fmt里判断是否为reference_wrapper

 参考：

一窥模板的替换和匹配方式：偏特化的参数比泛化版本的还要多：判断是不是std::pair＜,＞。-CSDN博客

C:\work\hchx\HchxKernel\PublicAPI\fmt\args.h

template <typename T> struct is_reference_wrapper : std::false_type {};
template <typename T>
struct is_reference_wrapper<std::reference_wrapper<T>> : std::true_type {};

 一个引用的地方：

template <typename T>
  using stored_type = conditional_t<detail::is_string<T>::value &&
                                        !has_formatter<T, Context>::value &&
                                        !detail::is_reference_wrapper<T>::value,
                                    std::basic_string<char_type>, T>;

reference_wrapper的定义

template<class _Ty>
	class reference_wrapper
		: public _Weak_types<_Ty>::type
{	// stand-in for an assignable reference
public:
	static_assert(is_object_v<_Ty> || is_function_v<_Ty>,
		"reference_wrapper<T> requires T to be an object type or a function type.");

	using type = _Ty;

	template<class _Uty,
		enable_if_t<conjunction_v<
			negation<is_same<remove_cv_t<remove_reference_t<_Uty>>, reference_wrapper>>,
			_Refwrap_has_ctor_from<_Ty, _Uty>>, int> = 0>
		reference_wrapper(_Uty&& _Val)
			_NOEXCEPT_COND(_NOEXCEPT_OPER(_Refwrap_ctor_fun<_Ty>(_STD declval<_Uty>())))
			{	// construct
			_Ty& _Ref = _STD forward<_Uty>(_Val);
			_Ptr = _STD addressof(_Ref);
			}

	operator _Ty&() const noexcept
		{	// return reference
		return (*_Ptr);
		}

	_NODISCARD _Ty& get() const noexcept
		{	// return reference
		return (*_Ptr);
		}

	template<class... _Types>
		auto operator()(_Types&&... _Args) const
		-> decltype(_STD invoke(get(), _STD forward<_Types>(_Args)...))
		{	// invoke object/function
		return (_STD invoke(get(), _STD forward<_Types>(_Args)...));
		}

private:
	_Ty * _Ptr;
	}