Why g++ does not enable RVO here?

问题

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?

回答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.

回答2:

The standard allows copy elision only in case where you pass a temporary as a function argument.

The two elisions you're expecting are bolded below:

[C++11: 12.8/31]: When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the copy/move constructor and/or destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization. This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

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

in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object (15.1) can be omitted by constructing the automatic object directly into the exception object

when a temporary class object that has not been bound to a reference (12.2) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

when the exception-declaration of an exception handler (Clause 15) declares an object of the same type (except for cv-qualification) as the exception object (15.1), the copy/move operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration. [..]

It didn't happen for the return value because the non-volatile name was a function parameter.

It has happened for the construction into create's parameter, otherwise you'd have seen:

Klass()
Klass(const Klass& right)
create(Klass a)
Klass(const Klass& right)

来源：https://stackoverflow.com/questions/9293726/why-g-does-not-enable-rvo-here

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