Typedef declaration syntax

Question

A few days ago I looked at the boost sources and found an interesting typedef .

There is code from "boost \ detail \ none_t.hpp":

namespace boost {

namespace detail {

struct none_helper{};

typedef int none_helper::*none_t ;

} // namespace detail

} // namespace boost

I have not seen such syntax before and cannot explain the meaning of it.

This typedef introduces the name "none_t" as a pointer to an int in the boost :: detail namespace.

What is syntax?

What's the difference between " typedef int none_helper :: * none_t " and, for example, " typedef int * none_t "?

+2

mt_serg Apr 19 10 at 12:35

3 answers

typedef int* none_t;

introduces a type alias for a pointer to an integer.
typedef int non_helper::*none_t;

introduces a type alias for a pointer to an integer member of the class non_helper

.

+1

Nikolai Fetissov Apr 19 10 at 12:40

none_t

is a pointer to a member variable of type int

none_helper

.

struct none_helper
{
  int x1;
  int x2;
};

int none_helper::* ptm = &none_helper::x1;
^^^^^^^^^^^^^^^^^^
     none_t

+1

Kirill V. Lyadvinsky Apr 19 10 at 12:41

Georg Fritzsche · Accepted Answer · 2010-04-19T12:40:35+0000

The syntax is for a pointer to a member - here it's typedefs none_t

as a pointer to a data member int

none_helper

.

The syntax can be used for example. in the following way:

 struct X { int i; };

 typedef int X::*PI;
 PI pi = &X::i;
 X* x = foo();
 x->*pi = 42;

InformIT has an article on Member Pointers for more details.