mysql::meta::Not_decayed Concept Reference

false if Args is exactly one type, say A, and std::decay_t<A> equals Type. More...

#include <not_decayed.h>

Concept definition

template<class Type, class... Args>
concept mysql::meta::Not_decayed =  (!detail::Is_decayed_helper<Type, Args...>::value)

Detailed Description

false if Args is exactly one type, say A, and std::decay_t<A> equals Type.

The use case is to constrain constructors taking (variadic) forwarding reference arguments, so they cannot be invoked as copy constructor. Here is a typical example:

template <class T>
class Wrapper {
 public:
  // Copy constructor
  Wrapper(const Wrapper &) = default;
 
  // Forwarding constructor, invoking the constructor of m_wrapped.
  template <class... Args>
      requires Not_decayed<Wrapper<T>, Args_t...>
  Wrapper(Args &&... args) : m_wrapped(std::forward<Args>(args)...) {}
 
 private:
  T m_wrapped;
};
 
Wrapper<int> w1(1);  // invokes forwarding constructor
Wrapper<int> w2(w1); // invokes copy constructor

Here, we expect that w2's copy constructor is invoked. If we would omit the Not_decayed constraint, the forwarding constructor would have been "better" according to C++'s overload resolution rules. The Not_decayed constraint excludes the forwarding constructor from the candidates and thus makes the copy constructor be the only viable overload.

See also https://akrzemi1.wordpress.com/2013/10/10/too-perfect-forwarding/