CODEWITHSUNDEEP
c++macrosc-preprocessorvariadic-macros
Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42929

Find out the type of __VA_ARGS__ in a variadic macro

Supposedly, I have a variadic macro (e.g., MY_MACRO(...)) and I call it the following way:

MY_MACRO(std::pair<int, int> const &p)

Now, __VA_ARGS__ in my macro's body would be std::pair<int, int> const &p.

Is there a way to figure out the type of __VA_ARGS__?

Presumably, I would be grateful if something like decltype(std::pair<int, int> const &p) worked and yield std::pair<int, int> const&, so in my variadic macro's body decltype(__VA_ARGS__) would yield std::pair<int, int> const& as well. Unfortunately this doesn't work.

Upvotes: 3

Views: 1476

Answers (1)

HolyBlackCat
HolyBlackCat

Reputation: 96821

You can use __VA_ARGS__ as a lambda parameter, then convert that lambda to a function pointer and extract the parameter type:

template <typename T> struct func_param {};
template <typename T> struct func_param<void(*)(T)> {using type = T;};

#define FOO(...) \
    do \
    { \
        auto lambda = +[]([[maybe_unused]] __VA_ARGS__) {}; \
        using type = func_param<decltype(lambda)>::type; \
        /* Do something with `type`. */\
    } \
    while (0);

Upvotes: 6

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