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Empirically determine value category of C++11 expression?

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*

[decltype][1] 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


  [1]: https://en.cppreference.com/w/cpp/language/decltype

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3

I didn't realize decltype "mapped" expression value categories (lvalue, xvalue, prvalue) to reference types (lvalue reference to T, rvalue reference to T, T). Thanks.

Andrew Tomazos
– Andrew Tomazos

2013-05-19 18:39:11 +00:00
Commented May 19, 2013 at 18:39
1

@user1131467: You have to use decltype with the expression wrapped in double-parenthesis like that. That changes how decltype deduces the expression's type.

Nicol Bolas
– Nicol Bolas

2013-05-19 18:53:48 +00:00
Commented May 19, 2013 at 18:53
decltype(...) is documented in N3485 7.1.6.2/4

Andrew Tomazos
– Andrew Tomazos

2013-05-19 19:12:50 +00:00
Commented May 19, 2013 at 19:12
@NicolBolas: Only if it is an id-expression. For example decltype(move(x)) will produce int&& as expected.

Andrew Tomazos
– Andrew Tomazos

2013-05-19 21:16:56 +00:00
Commented May 19, 2013 at 21:16
@Nikos "named rvalue references are lvalues".

user202729
– user202729

2020-02-07 08:03:18 +00:00
Commented Feb 7, 2020 at 8:03

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Empirically determine value category of C++11 expression?

Answer*