mirror/meta_prog/optional.hpp

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#ifndef MIRROR_META_PROG_OPTIONAL_1104130956_HPP
#define MIRROR_META_PROG_OPTIONAL_1104130956_HPP

#include <mirror/meta_prog/forward_decl.hpp>
#include <mirror/meta_prog/nil_type.hpp>
#include <mirror/meta_prog/traits.hpp>

MIRROR_NAMESPACE_BEGIN
namespace mp {


template <typename T>
struct optional
{
    typedef optional type;
};

typedef optional<nil_t> nil_optional;

#ifdef MIRROR_DOCUMENTATION_ONLY


template <typename Optional>
struct nil { };
#else
// The default implementation of the empty meta-function
template <typename X>
struct nil : nil<typename X::type>::type
{
    MIRROR_ASSERT_RETURNS_OPTIONAL(X);
};
#endif

template <>
struct nil<nil_optional>
 : std::true_type
{ };

template <typename T>
struct nil<optional<T> >
 : std::false_type
{ };

#ifdef MIRROR_DOCUMENTATION_ONLY


template <typename Optional>
struct get
{
    typedef unspecified_type type;
};
#else
template <typename X>
struct get
{
    MIRROR_ASSERT_RETURNS_OPTIONAL(X);
    typedef typename get<typename X::type>::type type;
};
#endif

template <typename T>
struct get<optional<T> >
{
    typedef T type;
};

namespace aux {

template <typename Expr, typename IsOptional>
struct as_optional;

template <typename Optional>
struct as_optional<Optional, std::true_type>
{
    typedef Optional type;
};

template <typename Expr>
struct as_optional<Expr, std::false_type>
{
    MIRROR_ASSERT_RETURNS_OPTIONAL(Expr);
    typedef typename Expr::type type;
};

} // namespace aux

template <typename Expr>
struct as_optional : public aux::as_optional<
    Expr,
    typename is_optional<Expr>::type
>{ };


} // namespace mp
MIRROR_NAMESPACE_END

#endif //include guard