在C ++中为每个可变参数宏参数加上一个函数参数的名称

Question

I have a macro that is used inside of a class definition for defining as_tuple() member functions.

#define DEFINE_AS_TUPLE(...) \
    auto as_tuple() const \
    { \
        return std::tie(__VA_ARGS__); \
    } \
    auto as_tuple() \
    { \
        return std::tie(__VA_ARGS__); \
    }

but I'd like to have a macro for defining class_as_tuple functions outside of a class. In common it should look like this:

#define DEFINE_AS_TUPLE_OUTSIDE_OF_CLASS(ClassName, ...) \
    inline auto class_as_tuple(const ClassName & val) \
    { \
        return std::tie(__VA_ARGS__); \
    } \
    inline auto class_as_tuple(ClassName & val) \
    { \
        return std::tie(__VA_ARGS__); \
    } \

but it is not clear how to prepend VA_ARGS with 'val.' in std::tie() call.

To make the question more clear I provided an example of how my first macro is used:

class B
{
public:

    B() : m_set{ 0, 1, 2 }, m_v{3, 4}
    {
    }

    DEFINE_AS_TUPLE(m_set, m_v, m_u8, m_b)

private:

    std::set<int> m_set;
    std::vector<int> m_v;

    uint8_t m_u8 = 25;

    bool m_b = true;
};

but I also need:

DEFINE_AS_TUPLE_OUTSIDE_OF_CLASS(B, m_set, m_v, m_u8, m_b)

Answer 1

The pragmatic approach would be to type out (or possibly generate) a macro for each number of parameters you care about. Given that you might not need to "tuplify" more than eg 10 members and that this pattern is apparently common enough in your code to warrant a macro like this, writing them out by hand may be the easiest way. For example:

#define DEFINE_AS_TUPLE_OUTSIDE_OF_CLASS_2(ClassName, a, b) \
    inline auto class_as_tuple(const ClassName& val) \
    { \
        return std::tie(val.a, val.b); \
    } \
    inline auto class_as_tuple(ClassName& val) \
    { \
        return std::tie(val.a, val.b); \
    }

#define DEFINE_AS_TUPLE_OUTSIDE_OF_CLASS_3(ClassName, a, b, c) \
    inline auto class_as_tuple(const ClassName& val) \
    { \
        return std::tie(val.a, val.b, val.c); \
    } \
    inline auto class_as_tuple(ClassName& val) \
    { \
        return std::tie(val.a, val.b, val.c); \
    }

However, this does not work in your given example as all the members are private. You'd have to make these into friend functions, so you'd need a macro for that, in which case you could just write class_as_tuple using val.as_tuple() ...