decltype

#include <iostream>
#include <type_traits>

struct A { double x; };
const A* a;

decltype(a->x) y;       //  y  double ( )
decltype((a->x)) z = y; //  z  const double& ( lvalue)

template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u) //      
                                      //     ,
                                      //   ++14
{
    return t + u;
}

const int& getRef(const int* p) { return *p; }
static_assert(std::is_same_v<decltype(getRef), const int&(const int*)>);
auto getRefFwdBad(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdBad), int(const int*)>,
    "  auto    .");
decltype(auto) getRefFwdGood(const int* p) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood), const int&(const int*)>,
    " decltype(auto)    .");

// :
auto getRefFwdGood1(const int* p) -> decltype(getRef(p)) { return getRef(p); }
static_assert(std::is_same_v<decltype(getRefFwdGood1), const int&(const int*)>,
    " decltype( return)     .");

int main()
{
    int i = 33;
    decltype(i) j = i * 2;
    static_assert(std::is_same_v<decltype(i), decltype(j)>);
    assert(i == 33 && 66 == j);

    auto f = [i](int a, int b) -> int { return a * b + i; };
    auto h = [i](int a, int b) -> int { return a * b + i; };
    static_assert(!std::is_same_v<decltype(f), decltype(h)>,
        " -     ");

    decltype(f) g = f;
    std::cout << f(3, 3) << ' ' << g(3, 3) << '\n';
}