Re: [boost] [MPL] meta-functions for STL templates

From: Daryle Walker (darylew_at_[hidden])
Date: 2006-11-12 15:42:28

On 11/11/06 9:10 PM, "Sascha Krissler" <boost-dev_at_[hidden]>
wrote:

> i had this little idea.
> when yo want to customize a class with a container like
> 
> template <typename T, typename Container>
> struct Foo;
> 
> then you must instanciate it with:
> 
> Foo<int, std::vector<int> >
> 
> the second int is redundant.
[TRUNCATE failed solution with a template-based template parameter and a
better solution with MPL reading or writing template parameters]

No, it's not redundant.  STL stuff involving containers is set up so a
direct, non-template, class can be used as a container.  Solutions involving
certain template games will break that philosophy.

I use asserts to make sure a container is compliant with what I need.  Here
is an example from some code I'm putting in the Sandbox:

//========================================================================
#include <memory>                         // for std::allocator
#include <boost/type_traits/is_same.hpp>  // for boost::is_same
#include <boost/static_assert.hpp>        // for BOOST_STATIC_ASSERT

namespace boost { namespace math {
//...

template < int Radix, class Allocator = ::std::allocator<int> >
class big_radix_whole
{
    // Pre-conditions
    // (SNIP constraints on "Radix")
    BOOST_STATIC_ASSERT( (is_same<int,
     typename Allocator::value_type>::value) );

    // ...
};

//...
} }
//========================================================================

Now I can use a non-template allocator class like:

//========================================================================
#include <cstddef>

class my_int_allocator  // Note: _no_ "template" preamble!
{
public:
    typedef int                    value_type;
    typedef std::size_t             size_type;
    typedef std::ptrdiff_t    difference_type;
    typedef int *                pointer;
    typedef int const *    const_pointer;
    typedef int &              reference;
    typedef int const &  const_reference;

    // You're screwed if you need "rebind".

    // Put address, max_size, allocate, deallocate, etc. here...
};
//========================================================================

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com