function template
<utility>

std::declval

template <class T>  typename add_rvalue_reference<T>::type declval() noexcept;
Declaration value
Returns an rvalue reference to type T without referring to any object.

This function shall only be used in unevaluated operands (such as the operands of sizeof and decltype).

T may be an incomplete type.

This is a helper function used to refer to members of a class in unevaluated operands, especially when either the constructor signature is unknown or when no objects of that type can be constructed (such as for abstract base classes).

Parameters

none

Return value

An rvalue reference to type T.
The return type uses add_rvalue_reference to return T&& when T is an object type.

Example

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
// declval example
#include <utility>      // std::declval
#include <iostream>     // std::cout

struct A {              // abstract class
  virtual int value() = 0;
};

class B : public A {    // class with specific constructor
  int val_;
public:
  B(int i,int j):val_(i*j){}
  int value() {return val_;}
};

int main() {
  decltype(std::declval<A>().value()) a;  // int a
  decltype(std::declval<B>().value()) b;  // int b
  decltype(B(0,0).value()) c;   // same as above (known constructor)
  a = b = B(10,2).value();
  std::cout << a << '\n';
  return 0;
}

Output:
20


Data races

Calling this function introduces no data races.

Exceptions

No-throw guarantee: this function never throws exceptions.

See also