rvalue references (T&&)

After programming in C++ for about 4/5ths of the time that it has been in existence, you would think I would stop making this mistake: 

extern int foo(std::string &);
 . . . 
void bar (void)
{
  string s, t;
   . . .
  int i = foo(s + t);
}

The mistake, of course, is that a non-const reference (whether a formal parameter or not) cannot be bound to a temporary object, nor any other rvalue. On a fifty-fifty cocktail of intant-rewrite-in-a-can and Homer-Simpson-doh, I hastily rearrange to the code to say: 

void bar (void)
{
  string s, t, st;
   . . .
  st = s + t;
  int i = foo(st);
}

No longer. Instead, the signature of foo() can be rephrased as: 

extern int foo(std::string &&);

which will allow foo to bind to the temporary created by concatenating s and t.