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 decltype expressions without the need to go through constructors.

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 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 <utility>
#include <iostream>
 
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