Question

the following code compiles with Visual Studio 2008 but not with g++ on Mac OSX:

class A
{
public:
  A CreateA()
  {
    A a;
    return a;
  }
  };

class B
{
public:
  void ComputeWithA
    (
    A &a // Here, removing the reference solves the problem
    )
  {
    return;
  }
};

int main ()
{
  A a;
  B b;

  b.ComputeWithA(a); // Works
  b.ComputeWithA(a.CreateA()); // Invalid arguments 'Candidates are: void ComputeWithA(A &)'

  return 0;
}

Why this reference related issue? Any explanation would be greatly appreciated.

Best regards.

Answer 1

a.CreateA() gives you a R-Value (i.e. a temporary). ComputeWithA wants a reference, aka L-Value. The standard says that you can’t convert R- into L-Values, so the MSVC is wrong here.

However, you can take a const reference since this case is explicitly allowed:

void ComputeWithA(A const &a) // add a const and everything works fine