meta::swappable

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

template <typename T>
concept swappable = requires(T& a, T& b) {
   /* using std::swap; */ swap(a, b);
};

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

---

Differences with the C++20 version

• The implementation does not rely on std::ranges::swap, but on the usual customization point idiom, i.e. using std::swap; swap(/* ... */);.

• The std::swappable_with concept cannot be implemented using std::swap, since it constrains its parameters to have the exact same type.

Concept emulation

namespace mgs {
namespace meta {

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

template <typename T>
constexpr auto is_swappable_v = is_swappable<T>::value;

template <typename T,
          typename = std::enable_if_t<is_swappable_v<T>>>
using swappable = T;

} // namespace meta
} // namespace mgs

Example

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

using namespace mgs::meta;

struct dummy_swappable {
  // prevent from being used by default std::swap
  dummy_swappable(dummy_swappable const&) = delete;
};

void swap(dummy_swappable&, dummy_swappable&);

static_assert(is_swappable_v<int>, "");
static_assert(is_swappable_v<dummy_swappable>, "");