meta::boolean¶

Defined in header <mgs/meta/concepts/boolean.hpp>.

template <typename B>
concept boolean =
  meta::movable<std::remove_cvref_t<B>> &&
  requires(std::remove_reference_t<B> const& b1,
           std::remove_reference_t<B> const& b2, bool const a) {
    { b1 } -> meta::convertible_to<bool>;
    { !b1 } -> meta::convertible_to<bool>;
    { b1 &&  a } -> meta::same_as<bool>;
    { b1 ||  a } -> meta::same_as<bool>;
    { b1 && b2 } -> meta::same_as<bool>;
    {  a && b2 } -> meta::same_as<bool>;
    { b1 || b2 } -> meta::same_as<bool>;
    {  a || b2 } -> meta::same_as<bool>;
    { b1 == b2 } -> meta::convertible_to<bool>;
    { b1 ==  a } -> meta::convertible_to<bool>;
    {  a == b2 } -> meta::convertible_to<bool>;
    { b1 != b2 } -> meta::convertible_to<bool>;
    { b1 !=  a } -> meta::convertible_to<bool>;
    {  a != b2 } -> meta::convertible_to<bool>;
  };

Pre-C++20 implementation of the std::boolean concept.

Concept emulation¶

namespace mgs {
namespace meta {

template <typename T>
struct is_boolean { /* ... */ };

template <typename T>
constexpr auto is_boolean_v = is_boolean<T>::value;

template <typename T,
          typename = std::enable_if_t<is_boolean_v<T>>>
using boolean = T;

} // namespace meta
} // namespace mgs

Example¶

#include <mgs/meta/concepts/boolean.hpp>

using namespace mgs::meta;

static_assert(is_boolean_v<bool>, "");
static_assert(is_boolean_v<std::true_type>, "");
static_assert(!is_boolean_v<int*>, "");

See also¶