not_decayed.h

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// Copyright (c) 2025, Oracle and/or its affiliates.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0,
// as published by the Free Software Foundation.
//
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License, version 2.0, for more details.
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#ifndef MYSQL_META_NOT_DECAYED_H
#define MYSQL_META_NOT_DECAYED_H
 
/// @file
/// Experimental API header
 
#include <concepts>     // same_as
#include <type_traits>  // bool_constant
 
/// @addtogroup GroupLibsMysqlMeta
/// @{
 
namespace mysql::meta::detail {
 
template <class T, class... Args>
struct Is_decayed_helper : public std::false_type {};
 
template <class T, class U>
struct Is_decayed_helper<T, U>
    : public std::bool_constant<std::same_as<T, std::decay_t<U>>> {};
 
}  // namespace mysql::meta::detail
 
namespace mysql::meta {
 
/// 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:
///
/// @code
/// 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
/// @endcode
///
/// 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/
template <class Type, class... Args>
concept Not_decayed = (!detail::Is_decayed_helper<Type, Args...>::value);
 
}  // namespace mysql::meta
 
// addtogroup GroupLibsMysqlMeta
/// @}
 
#endif  // ifndef MYSQL_META_NOT_DECAYED_H