Re: Can anyone explain what the argument in: __test_type(int _Tp::*); means ?
Ivan Novick wrote:
namespace __gnu_internal
{
typedef char __one;
typedef char __two[2];
template<typename _Tp>
__one __test_type(int _Tp::*);
template<typename _Tp>
__two& __test_type(...);
} // namespace __gnu_internal
There are 2 versions of the template member function __test_type.
I read it as the first will get instantiated if you pass in a member
function pointer of the _Tp class and the second will get instantiated
for any other template argument and takes a variable number of
parameters.
Almost. The first will get instantiated if _Tp is a class type,
so that the type int _Tp::* is a valid type. The second will
always be instantiated. Then function overload resolution takes
place: if you call the function with something that can be
converted to a pointer to member, and the first is instantiated,
it will be chosen. Otherwise, the second will be chosen.
Typically, this is used in something like:
template< bool B > void f(...) { ... }
f< sizeof( __test_type< T >( 0 ) ) == sizeof( __one) >()
The result is to generate a call f<true> if T is a class type,
and f<false> otherwise.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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