Home/ Questions/Q 1912
Answered
Simmon
Asked: 2022-07-21T06:58:59+00:00

Require certain specialization of template as a template parameter

I have a datastructure meta_array which is instantiated with a variant as type T parameter. The requirement is that the std::variant specialization must contain the type meta_array_head_t as this is is used for tracking information. Can I somehow state this as a requirement using type traits (C++17)?

CompilerExplorer:

#include <cstdio>
#include <variant>
#include <array>

struct meta_array_head_t {
    // end_;
    // remaining_;
    // prev_;
};

template <typename T, size_t S> // make sure T = std::variant<... , meta_array_head_t, ...>
struct meta_array
{
    std::array<T, S> data_; 
};


using val = std::variant<std::monostate, int, double, meta_array_head_t>;

int main()
{
    meta_array<val, 100> marray;
}
c++
Share

1 Answer

  1. Best Answer

    First, determine that T is of type std::variant, then use fold expression to detect whether the alternative type contains meta_array_head_t.

    #include <variant>

template<typename T>
struct meta_variant : std::false_type { };

template<typename... Ts>
struct meta_variant<std::variant<Ts...>> : 
  std::bool_constant<((std::is_same_v<Ts, meta_array_head_t> || ...))> { };

template <typename T, size_t>
struct meta_array {
  static_assert(meta_variant<T>::value, 
    "T must be a std::variant specialization "
    "containing an alternative type of meta_array_head_t.");
};

    Demo

    • 0

Leave an answer

You must login to add an answer.

Need An Account,