std::declval

template< class T >
typename std::add_rvalue_reference<T>::type declval() noexcept;

Converts any type T to a reference type, making it possible to use member functions in the operand of the decltype specifier without the need to go through constructors.

std::declval is commonly used in templates where acceptable template parameters may have no constructor in common, but have the same member function whose return type is needed.

Note that std::declval can only be used in unevaluated contexts and is not required to be defined; it is an error to evaluate an expression that contains this function. Formally, the program is ill-formed if this function is odr-used.

Parameters

(none).

Return value

Cannot be called and thus never returns a value. The return type is T&& unless T is (possibly cv-qualified) void, in which case the return type is T.

Possible implementation

template<typename T>
constexpr bool always_false = false;
 
template<typename T>
typename std::add_rvalue_reference<T>::type declval() noexcept
{
    static_assert(always_false<T>, "declval not allowed in an evaluated context");
}

Example

#include <iostream>
#include <utility>
 
struct Default
{
    int foo() const { return 1; }
};
 
struct NonDefault
{
    NonDefault() = delete;
    int foo() const { return 1; }
};
 
int main()
{
    decltype(Default().foo()) n1 = 1;                   // type of n1 is int
//  decltype(NonDefault().foo()) n2 = n1;               // error: no default constructor
    decltype(std::declval<NonDefault>().foo()) n2 = n1; // type of n2 is int
    std::cout << "n1 = " << n1 << '\n'
              << "n2 = " << n2 << '\n';
}

n1 = 1
n2 = 1

See also