19.15.txt

Exercise 19.15: What is the difference between an ordinary function pointer and
a pointer to a member function?

By Faisal Saadatmand

As is the case with a data member pointer, a function member pointer does not
point to an object in memory (in this case a function), but rather it points to
a non-static member function of a class. As such, it, too, requires a special
syntax.

	// declaration
	[return type] ([class type]::*[function name]) ([parameters]) [qualifier];

	// definition
	[return type] ([class type]::*[function name]) ([parameters]) [qualifier]
											= &[class type]::[function name];
for example,

	char (Screen::*pmf)(Screen::pos, Screen::pos) const = &Screen::get;

	// we can use auto to simplify the declaration (if the name is not ambiguous)
	auto pmf = &Screen::get_cursor;  // OK; no overloads
	auto pmg = &Screen::get; // error; ambiguous: get() or get(Screen::pos, Screen::pos)?

Moreover, "unlike ordinary function pointers, there is no automatic conversion
between a member function and a pointer to that member." (p. 838) Thus, the
address-of, &, operator must be used to assign a member function to a member
function pointer.

Reading ahead, another an important distinction between the two, a member
function is not a callable object (section 19.4.3, p. 841). This means we
cannot use a member function pointer in library algorithms that expect a
callable object as we normally would with a function pointer. We can, however,
use the library 'function' template (p. 842), mem_fn or bind (p. 843) to
generate a callable object from a member function pointer (p. 842) that we
could then pass to an algorithm.