在cppreference.com中解释decltype关键字时，有这样的一段代码：
#include <iostream>
struct A { double x; };
const A* a = new A{ 0 };
decltype(a->x) y; // type of y is double (declared type)
decltype((a->x)) z = y; // type of z is const double& (lvalue expression)
template<typename T, typename 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) g = f; // the type of a lambda function is unique and unnamed
i = f(2, 2);
j = g(3, 3);
std::cout << "i = " << i << ", "
<< "j = " << j << '\n';
}
其中
template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u); // return type depends on template parameters
这段代码是什么意思？
求详解。
