meta::convertible_to

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

template <typename From, typename To>
concept convertible_to =
  std::is_convertible_v<From, To> &&
  requires(From (&f)()) {
    static_cast<To>(f());
  };

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

---

Differences with the C++20 version

• Checks template arguments before calling std::is_convertible_v, which is not SFINAE-friendly.

Concept emulation

namespace mgs {
namespace meta {

template <typename From, typename To>
struct is_convertible_to { /* ... */ };

template <typename From, typename To>
constexpr auto is_convertible_to_v = is_convertible_to<From, To>::value;

template <typename From, typename To,
          typename = std::enable_if_t<is_convertible_to_v<From, To>>
using convertible_to = From;

} // namespace meta
} // namespace mgs

Example

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

using namespace mgs::meta;

struct A {};
struct B { explicit operator A(); };
struct C { operator A(); };

static_assert(is_convertible_to_v<C, A>, "");
static_assert(!is_convertible_to_v<B, A>, "");