Propagate C++ standard to export

Usage
1

Hi,

I have a project that uses std::string_view in the interfaces. Therefore I need to propagate the standard to the export so that users using find_package() will automatically set their standard right.

How to do this?
As my project has hundreds of exported target, how can I set this for all exported targets?

Thanks!

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2

Use an INTERFACE target my_config_target with only set some include, defines, properties, …

and use target_link_libraies(xxx PUBLIC my_config_target) on all other targets.

For an example see cpp_vcpkg_project/CMakeLists.txt at main · aminya/cpp_vcpkg_project (github.com)

3

Be careful with this. I used a project that exports -std=c++17 as compile flags, but we want to build with C++20. The resulting compilation command includes our -std=c++20 followed by the imported -std=c++17, meaning no matter what we do, we can’t use C++20 features.

I don’t have the answer, but it would be better to specify the features you require or just let a project that doesn’t support a new enough standard fail to compile when they try to use your project.

4

The right way to apply that constraint is with compile features:

target_compile_features(someTarget PUBLIC cxx_std_17)

That will make the someTarget compile with at least C++17, and anything that links to someTarget will also compile with at least C++17. Compile features support transitive requirements just like compile options, compile definitions and header search paths.

Unfortunately, you can’t set a variable to define a default value for compile features. If specifying them individually is tedious for you to do, then the alternative is you iterate over all targets within a directory and its children and you add something like the above call for each target. You’ll need the BUILDSYSTEM_TARGETS and SUBDIRECTORIES directory properties to iterate over and to walk the directory structure (there are caveats, but if you’re not loading a source directory multiple times, you’ll be fine). It’s a bit of a brute force approach, and I wouldn’t recommend doing it this way. Even if I have to manually edit a project once to update 100 targets, it’s not hard and you only need to do it once, and it is clearer what you’re doing.

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