c++ - Empirically determine value category of C++11 expression? -

each expression in c++11 has value category. 1 of lvalue, xvalue or prvalue.

is there way write macro that, given expression argument, produce string "lvalue", "xvalue" or "prvalue" appropriate?

for example:

int main() {     int x;      cout << value_cat(x) << endl; // prints lvalue     cout << value_cat(move(x)) << endl; // prints xvalue     cout << value_cat(42) << endl; // prints prvalue } 

how value_cat implemented?

decltype can return declared type of entity (hence name), can used query type of expression. however, in latter case resulting type 'adjusted' according value category of expression: lvalue expression results in lvalue reference type, xvalue in rvalue reference type, , prvalue in type. can use our benefit:

template<typename t> struct value_category {     // or can integral or enum value     static constexpr auto value = "prvalue"; };  template<typename t> struct value_category<t&> {     static constexpr auto value = "lvalue"; };  template<typename t> struct value_category<t&&> {     static constexpr auto value = "xvalue"; };  // double parens ensuring inspect expression, // not entity #define value_category(expr) value_category<decltype((expr))>::value