[Boost-users] [utility] question about enable_if

From: Christian Henning (chhenning_at_[hidden])
Date: 2008-02-15 10:47:56

Hi there, I'm having a problem with using enable_if. For the sake of
simplicity I have created the following test code which compiles fine.

#include <string>

#include <boost/mpl/and.hpp>
#include <boost/utility/enable_if.hpp>

using namespace std;
using namespace boost;

struct reader {};

template< typename String > struct is_string : public mpl::false_ {};
template<> struct is_string<std::string> : public mpl::true_ {};

template< typename Device > struct is_device : public mpl::false_ {};
template<> struct is_device<reader> : public mpl::true_ {};

template< typename String>
struct enable_if_helper
{
typedef typename enable_if< typename is_string< String >::type >::type type;
};

template<typename Device >
void
foo( typename enable_if< typename is_device<Device>::type >::type* ptr = 0 )
{}

template<typename String>
void foo( typename enable_if< typename is_string<String>::type
>::type* ptr = 0 )
{ foo<reader>(); }

int main(int argc, _TCHAR* argv[])
{
   foo<std::string>();
        return 0;
}

If I changed the string foo() interface to use the enable_if_helper
the code doesn't compile anymore. Why is that?

I've changed the foo ( with the string template parameter ) to the following:

template<typename String>
void foo( typename enable_if_helper< String >::type* ptr = 0 )
{ foo<reader>(); }

Anyone can help me out here? I'm using Visual Studio 8.

Thanks,
Christian