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### Internal Support Code

There are some traits classes which authors of new backends should be aware of:

    namespace boost{ namespace multiprecision{ namespace detail{

    template<typename From, typename To>
    struct is_explicitly_convertible;

    }}}

Inherits from `boost::integral_constant<bool,true>` if type `From` has an explicit conversion from `To`.

For compilers that support C++11 SFINAE-expressions this trait should "just work". Otherwise it inherits from `boost::is_convertible<From, To>::type`, and will need to be specialised for Backends that have constructors marked as `explicit`.

    template <class From, class To>
    struct is_lossy_conversion
    {
       static const bool value = see below;
    };

Member `value` is true if the conversion from `From` to `To` would result in a loss of precision, and `false` otherwise.

The default version of this trait simply checks whether the kind of conversion (for example from a floating-point to an integer type) is inherently lossy. Note that if either of the types `From` or `To` are of an unknown number category (because `number_category` is not specialised for that type) then this trait will be `true`.

    template<typename From, typename To>
    struct is_restricted_conversion
    {
       static const bool value = see below;
    };

Member `value` is `true` if `From` is only explicitly convertible to `To` and not implicitly convertible, or if `is_lossy_conversion<From, To>::value` is `true`. Otherwise `false`.

Note that while this trait is the ultimate arbiter of which constructors are marked as `explicit` in class `number`, authors of backend types should generally specialise one of the traits above, rather than this one directly.

    template <class T>
    is_signed_number;
    template <class T>
    is_unsigned_number;

These two traits inherit from either `mpl::true_` or `mpl::false_`, by default types are assumed to be signed unless `is_unsigned_number` is specialized for that type.