Regarding C++ language standard

[stalkyr]

I want to build libraries with C++17 API.

Some library, mongo-cxx-driver for example, can build with C++11 with polyfills or build with C++17 directly without depending on others. We can use definition flag in portfile to choose it.

In order to choose, one way is using vcpkg feature (vcpkg install mongo-cxx-driver[boost]), this is current implementation on master branch. But for unknown reason feature for C++17 was not included, so I made a PR ( #22781 ) to add one more feature.

The problem is the current policy no longer allow using features to choose polyfills ( https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md#do-not-use-features-to-control-alternatives-in-published-interfaces ), so my PR was not accepted.

So here is the other way I thought,
For anyone who want to specify C++ language standard, he can create a custom triplet with

set(VCPKG_CMAKE_CONFIGURE_OPTIONS "-DCMAKE_CXX_STANDARD=17")

Then in library portfile

if("-DCMAKE_CXX_STANDARD=17" IN_LIST VCPKG_CMAKE_CONFIGURE_OPTIONS)
    set(BSONCXX_POLY STD)
else()
    # fallback default
    set(BSONCXX_POLY MNMLSTC)
endif()

I am not sure if it is better way? Will it accept into master branch? (So I don't need to write overlay every time)

[Neumann-A]

Will it accept into master branch?

Simple answer: No. Portfiles should not assume something is injected into VCPKG_CMAKE_CONFIGURE_OPTIONS. If you want to inject something use the toolchain to do so.

[stalkyr]

I guessed so.

Is there any plan to specify C++ language standard in formal way? I think it will be very helpful.

[Neumann-A]

Set it in the toolchain and add the compiler flag for it.

[JackBoosY]

We accept setting the c/cxx standard as a feature, but enabling this feature must ensure that this setting is passed downstream(user's custom project) and does not cause changes to the c/cxx standard on ports that depend on it.

[dg0yt]

I'm about to update mongo-cxx-driver to v3.7.0. AFAIU the current feature design in this port is broken. But what could (be made) work with the update, given upstream's automatic choice:

Create a custom triplet with:

if(PORT STREQUAL "mongo-cxx-driver")
    list(APPEND VCPKG_CMAKE_CONFIGURE_OPTIONS "-DCMAKE_CXX_STANDARD=17")
endif()

(For this port, other BSONCXX_POLY_USE_<variant> alternatives might be selected in a similar way, coupling the ABI to the triplet (right) instead of to features (wrong).

[dg0yt]

Proposed: #27565. See the comment in the portfile.