AppleClang 15: no member named 'fetch_add' in 'std::atomic<float>'

Question

I have a C++20 program that can be successfully built in Ubuntu 20 with GCC 10 or in Visual Studio 2019. And now I need to compile it for macOS 13.7.2 x64 (currently available on GitHub-hosted runner), which possesses AppleClang 15 compiler.

The program uses std::atomic<float> data types, and fetch_add operation for them as in the example:

#include <atomic>

int main() {
    std::atomic<float> a{0};
    a.fetch_add(1);
}

Unfortunately, it fails to compile in AppleClang 15 with the

error: no member named 'fetch_add' in 'std::atomic'

Online demo: https://gcc.godbolt.org/z/8WvGrczq7

I would like to keep std::atomic<float> data types in the program (to minimize the changes), but replace fetch_add with something else available (probably less performant). Is there such workaround for old Clangs with libc++?

Drew Dormann · Accepted Answer · 2024-12-31 18:34:09Z

4

Is there such workaround for old Clangs?

As commented, std::compare_exchange_weak is available for your clang version.

#include <atomic>

float fetch_add(std::atomic<float>& a, float val) {
    float expected = a;
    while( !a.compare_exchange_weak(expected, expected + val)) {}
    return expected;
}

int main() {
    std::atomic<float> a{0};
    fetch_add(a,1);
}

answered Dec 31, 2024 at 18:34

Drew Dormann

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Comments

Marshall Clow · Accepted Answer · 2025-01-02 22:41:11Z

2

The libc++ status page for C++20 shows that P0020 (the paper that added std::fetch_add) was implemented for clang 18.

[Later: I was mistaken - I said P0019, which added atomic_ref, which has its own fetch_add calls. Thanks to @Nate for the correction. ]

edited Jan 2 at 22:41

answered Dec 31, 2024 at 16:54

Marshall Clow

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1 Comment

Alan Birtles Dec 31, 2024 at 17:46

Note that the latest apple clang version (16) is based on llvm 17 en.m.wikipedia.org/wiki/…

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