obtain them differently you may need to adjust some CMake files.

Conan quick intro

Install or upgrade Conan with:

pip install -U conan

Then create a profile for your environment in ~/.conan2/profiles. Example:

~/.conan2/profiles/gcc12

[settings]
arch=x86_64
build_type=Release
compiler=gcc
compiler.cppstd=20
compiler.libcxx=libstdc++11
compiler.version=15
os=Linux

[conf]
tools.build:compiler_executables={"c": "gcc-15", "cpp": "g++-15"}

Setting the language version

mp-units requires at least C++20. Setting it in the profile avoids repeating -s compiler.cppstd=20 in the Conan command line command each time.

Using Ninja as a CMake generator for Conan

Prefer Ninja. You can set the tools.cmake.cmaketoolchain:generator in ~/.conan2/global.conf:

~/.conan2/global.conf

tools.cmake.cmaketoolchain:generator="Ninja Multi-Config"

Separate build folders for different configurations

Optionally set tools.cmake.cmake_layout:build_folder_vars to get separate build folders per compiler/std version:

~/.conan2/global.conf

tools.cmake.cmake_layout:build_folder_vars=["settings.compiler", "settings.compiler.version", "settings.compiler.cppstd"]

Then use a configuration-specific preset name in the Conan instructions provided below rather than the default conan-default and conan-release (e.g., conan-gcc-13-23 and conan-gcc-13-23-release)

CMake with Presets support

Use CMake 3.23+ to benefit from Conan-generated presets. Otherwise, you need to do something like:

mkdir build && cd build
cmake .. -G "Ninja Multi-Config" -DCMAKE_TOOLCHAIN_FILE=<path_to_generators_dir>/conan_toolchain.cmake
cmake --build . --config Release

Build options¶

Note

Most of the below options are related to the C++ language features available in the compilers. Please refer to the C++ compiler support chapter to learn more about which C++ features are required for each option and which compilers support them.

Conan options¶

`cxx_modules`¶

2.2.0 · True/False (Default: auto from settings)

Adds C++ modules to the default targets.

`import_std` ¶

2.3.0 · True/False (Default: False)

Enables import std; usage.

`std_format`¶

2.2.0 · True/False (Default: auto from settings)

Enables std::format and associated facilities for text formatting. If not supported, falls back to {fmt}.

`no_crtp`¶

2.2.0 · True/False (Default: auto from settings)

Avoids CRTP in quantity_spec definitions.

`contracts`¶

2.2.0 · none/gsl-lite/ms-gsl (Default: see below)

Enables precondition checks and additional assertions.

If import_std defaults to True, contracts defaults to none; otherwise gsl-lite.

`freestanding`¶

2.2.0 · True/False (Default: False)

Configures the library in the freestanding mode. When enabled, the library's source code will build with the compiler's -ffreestanding compilation option without any issues.

`natural_units`¶

2.5.0 · ON/OFF (Default: ON)

Enables experimental natural units systems.

CMake options to set when Conan is not being used

CMake options¶

Conan sets all the below CMake options automatically based on its configuration (described above). Manual setting of the below CMake options is only needed when Conan is not being used.

MP_UNITS_BUILD_AS_SYSTEM_HEADERS

2.2.0 · ON/OFF (Default: OFF)

Exports library as system headers.

MP_UNITS_BUILD_CXX_MODULES

2.2.0 · ON/OFF (Default: OFF)

Adds C++ modules to the list of default targets.

MP_UNITS_API_STD_FORMAT

2.2.0 · ON/OFF (Default: auto)

Enables the usage of std::format and associated facilities for text formatting. If it is not supported, then the {fmt} library is used instead.

MP_UNITS_API_NO_CRTP

2.2.0 · ON/OFF (Default: auto)

Removes the need for the usage of the CRTP idiom in the quantity_spec definitions.

MP_UNITS_API_CONTRACTS

2.2.0 · NONE/GSL-LITE/MS-GSL (Default: GSL-LITE)

Enables checking of preconditions and additional asserts in the code.

MP_UNITS_API_FREESTANDING

2.2.0 · ON/OFF (Default: OFF)

Configures the library in the freestanding mode. When enabled, the library's source code should build with the compiler's -ffreestanding compilation option without any issues.

MP_UNITS_API_NATURAL_UNITS

2.5.0 · ON/OFF (Default: ON)

Enables experimental natural units systems support.

MP_UNITS_INSTALL

2.5.0 · ON/OFF (Default: ON)

Creates an installable target. Users may want to turn this off for example when consuming the library via CMake's add_subdirectory or similar mechanisms.

Installation and reuse¶

There are many ways to install or reuse mp-units. Below we list a few common approaches.

Important: Prefer using Conan if possible

The easiest and most recommended way to obtain mp-units is with the Conan package manager. See Conan + CMake (release) for a detailed instruction.

Conan + CMake (release)¶

Tip

If you are new to the Conan package manager you may want to read Obtaining Dependencies and refer to the Consuming packages chapter of the official Conan documentation for more information.

mp-units releases are hosted on Conan-Center. The following steps may be performed to obtain an official library release:

Create Conan configuration file (either conanfile.txt or conanfile.py) in your project's top-level directory and add mp-units as a dependency of your project. For example, the simplest file may look as follows:

conanfile.txtconanfile.py

[requires]
mp-units/2.4.0

[options]
# The below mp-units options are set to defaults by Conan.
# Uncomment and set to an explicit value to override the defaults.
#
# mp-units*:cxx_modules=True
# mp-units*:import_std=False
# mp-units*:std_format=True
# mp-units*:no_crtp=True
# mp-units*:contracts=gsl-lite
# mp-units*:freestanding=False

[layout]
cmake_layout

[generators]
CMakeToolchain
CMakeDeps

from conan import ConanFile
from conan.tools.build import can_run
from conan.tools.cmake import CMake, cmake_layout

class MPUnitsTestConan(ConanFile):
    settings = "os", "arch", "compiler", "build_type"
    generators = "CMakeDeps", "CMakeToolchain"

    def requirements(self):
        self.requires(
            "mp-units/2.4.0",
            options={
                # The below mp-units options are set to defaults by Conan.
                # Uncomment and set to an explicit value to override the defaults.
                #
                # "cxx_modules": False,
                # "import_std": False,
                # "std_format": True,
                # "no_crtp": True,
                # "contracts": "gsl-lite",
                # "freestanding": False,
            },
        )

    def layout(self):
        cmake_layout(self)

    def build(self):
        cmake = CMake(self)
        cmake.configure()
        cmake.build()
        if can_run(self):
            cmake.ctest(cli_args=["--output-on-failure"])

Import mp-units and its dependencies definitions with find_package:
```
find_package(mp-units REQUIRED)
```

Link your CMake targets with mp-units:

target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)

Download, build, and install Conan dependencies before running the CMake configuration step:

conanfile.txt or conanfile.pyconanfile.py only

conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing
cmake --preset conan-default
cmake --build --preset conan-release
cmake --build --preset conan-release --target test

conan build . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing

Conan + CMake (Live At Head)¶

This chapter describes the procedure to Live At Head, which means using the latest stable version of mp-units all the time.

Note

Please note that even though the Conan packages that you will be using are generated ONLY for builds that are considered stable (passed our CI tests), some minor regressions may happen (CI and C++ build environments are not perfect yet). Also, please expect that the library interface might, and probably will, change occasionally. Even though we do our best, such changes might not be reflected in the project's documentation right away.

The procedure is similar to the one described in Conan + CMake (release) with the following differences:

Before starting the previous procedure, add mp-units remote to your Conan configuration:

conan remote add conan-mpusz https://mpusz.jfrog.io/artifactory/api/conan/conan-oss

In your Conan configuration file, provide the package identifier of the mpusz/testing stream:

conanfile.txtconanfile.py

[requires]
mp-units/2.5.0@mpusz/testing

[options]
# The below mp-units options are set to defaults by Conan.
# Uncomment and set to an explicit value to override the defaults.
#
# mp-units*:cxx_modules=True
# mp-units*:import_std=False
# mp-units*:std_format=True
# mp-units*:no_crtp=True
# mp-units*:contracts=gsl-lite
# mp-units*:freestanding=False

[layout]
cmake_layout

[generators]
CMakeToolchain
CMakeDeps

from conan import ConanFile
from conan.tools.build import can_run
from conan.tools.cmake import CMake, cmake_layout

class MPUnitsTestConan(ConanFile):
    settings = "os", "arch", "compiler", "build_type"
    generators = "CMakeDeps", "CMakeToolchain"

    def requirements(self):
        self.requires(
            "mp-units/2.5.0@mpusz/testing",
            options={
                # The below mp-units options are set to defaults by Conan.
                # Uncomment and set to an explicit value to override the defaults.
                #
                # "cxx_modules": False,
                # "import_std": False,
                # "std_format": True,
                # "no_crtp": True,
                # "contracts": "gsl-lite",
                # "freestanding": False,
            },
        )

    def layout(self):
        cmake_layout(self)

    def build(self):
        cmake = CMake(self)
        cmake.configure()
        cmake.build()
        if can_run(self):
            cmake.ctest(cli_args=["--output-on-failure"])

Tip

The identifiers of the latest packages can always be found in the project's README file or on the project's Artifactory.

Force Conan to check for updated recipes with -u:

conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing -u

Alternative installation scenarios

Copy¶

As mp-units is a C++ header-only library you can simply copy all needed src/*/include subdirectories to your source tree.

Note

In such a case, you are on your own to ensure all the dependencies are installed and their header files can be located during the build. Please also note that some compiler-specific flags are needed to make the code compile without issues.

Copy + CMake¶

If you copy the mp-units library source code from the project's ./src directory (not the entire repo from its root), you can reuse CMake targets defined by the library. To do so, you should use CMakeLists.txt file from the ./src directory:

add_subdirectory(<path_to_mp_units_lib_folder>)
# ...
target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)

Note

You are still on your own to make sure all the dependencies are installed and their header and CMake configuration files can be located during the build.

Important: Library users should not use the top-level CMake file

Top level CMakeLists.txt file should only be used by mp-units developers and contributors as an entry point for the project's development. ./src/CMakeLists.txt contains only a pure library definition and should be used by the customers that prefer to use CMake's add_subdirectory() to handle the dependencies.

To learn more about the rationale, please check our FAQ.

CPM or FetchContent (CMake)¶

mp-units can be consumed via CPM or CMake's built-in FetchContent mechanism. An example CPM configuration is shown below.

CPMFindPackage(
    NAME mp-units
    VERSION 2.4.0
    GIT_TAG 6e06eddf205deaf6c2f2f63500c8c70ec220a99f
    GITHUB_REPOSITORY mpusz/mp-units
    SOURCE_SUBDIR src
    SYSTEM YES
    EXCLUDE_FROM_ALL YES
    OPTIONS
        "MP_UNITS_BUILD_AS_SYSTEM_HEADERS ON"
        "MP_UNITS_INSTALL OFF"
)
# ...
target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)

Note

This is essentially a shortcut way to do what Copy + CMake mentions above, so the same prerequisites apply.

Install¶

If you don't want to use Conan in your project and just want to install the mp-units library on your file system, and use find_package(mp-units) from another repository to find it; it is enough to perform the following steps:

conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing
mv CMakeUserPresets.json src
cd src
cmake --preset conan-default -DCMAKE_INSTALL_PREFIX=<your_installation_path>
cmake --build --preset conan-release --target install

Installation and Usage¶

Obtaining dependencies¶

Build options¶

Conan options¶

cxx_modules¶

import_std ¶

std_format¶

no_crtp¶

contracts¶

freestanding¶

natural_units¶

CMake options¶

Installation and reuse¶

Conan + CMake (release)¶

Conan + CMake (Live At Head)¶

Copy¶

Copy + CMake¶

CPM or FetchContent (CMake)¶

Install¶

`cxx_modules`¶

`import_std` ¶

`std_format`¶

`no_crtp`¶

`contracts`¶

`freestanding`¶

`natural_units`¶