C++ Function Template instantiaion with implicit parameters

Question

I can't figure out why the following code compiles fine:

#include <iostream>                                                                 

void bar(int x) {                                                                   
  std::cout << "int " << x << std::endl;                                            
}                                                                                   

void bar(double x) {                                                                
  std::cout << "double " << x << std::endl;                                         
}                                                                                   

template <typename A, typename B>    // Note the order of A and B.                                               
void foo(B x) {                                                                     
  bar((A)x);                                                                        
}                                                                                   

int main() {                                                                        
  int x = 1;                                                                        
  double y = 2;                                                                     

  foo<int>(x);        // Compiles OK.                                                              
  foo<double>(y);     // Compiles OK.                                               

  return 0;                                                                         
}

But if I switch the order of A and B as below, then it won't compile:

#include <iostream>                                                                 

void bar(int x) {                                                                   
  std::cout << "int " << x << std::endl;                                            
}                                                                                   

void bar(double x) {                                                                
  std::cout << "double " << x << std::endl;                                         
}                                                                                   

template <typename B, typename A>    // Order of A and B are switched.
void foo(B x) {                                                                     
  bar((A)x);                                                                        
}                                                                                   

int main() {                                                                        
  int x = 1;                                                                        
  double y = 2;                                                                     

  foo<int>(x);        // error: no matching function for call to ‘foo(int&)’
  foo<double>(y);     // error: no matching function for call to ‘foo(double&)’                                                              

  return 0;                                                                         
}      

EDIT: Ad-hoc explanations are welcome, but would be better if someone can point out exact what the spec. says. Thanks!

Answer 1

In the first one, the compiler knows that A is int because you specifically tell it so with foo<int>, and it knows that B is also int because of the parameter that you pass it. So both A and B are known or can be deduced (you could say: A is supplied, B is implied).

However, in the second one, since B comes first and A doesn't appear in the parameter list, the compiler can't tell what A is and gives you an error. You're explicitly telling it what B is with foo<int>, and then the parameter you pass is also a B which, at the call, is an int which agrees with your previous explicit definition of B, but no mention is made of A, implicitly or explicitly, so the compiler must stop and error.

You don't really need the standard for this, it's just common sense. What on earth would A be in the second one?

Thanks for asking this question though, because I didn't realise you could explicitly specify some parameters and implicitly specify others in the parameter list before this.