downcasted_to_.hpp

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#pragma once    // Source encoding: UTF-8 with BOM (π is a lowercase Greek "pi").
#include <cppx-core/language/syntax/type-assemblers.hpp>        // cppx::P_
#include <cppx-core/language/tmp/basic-Enable_if_.hpp>          // cppx::Enable_if_
#include <cppx-core/language/tmp/basic-type-traits.hpp>         // cppx::is_base_and_derived_

namespace cppx
{
    namespace impl
    {
        template< class Derived, class Type >
        inline auto downcasted_to_( P_<Type> p )
            -> P_<Derived>
        {
            const auto p_derived = dynamic_cast<P_<Derived>>( p );
            return static_cast<P_<Type>>( p_derived ) == p? p_derived : nullptr;
        }
    }

    template<
        class Derived,
        class Type,
        class = Enable_if_<is_base_and_derived_<Type, Derived>>
        >
    inline auto downcasted_to_( P_<Type> p )
        -> P_<Derived>
    { return impl::downcasted_to_<Derived>( p ); }

    // const
    template<
        class Derived,
        class Type,
        class = Enable_if_<is_base_and_derived_<Type, Derived>>
        >
    inline auto downcasted_to_( P_<const Type> p )
        -> P_<const Derived>
    { return impl::downcasted_to_<const Derived>( p ); }
}  // namespace cppx