FAQ 38.01 What is the type of a pointer to a nonstatic member function?

The most important thing to understand is that the type is different from that of a pointer to a C-style (non-member) function. Simply understanding that they are completely different and have incompatible types will prevent the most common and dangerous errors with pointers to member functions.

A pointer to the nonstatic member function with signature void Fred::f(int) has type void(Fred::*)(int). In particular, the type of the pointer to a nonstatic member function includes the class of the member function because nonstatic member functions have an implicit parameter that points to the object (the this pointer).

Here's an example.

 #include <iostream> using namespace std; class Fred { public:   void f(int i) throw();   void g(int i) throw();   void h(int i) throw(); }; void Fred::f(int i) throw()   { cout << "Fred::f(int); i=" << i << '\n'; } void Fred::g(int i) throw()   { cout << "Fred::g(int); i=" << i << '\n'; } void Fred::h(int i) throw()   { cout << "Fred::h(int); i=" << i << '\n'; } typedef void(Fred::*FredMemberPtr)(int); 

Note the use of the typedef. Because of the rather obscure syntax of pointers to nonstatic member functions, it is highly recommended that a typedef be used to represent the pointer type.

In the following example, a pointer p is created to point to Fred::g. This pointer is then used to call the member function.

 void sample(Fred& x, FredMemberPtr p) throw() { (x.*p)(42); }                                      <-- 1 int main() {   FredMemberPtr p = &Fred::g;   Fred x;   sample(x, p); } 

(1) If p is &Fred::g, this is the same as x.g(42)

The output of this program is as follows.

 Fred::g(int); i=42 

A pointer to a nonstatic member function of class Fred has a totally different type from a pointer to a function. For example, the pointer type void(Fred::*)(int) is totally different from the pointer type void(*)(int). Do not use a cast to try to convert between the two types. You have been warned.

A pointer to a static member function of class Fred has the same type as a pointer to a C-like function. In other words, a C-like function or static member function can be converted to the same pointer to function type, such as void(*)(int). But a pointer to a nonstatic member function cannot be converted to a normal pointer to a function type.