Type of an expression

expression-type.cpp

#include <iostream>
#include <utility>


// based on: https://medium.com/@barryrevzin/value-categories-in-c-17-f56ae54bccbe
// LIVE: http://coliru.stacked-crooked.com/a/38321a888132b051


template <class T>
struct E { static constexpr auto value = "prvalue\n"; };

template <class T>
struct E<T&> { static constexpr auto value = "lvalue\n"; };

template <class T>
struct E<T&&> { static constexpr auto value = "xvalue\n"; };


struct A { int i; };

int main ()
{
    int&& r = 42;

    // type of X expression is found by decltype( (X) )

    std::cout << E<decltype( (4) )>::value; // prvalue
    std::cout << E<decltype( (r) )>::value; // lvalue
    std::cout << E<decltype( (std::move(r)) )>::value; // xvalue

    std::cout << E<decltype( (A{}) )>::value; // prvalue
    std::cout << E<decltype( (A{}.i) )>::value; // xvalue (but printed prvalue)

}