Question

Consider that TEST code:

#include <iostream>
using namespace std;

class Klass
{
public:
  Klass()
  {
    cout << "Klass()" << endl;
  }

  Klass(const Klass& right)
  {
    cout << "Klass(const Klass& right)" << endl;
  }
};

Klass create(Klass a)
{
  cout << "create(Klass a)" << endl;
  return a;
}

int main()
{
  const Klass result = create(Klass());
}

Compiling with:

g++ -O3 rvo.cpp   -o rvo

The output is:

$ ./rvo
Klass()
create(Klass a)
Klass(const Klass& right)

I was expecting the compiler to use the RVO mechanism in order elide every COPY CTOR call, to avoid copying the return value AND the parameter of the function create(). Why isn’t it the case?

Answer 1

The copy you see is a copy for the “return” statement in the “create” function. It cannot be eliminated by RVO, as it is not possible to construct the return value directly. You requested to “return a”. A copy is needed here; there is no way to return an object without it.

In a standard speak, following condition of [C++11: 12.8/31] is not met

in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function’s return value

As for the reasons, it is not an arbitrary rule, it makes sense from implementation point of view, as this is what is not possible to do with a function parameters:

constructing the automatic object directly into the function’s return value

You are copying the function parameter. You cannot elide this copy without inlining, as the parameter already exists before you enter the function, therefore you cannot construct that object into the return value directly instead.