Re: [Boost-users] MPL if_ not following true branch

Subject: Re: [Boost-users] MPL if_ not following true branch
From: joaquin_at_[hidden]
Date: 2009-05-18 03:48:28

Cory Nelson escribió:
> This code doesn't seem to ever follow the true branch of the if_...
> it's like the eval_if is clobbering it.  Can someone tell me what I'm
> doing wrong (other than not using a map.. this is a reduction of some
> other code that can't use a map)?
>
> #include <iostream>
> #include <typeinfo>
> #include <boost/mpl/if.hpp>
> #include <boost/mpl/eval_if.hpp>
> #include <boost/mpl/identity.hpp>
> #include <boost/type_traits/is_same.hpp>
> #include <boost/static_assert.hpp>
>
> template<typename T>
> struct choose_type
> {
> 	struct int_tag {};
> 	struct float_tag {};
>
> 	template<typename InvalidT>
> 	struct invalid_type_specified
> 	{
> 		BOOST_STATIC_ASSERT(sizeof(InvalidT) == 0);
> 	};
>
> 	typedef typename boost::mpl::if_<
> 		boost::is_same<T, int>,
> 		int_tag,
> 		typename boost::mpl::eval_if<
> 			boost::is_same<T, float>,
> 			boost::mpl::identity<float_tag>,
> 			invalid_type_specified<T>
> 		>::type
> 	>::type type;
> };
>   
The problem with your code is that the else part of your mpl::if_ expression
is always evaluated (it has the form mpl::eval_if<...>::type), so when you
you instantiate choose_type<int> the mpl::eval_if_  is also instantiated and
static asserts (as int is not float). Try this instead:

struct choose_type
{
    struct int_tag {};
    struct float_tag {};

    template<typename InvalidT>
    struct invalid_type_specified
    {
        BOOST_STATIC_ASSERT(sizeof(InvalidT) == 0);
    };

    typedef typename boost::mpl::eval_if<
        boost::is_same<T, int>,
        boost::mpl::identity<int_tag>,
        typename boost::mpl::eval_if<
            boost::is_same<T, float>,
            boost::mpl::identity<float_tag>,
            invalid_type_specified<T>
        >
    >::type type;
};

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo