12 Templates [temp]

12.6 Template declarations [temp.decls]

12.6.6 Function templates [temp.fct]

12.6.6.2 Partial ordering of function templates [temp.func.order]

1
#
If a function template is overloaded, the use of a function template specialization might be ambiguous because template argument deduction may associate the function template specialization with more than one function template declaration.
Partial ordering of overloaded function template declarations is used in the following contexts to select the function template to which a function template specialization refers:
2
#
Partial ordering selects which of two function templates is more specialized than the other by transforming each template in turn (see next paragraph) and performing template argument deduction using the function type.
The deduction process determines whether one of the templates is more specialized than the other.
If so, the more specialized template is the one chosen by the partial ordering process.
If both deductions succeed, the partial ordering selects the more constrained template as described by the rules in [temp.constr.order].
3
#
To produce the transformed template, for each type, non-type, or template template parameter (including template parameter packs thereof) synthesize a unique type, value, or class template respectively and substitute it for each occurrence of that parameter in the function type of the template.
[ Note
:
The type replacing the placeholder in the type of the value synthesized for a non-type template parameter is also a unique synthesized type.
— end note
 ]
If only one of the function templates M is a non-static member of some class A, M is considered to have a new first parameter inserted in its function parameter list.
Given cv as the cv-qualifiers of M (if any), the new parameter is of type “rvalue reference to cv A” if the optional ref-qualifier of M is && or if M has no ref-qualifier and the first parameter of the other template has rvalue reference type.
Otherwise, the new parameter is of type “lvalue reference to cv A”.
[ Note
:
This allows a non-static member to be ordered with respect to a non-member function and for the results to be equivalent to the ordering of two equivalent non-members.
— end note
 ]
[ Example
: 
struct A { };
template<class T> struct B {
  template<class R> int operator*(R&);              // #1
};

template<class T, class R> int operator*(T&, R&);   // #2

// The declaration of B​::​operator* is transformed into the equivalent of
// template<class R> int operator*(B<A>&, R&);      // #1a

int main() {
  A a;
  B<A> b;
  b * a;                                            // calls #1a
}
— end example
 ]
4
#
Using the transformed function template's function type, perform type deduction against the other template as described in [temp.deduct.partial].
[ Example
: 
template<class T> struct A { A(); };

template<class T> void f(T);
template<class T> void f(T*);
template<class T> void f(const T*);

template<class T> void g(T);
template<class T> void g(T&);

template<class T> void h(const T&);
template<class T> void h(A<T>&);

void m() {
  const int* p;
  f(p);             // f(const T*) is more specialized than f(T) or f(T*)
  float x;
  g(x);             // ambiguous: g(T) or g(T&)
  A<int> z;
  h(z);             // overload resolution selects h(A<T>&)
  const A<int> z2;
  h(z2);            // h(const T&) is called because h(A<T>&) is not callable
}
— end example
 ]
5
#
[ Note
:
Since partial ordering in a call context considers only parameters for which there are explicit call arguments, some parameters are ignored (namely, function parameter packs, parameters with default arguments, and ellipsis parameters).
[ Example
: 
template<class T> void f(T);                            // #1
template<class T> void f(T*, int=1);                    // #2
template<class T> void g(T);                            // #3
template<class T> void g(T*, ...);                      // #4

int main() {
  int* ip;
  f(ip);                                                // calls #2
  g(ip);                                                // calls #4
}
— end example
 ]
[ Example
: 
template<class T, class U> struct A { };

template<class T, class U> void f(U, A<U, T>* p = 0);   // #1
template<         class U> void f(U, A<U, U>* p = 0);   // #2
template<class T         > void g(T, T = T());          // #3
template<class T, class... U> void g(T, U ...);         // #4

void h() {
  f<int>(42, (A<int, int>*)0);                          // calls #2
  f<int>(42);                                           // error: ambiguous
  g(42);                                                // error: ambiguous
}
— end example
 ]
[ Example
: 
template<class T, class... U> void f(T, U...);          // #1
template<class T            > void f(T);                // #2
template<class T, class... U> void g(T*, U...);         // #3
template<class T            > void g(T);                // #4

void h(int i) {
  f(&i);                                                // error: ambiguous
  g(&i);                                                // OK: calls #3
}
— end example
 ]
— end note
 ]