When should RVO kick-in?

Question

From the following code, If RVO has happened, I expect to see the 2 addresses pointing to the same location, however this is not the case (my compiler is MS VC9.0)

Answer 1

RVO generally applies when you return an unnamed temporary, but not if you return a previously created object.

std::string foo() {
  return std::string("hello world"); // RVO
}

std::string foo() {
  std::string str("hello world");
  bar();
  return str; // Not RVO
}

std::string foo(std::string str) {
  return str; // Not RVO
}

A more general version is NRVO (Named return value optimization), which also works on named variables.

std::string foo() {
  std::string str("hello world");
  bar();
  return str; // NRVO
}

std::string foo(std::string str) {
  return str; // Not NRVO, as far as I know. The string is constructed outside the function itself, and that construction may be elided by the compiler for other reasons.
}

std::string foo(std::string str) {
  std::string ret;
  swap(ret, str);
  return ret; // NRVO. We're returning the named variable created in the function
}