Why does std::atomic constructor behave different in C++14 and C++17

Question

I'm working in a project with C++11 and I tried following code

#include <atomic>

struct A {
    std::atomic_int idx = 1;

};

int main() {
    return 0;
}

I get the compiler error

error: use of deleted function 'std::__atomic_base<_IntTp>::__atomic_base(const std::__atomic_base<_IntTp>&) [with _ITp = int]'
 std::atomic_int idx = 1;
                       ^

The same result is with C++14. When I switch to C++17 it works: wandbox

I checked cppreference for differences:

• std::atomic
• std::atomic<T>::operator=
• std::atomic<T>::atomic

But there is no difference documented between C++14 and C++17. Why does it work with C++17 and not with C++14?

Answer 1

Because in C++17 there is a guaranteed RVO. In C++14 statements like Foo x = Foo(args) and Foo x (args) are technically not the same, but they are in C++17.

struct Foo {
    Foo() = default;
    Foo(const Foo&) = delete;
};

int main() {
    // Works in C++17 and C++20, fails in C++14 and before
    Foo foo = Foo(); 
}

You can read more about this here: https://en.cppreference.com/w/cpp/language/copy_elision

In particular the section (since C++17):

T x = T(T(f())); // only one call to default constructor of T, to initialize x

To make C++14 code work, you can use

std::atomic_int idx { 1 };