CODEWITHSUNDEEP
c++gccc++17copy-elisiondialect
alfC
alfC

Reputation: 16300

"fno-elide-constructors" not working in GCC?

Should this code, compiled with -fno-elide-constructors print "cctor"?

#include<iostream>

struct non_auto
{
  non_auto()=default;
private:
  non_auto(const non_auto&)
  {std::cout << "cctor" << std::endl;}
};

non_auto foo()
{
  return non_auto{};
}

int main()
{
  auto z = foo();
}

It turns out, for all the versions of GCC I tried, it didn't print anything and effectively elided the constructor.

https://godbolt.org/z/rPTbzzza8

What am I missing? Is it that elision can happen anyway, even with this option?

UPDATE: I am using C++17, it seems that in C++14 works but it is not very interesting because I was trying this flag to reproduce the behavior of C++14 in C++17.

Upvotes: 0

Views: 196

Answers (1)

JaMiT
JaMiT

Reputation: 17063

Going by the gcc documentation:

In C++17, the compiler is required to omit these temporaries

When compiling for C++17 or later, this flag is largely ineffective as far as RVO (returning a temporary object) goes because the language mandates that the temporaries be omitted. There is nothing to copy, so there is no copy to keep around (or to elide away; there is no option anymore).

Note: In contrast, this flag does still affect NRVO (returning a variable). However, in the case of non_auto, this would have to be done in a (possibly static) member function, since the copy constructor is private.

Upvotes: 1

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