How to determine programmatically if an expression is prvalue or lvalue or xvalue in C++? [duplicate]

Question

Each expression in C++11 has a value category. One of lvalue, xvalue or prvalue.

Is there a way to write a macro that, given any expression as an argument, will produce a string "lvalue", "xvalue" or "prvalue" as 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 could VALUE_CAT be implemented?

Answer 1

decltype can return the declared type of an entity (hence the name), but can also be used to query the type of an expression. However, in the latter case the resulting type is 'adjusted' according to the value category of that expression: an lvalue expression results in an lvalue reference type, an xvalue in an rvalue reference type, and a prvalue in just the type. We can use this to our benefit:

template<typename T>
struct value_category {
    // Or can be an 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 for ensuring we inspect an expression,
// not an entity
#define VALUE_CATEGORY(expr) value_category<decltype((expr))>::value

Answer 2

#ifndef _TPF_TYPE_NAME_H
#define _TPF_TYPE_NAME_H

template <typename T>
constexpr bool is_lvalue_helper = std::is_lvalue_reference<T>::value;

template <typename T>
constexpr bool is_xvalue_helper = std::is_rvalue_reference<T>::value;

template <typename T>
constexpr bool is_prvalue_helper = !(is_lvalue_helper<T> || is_xvalue_helper<T>);

template <typename T>
constexpr bool is_rvalue_helper = is_xvalue_helper<T> || is_prvalue_helper<T>;

template <typename T>
constexpr bool is_glvalue_helper = is_xvalue_helper<T> || is_lvalue_helper<T>;

#define is_lvalue(type_instance) is_lvalue_helper<decltype((type_instance))>
#define is_xvalue(type_instance) is_xvalue_helper<decltype((type_instance))>
#define is_prvalue(type_instance)is_prvalue_helper<decltype((type_instance))>
#define is_rvalue(type_instance) is_rvalue_helper<decltype((type_instance))>
#define is_glvalue(type_instance)is_glvalue_helper<decltype((type_instance))>

#endif // end of _TPF_TYPE_NAME_H