Re: [boost] has_member?

From: Vladimir Prus (ghost_at_[hidden])
Date: 2002-03-22 03:53:54

Aleksey Gurtovoy wrote:
> Seems that something like the following should give us an ability to check
> for presence of a particulary named member [function/data] in a class:
>
> typedef char (&no_tag)[1];
> typedef char (&yes_tag)[2];
>
> template< typename T > no_tag has_member_foo_helper(...);
>
> template< typename T >
> yes_tag has_member_foo_helper(int, void (T::*)() = &T::foo);
>
> template< typename T >
> struct has_member_foo
> {
>     BOOST_STATIC_CONSTANT(bool
>         , value = sizeof(has_member_foo_helper<T>(0)) == sizeof(yes_tag)
>         );
> };
>
> struct my {};
> struct her { void foo(); };
>
> int main()
> {
>     BOOST_STATIC_ASSERT(!has_member_foo<my>::value);
>     BOOST_STATIC_ASSERT(has_member_foo<her>::value);
>
>     return 0;
> }

With both g++ 3.0 and online Comeau compiler it just says that "my" has no 
member "foo", which is supposedly not what you want? I don't understand how 
this can work at all: if default parameter is never instantiated (which is 
what my inderstanding of std::14.7.1/[1-2] suggests, but contrary to 
compiler's opinion), there's not way to check if the member is present. 
Otherwise, you'll have compile error.

- Volodya