Resolved: When GCC does not provide __cpp_lib_uncaught_exceptions feature?

Question:

#ifndef __cpp_lib_uncaught_exceptions
namespace std {
    int uncaught_exceptions() noexcept {
        return std::uncaught_exception();
    }
}
#endif

[ 89%] Linking CXX executable AdsLibTest.bin
/usr/lib/gcc/x86_64-alpine-linux-musl/11.2.1/../../../../x86_64-alpine-linux-musl/bin/ld: /usr/lib/gcc/x86_64-alpine-linux-musl/11.2.1/../../../../lib/libstdc++.a(eh_catch.o): in function `std::uncaught_exceptions()':
/home/buildozer/aports/main/gcc/src/gcc-11.2.1_git20220219/libstdc++-v3/libsupc++/eh_catch.cc:149: multiple definition of `std::uncaught_exceptions()'; CMakeFiles/AdsLibTest.bin.dir/main.cpp.o:main.cpp:(.text+0x8d0): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [_deps/ads-build/AdsLibTest/CMakeFiles/AdsLibTest.bin.dir/build.make:98: _deps/ads-build/AdsLibTest/AdsLibTest.bin] Error 1

Answer:

1. __cpp_lib_uncaught_exceptions is only documented to be defined (when applicable) if you’ve #include-ed <version> or <exception>; you’ve included neither. Add #include <exceptions> above that feature test somewhere, and it should work.

2. Your code as written will redefine uncaught_exceptions() in every compilation unit that includes it when the macro is not defined, because you made the definition in a header and did not make it inline, static or both, so every .cpp file including your header ends up getting its own exportable definition. Without the necessary headers included in your header, whether that feature test macro is defined will depend on whether each .cpp file includes <exception>/<version>, and whether it does so before or after including your header. If they aren’t uniform, some files could get your header’s definition, while others get the built-in definition.

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