Empirically determine value category of C++11 expression?

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 pro

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
struct value_category {
    // Or can be an integral or enum value
    static constexpr auto value = "prvalue";
};

template
struct value_category {
    static constexpr auto value = "lvalue";
};

template
struct value_category {
    static constexpr auto value = "xvalue";
};

// Double parens for ensuring we inspect an expression,
// not an entity
#define VALUE_CATEGORY(expr) value_category::value