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## Build Time Configuration

There are times when you want to control whether a build target gets built or not, based on what features the compiler supports. For example, suppose you have a test file "test_constexpr_128.cpp" which requires three key features in order to build:

- The `constexpr` keyword as detected by BOOST_NO_CXX11_CONSTEXPR.
- User defined literals, as detected by BOOST_NO_CXX11_USER_DEFINED_LITERALS.
- The `__int128` data type, as detected by BOOST_HAS_INT128.

Clearly we know that if these features are not supported by the compiler, then there's simply no point in even trying to build the test program. The main advantages being:

- Faster compile times - build configuration uses lightweight tests the results of which are also cached.
- Less noise in build output - there's no reason to be faced with pages of template instantiation backtrace if we know the file can never compile anyway.
- Less noise in the online test results - the test will show up as blank, rather than as a fail in the online test matrix.
- A better experience for end users building all of Boost, if those libraries which can not be built for the current target compiler are simply skipped, rather than generating pages of error output.

Returning to our example, the test case is probably executed in it's Jamfile via the "run" rule:

    run test_constexpr_128.cpp ;

We now need to make this target conditional on the necessary features. We can do that by first importing the necessary rule at the start of the Jamfile:

    import path-to-config-lib/checks/config : requires ;

Assuming that the test case is in the usual directory:

    libs/yourlib/test

then the import rule will actually be:

    import ../../config/checks/config : requires ;

Then add a "requires" rule invocation to the requirements section of the target:

    run test_constexpr_128.cpp
       : : : #requirements:
       [ requires cxx11_constexpr cxx11_user_defined_literals int128 ] ;

Notice that multiple arguments can be added to the requires rule, and that these are always the same as the Boost.Config macro name, but in lower case and with the boost_no_ or boost_has_ prefix removed. You can also use any C++ standard feature-macro name with the leading underscores removed (see more below).

When building the above example, you will see at the start of the build process the results of the configuration, for example GCC in C++11 mode gives:

    - Boost.Config Feature Check: int128 : yes
    - Boost.Config Feature Check: cxx11_constexpr : yes
    - Boost.Config Feature Check: cxx11_user_defined_literals : yes

If you wish to make a build conditional on a C++ standard feature macro then you can specify these too, just remove the leading underscores from the name. For example:

    [ requires cpp_constexpr ]

To require C++11 style const-expressions. If you want to specify a macro from a particular standard, then you append an underscore followed by the (2 digit) year of the standard, for example:

    [ requires cpp_constexpr_17 ]

For C++17 constepxr. If you don't specify a standard then you get the first version that introduced the macro. In addition there are only standard-specific rules for each version bump of the macro, so:

    [ requires cpp_if_constexpr_17 ]

Is fine since the macro was introduced in C++17 and is the same as the un-versioned name, but:

    [ requires cpp_if_constexpr_20 ]

Will result in a build error since there is no C++20 version bump for `__cpp_if_constexpr`.

That's all there is to this handy feature, should at any time you be unsure of the feature-test names you can pass to the "requires" rule, then search for the Boost.Config macro of interest in libs/config/checks/Jamfiles.v2, and the name of the feature check will follow it.

And finally, this feature is built around the Boost.Build built in rule check-target-builds which can be used to perform more generalized build-time feature testing. The checks in this library are provided as a convenient shorthand without the need for you to write the test cases yourself.