decltype specifier

#include <iostream>

struct A {
   double x;
};
const A* a = new A();

decltype( a->x ) x3;       // type of x3 is double (declared type)
decltype((a->x)) x4 = x3;  // type of x4 is const double& (lvalue expression)

template <class T, class U>
auto add(T t, U u) -> decltype(t + u); // return type depends on template parameters

int main() 
{
    int i = 33;
    decltype(i) j = i*2;
   
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';

    auto f = [](int a, int b) -> int {
       return a*b;
    };
   
    decltype(f) f2{f}; // the type of a lambda function is unique and unnamed
    i = f(2, 2);
    j = f2(3, 3);
   
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';
}