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Showing content from https://cplusplus.com/declval below:

function template

<utility>

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

Declaration value

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).

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// 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;
}

common_type

Common type (class template)

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