std::declval 源码分析

C++
#include      // std::declval
#include     // std::cout
struct A {              // abstract classvirtual int value() = 0;
};
class B : public A {    // class with specific constructorint val_;
public:
  B(int i,int j):val_(i*j){}
  int value() {return val_;}
};
int main() {
  decltype(std::declval().value()) a;  // int a
  decltype(std::declval().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;
}