14 Templates [temp]

14.5 Template declarations [temp.decls]

14.5.6 Function templates [temp.fct] Partial ordering of function templates [temp.func.order]

If a function template is overloaded, the use of a function template specialization might be ambiguous because template argument deduction ([temp.deduct]) 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:

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.

To produce the transformed template, for each type, non-type, or template template parameter (including template parameter packs ([temp.variadic]) 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. If only one of the function templates is a non-static member, that function template is considered to have a new first parameter inserted in its function parameter list. The new parameter is of type “reference to cv A,” where cv are the cv-qualifiers of the function template (if any) and A is the class of which the function template is a member. [ Note: This allows a non-static member to be ordered with respect to a nonmember function and for the results to be equivalent to the ordering of two equivalent nonmembers.  — 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 ]

Using the transformed function template's function type, perform type deduction against the other template as described in [temp.deduct.partial].


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 ]

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 ]