NAME¶

std::declval - std::declval

Synopsis¶

Defined in header <utility>
template<class T> (since C++11)
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¶

// Run this code

#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';
}

Output:¶

n1 = 1
n2 = 1

See also¶

decltype specifier(C++11) obtains the type of an expression or an entity
result_of deduces the result type of invoking a callable object with
invoke_result a set of arguments
(C++11)(removed in C++20) (class template)
(C++17)