Copy elision with std::mutex

Question

This explanation of copy elision states that

Under the following circumstances, the compilers are required to omit the copy and move construction of class objects, even if the copy/move constructor and the destructor have observable side-effects. The objects are constructed directly into the storage where they would otherwise be copied/moved to. The copy/move constructors need not be present or accessible, as the language rules ensure that no copy/move operation takes place, even conceptually:

In a return statement, when the operand is a prvalue of the same class type (ignoring cv-qualification) as the function return type:

T f() { return T(); }
f(); // only one call to default constructor of T

My question is why does the following code not compile then:

#include <mutex> std::mutex createMutex() { return std::mutex(); } int main() { auto mutex = createMutex(); }

Example program with compile errors.

mutex as return value? WAT? You are doing something terribly wrong. — Marek R
– Marek R, Commented Sep 5, 2019 at 12:39

eerorika · Accepted Answer · 2019-09-05 12:27:45Z

My question is why does the following code not compile then

Because the reference that you quote says

(since C++17)

It does not apply to older C++ standards. You compiled the program with a C++14 compiler. In C++14, the returned object is moved, so the type must be movable, which std::mutex is not. The move may be elided as an optimisation, but that possibility does not remove the movability requirement.

The example is well-formed in C++17 and will compile using a compliant compiler.

@StaceyGirl I still had the timeout at the time I wrote my comment. Ofc I will accpet a good answer!

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Copy elision with std::mutex

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