The standard library utility declval is defined as: template<class T> add_rvalue_reference_t<T> declval() noexcept; To add a rvalue reference here seemed like a good idea, if you think...

Code as below or on godbolt compiles with gcc and MSVC but fails with clang. I couldn't find if/where it is forbidden in the standard. In my opinion it should be supported. So who is correct on thi...

I have been trying to understand the implementation of std::reference_wrapper, from here, which is as follows: namespace detail { template <class T> constexpr T& FUN(T& t) noexcept { ...

I see some code examples where the type used to instantiate the std::declval template function is specified as a reference type rather than just a type, as in: std::declval<T &>() as opp...

The code below comes from libstdc++-v3 std::type_traits, which is an implementation of std::declval: template<typename _Tp, typename _Up = _Tp&&> // template 1 _Up __declval(int); ...

Consider this example (coming from here): #include <type_traits> #include <iostream> template <typename U> struct A { }; struct B { template <typename F = int> A&l...

I am trying to do something like this (in c++11): #include <utility> template <typename T> struct base { using type = decltype( std::declval<T>().foo() ); }; struct bar : base...

Looking at libstdc++ source code, I found the following declval implementation: template<typename _Tp, typename _Up = _Tp&&> _Up __declval(int); // (1) template<typename _Tp> ...

I have a function and I need to test whether I can pass an argument of a given type to it. For example: template<typename T, auto F> decltype(F(declval<T>{})) foo(); Calling foo<i...

With c++17 we have fancy new is_invocable and fancy new prvalues that aren't really values. This permits you to create an object without having to first logically construct it, then elide the cons...

I am trying to understand how std::declval<T>() works. I know how to use it, and know what it does, mainly allows you to use decltype without constructing the object, like decltype(std::decl...

std::declval is a compile-time utility used to construct an expression for the purpose of determining its type. It is defined like this: template< class T > typename std::add_rvalue_referenc...

changing a type into a reference to a type, allows one to access the members of the type without creating an instance of the type. This seems to be true for both lvalue references and rvalue refere...