C++ Utility Library - declval Function

Description

It returns an rvalue reference to type T without referring to any object.

Declaration

Following is the declaration for std::declval function.

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

C++11

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

Parameters

a, b − These are two objects.

Return Value

It returns an rvalue reference to type T without referring to any object.

Exceptions

Basic guarantee − if the construction or assignment of type T throws.

Data races

Calling this function introduces no data races.

Example

In below example explains about std::declval function.

#include <utility>
#include <iostream>

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

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

int main() {
   decltype(std::declval<A>().value()) a;
   decltype(std::declval<B>().value()) b;
   a = b = B(100,20).value();
   std::cout << a << '\n';
   return 0;
}

Let us compile and run the above program, this will produce the following result −

2000