Re: Should this short (but abstruse) program compile?

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 14 Jul 2006 11:04:33 -0400

Message-ID:

<e98bq4$bdq$1@news.datemas.de>

Howard Gardner wrote:

/*

As it sits, this program won't compile for me. It will compile
using either of the versions of tpt that I've commented out.
Which versions SHOULD compile?

I think that the code is interesting because I think that it is
on the path to building a useful non-intrusive type introspection
facility. The version that uses "has" indicates the direction.

*/
#include <ostream>
#include <cstddef>

template< typename x >
x * faux_ptr();

//template< typename x, size_t = sizeof( int * ) >
// struct tpt{};

//template< typename x, size_t = sizeof( faux_ptr< int >() ) >
// struct tpt{};

//struct has{ int memvar; };
//template< typename x, size_t = sizeof( faux_ptr< has >()->memvar ) >
// struct tpt{};

template< typename x, size_t = sizeof( faux_ptr< x >() ) >
struct tpt{};

Why don't you simply make it

template< typename x, size_t = sizeof( x* ) >
struct tpt{};

Isn't it the same?

Incidentally, the original code compiles fine for me with VC++ 2005
and doesn't compile with online Comeau trial.

template< typename x >
bool fnc( tpt< x > );

It might be a mistake to try to rely on the default template argument
here. The syntax 'tpt< x >' here might not give the compiler good enough
a hint so it knows to use the default arg. Have you tried

template< typename x, size_t s >
bool fnc( tpt< x,s > );

? Comeau compiles the code in this case.

tpt< int > inst;

size_t size = sizeof( fnc( inst ) );

int main()
{
using namespace std;
cout << size << endl;
}

V
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