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

I don't know the full conditions, but I believe the fact that you are returning a parameter and not an instance created in the function is causing the problem in your example.

For me, the following showed the same address for both:

#include 
#include 

std::string foo()
{
   std::string s("rvo!");
   std::cout << "address: " << (void *)(&s) << std::endl;
   return s;
}

int main()
{
   const std::string s = foo();
   std::cout << "address: " << (void *)(&s) << std::endl;
   std::cout << s << std::endl;
   return 0;
}

Follow up to darid's comment

The codepad about page documents that for it use's the -fno-elide-constructors for C++. The documentation for this option form the g++ man page state:

The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

On my machine, compiling with -fno-elide-constructors prevents RVO, but compiling without allows it.