Special Deduction Situations

Special Deduction Situations

There are two situations in which the pair (A, P) used for deduction is not obtained from the arguments to a function call and the parameters of a function template. The first situation occurs when the address of a function template is taken. In this case, P is the parameterized type of the function template declarator, and A is the function type underlying the pointer that is initialized or assigned to. For example:

template<typename T> 
void f(T, T); 

void (*pf)(char, char) = &f; 

In this example, P is void(T, T) and A is void(char, char). Deduction succeeds with T substituted with char, and pf is initialized to the address of the specialization f<char>.

The other special situation occurs with conversion operator templates. For example:

class S { 
  public: 
    template<typename T, int N> operator T[N]&(); 
}; 

In this case, the pair (P, A) is obtained as if it involved an argument of the type to which we are attempting to convert and a parameter type that is the return type of the conversion operator. The following code illustrates one variation:

void f(int (&)[20]); 

void g(S s) 
{ 
    f(s); 
} 

Here we are attempting to convert S to int (&)[20]. Type A is therefore int[20] and type P is T[N]. The deduction succeeds with T substituted with int and N with 20.