12 Overloading [over]

12.3 Address of an overload set [over.over]

An id-expression whose terminal name refers to an overload set S and that appears without arguments is resolved to a function, a pointer to function, or a pointer to member function for a specific function that is chosen from a set of functions selected from S determined based on the target type required in the context (if any), as described below.
The target can be
The id-expression can be preceded by the & operator.
[Note 1:
Any redundant set of parentheses surrounding the function name is ignored ([expr.prim.paren]).
— end note]
If there is no target, all non-template functions named are selected.
Otherwise, a non-template function with type F is selected for the function type FT of the target type if F (after possibly applying the function pointer conversion ([conv.fctptr])) is identical to FT.
[Note 2:
That is, the class of which the function is a member is ignored when matching a pointer-to-member-function type.
— end note]
The specialization, if any, generated by template argument deduction ([temp.over], [temp.deduct.funcaddr], [temp.arg.explicit]) for each function template named is added to the set of selected functions considered.
Non-member functions and static member functions match targets of function pointer type or reference to function type.
Non-static member functions match targets of pointer-to-member-function type.
[Note 3:
If a non-static member function is chosen, the result can be used only to form a pointer to member ([expr.unary.op]).
— end note]
All functions with associated constraints that are not satisfied ([temp.constr.decl]) are eliminated from the set of selected functions.
If more than one function in the set remains, all function template specializations in the set are eliminated if the set also contains a function that is not a function template specialization.
Any given non-template function F0 is eliminated if the set contains a second non-template function that is more constrained than F0 according to the partial ordering rules of [temp.constr.order].
Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized than the function template of F1 according to the partial ordering rules of [temp.func.order].
After such eliminations, if any, there shall remain exactly one selected function.
[Example 1: int f(double); int f(int); int (*pfd)(double) = &f; // selects f(double) int (*pfi)(int) = &f; // selects f(int) int (*pfe)(...) = &f; // error: type mismatch int (&rfi)(int) = f; // selects f(int) int (&rfd)(double) = f; // selects f(double) void g() { (int (*)(int))&f; // cast expression as selector }
The initialization of pfe is ill-formed because no f() with type int(...) has been declared, and not because of any ambiguity.
For another example,
struct X { int f(int); static int f(long); }; int (X::*p1)(int) = &X::f; // OK int (*p2)(int) = &X::f; // error: mismatch int (*p3)(long) = &X::f; // OK int (X::*p4)(long) = &X::f; // error: mismatch int (X::*p5)(int) = &(X::f); // error: wrong syntax for // pointer to member int (*p6)(long) = &(X::f); // OK — end example]
[Note 4:
If f and g are both overload sets, the Cartesian product of possibilities is considered to resolve f(&g), or the equivalent expression f(g).
— end note]
[Note 5:
Even if B is a public base of D, we have D* f(); B* (*p1)() = &f; // error void g(D*); void (*p2)(B*) = &g; // error
— end note]