CODEWITHSUNDEEP
c++
SungJinKang
SungJinKang

Reputation: 429

Is There performance difference between this three function return ways (c++)

A Function1()
{
    return {1};
}

A Function2()
{
    return A{1};
}

A Function3()
{
    A a{1};
    return a;
}

i tried three ways. they all call just one assignment constructor.

i question There can be performance difference depending on which complier to use?

Upvotes: 1

Views: 78

Answers (2)

Jarod42
Jarod42

Reputation: 217085

There are subtle differences:

  • Function1 requires non explicit A's constructor. construct A "in place" (even before C++17). so copy/move A's constructor might even be marked as = delete, and program is well formed.

  • pre-C++17, Function2 will use move/copy constructor which might be elided (return value optimization: RVO) (copy/move constructor should be available though (marking as = delete won't compile)).
    Post C++17, no move/copy are done (similar to Function1, but work with explicit constructors).

  • Function3 will use move/copy constructor which might be elided (named return value optimization: NRVO).

With correct options, compilers will do the optimization, and all should be equivalent.

With options such as -fno-elide-constructors, pre-c++17, Function1 should win, post C++17, Function1 and Function2 should win.

Upvotes: 5

Blindy
Blindy

Reputation: 67362

The first two are identical, they will construct and return the object using A(1).

The last one, without NRVO, will create the object on the stack then copy-construct it to return. With NRVO it should be the same as the other two.

Upvotes: 3

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