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

Example¶

// Run this code

#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';
}

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)