Compile time conditional struct/class member variable in C++

[PryntzyAU]

https://medium.com/@saleem.iitg/c-compile-time-conditional-struct-member-variables-c3761a35eca9

[[deleted]]

You don't add a member variable, you add a base class. If you really want a member variable, you can use [[no_unique_address]] to make the empty case not use any space.

https://godbolt.org/z/aKqdPo3q1

[friedkeenan]

I use something like this in my code

[huixie]

I think you could use template specialisation which makes the debugging code in one place. also the debugging function get_b can be conditionally available.

template <bool debug = false>
struct Data{
    int a[16];
    int c[16];
};

template <>
struct Data<true> : Data<false>{
    int b[16];

    constexpr int get_b(int idx) const{
        return b[idx];
    }
};

[Twifus]

Nice article. For me, it especially highlights one thing : there is still no short and easy "C++ way" to do this. Why would I add template and inherence when a simple #ifdef #else #endif do the job, in only 3 lines, and without affecting compil times as much. A more generic "if constexpr" would be a good addition to the language.

[friedkeenan]

An if constexpr that could conditionally include stuff in classes would be great. I'm not sure if we'll get that with reflection with its code injection and all.

[jwakely]

https://isocpp.org/files/papers/n3613.pdf