CODEWITHSUNDEEP
c++metaprogrammingconstexpr
Mathieu Van Nevel
Mathieu Van Nevel

Reputation: 1486

Constexpr tricks

I think it's not possible but I'd like to ask you before give up about it.

I want something like a constexpr increment.

#include  <iostream>

constexpr int inc() {

  static int inc = 0;
  return inc++;
}

class Foo {

  static const int  Type = inc();
};

class Foo2 {

  static const int  Type = inc();
};

int main() {

  std::cout << "Foo1 " << Foo1::Type << st::endl;
  std::cout << "Foo2 " << Foo2::Type << st::endl;
  return 0;
}

I want to call it into some classes not manually (I use CRTP for that), to give a different type to each of them, but the type need to be const. There is anyway to achieve something like that in C++? (C++17 + TS)

Upvotes: 9

Views: 1198

Answers (1)

Mathieu Van Nevel
Mathieu Van Nevel

Reputation: 1486

So there is the solution by Filip Roseen called the constant-expression counter:

#include  <iostream>

template<int N>
struct flag {
  friend constexpr int adl_flag (flag<N>);
};

template<int N>
struct writer {
  friend constexpr int adl_flag (flag<N>) {
    return N;
  }

  static constexpr int value = N;
};

template<int N, int = adl_flag (flag<N> {})>
int constexpr reader (int, flag<N>) {
  return N;
}

template<int N>
int constexpr reader (float, flag<N>, int R = reader (0, flag<N-1> {})) {
  return R;
}

int constexpr reader (float, flag<0>) {
  return 0;
}

template<int N = 1>
int constexpr next (int R = writer<reader (0, flag<32> {}) + N>::value) {
  return R;
}

class Foo {

  public:
    static const int  Type = next();
};

class Foo2 {

  public:
    static const int  Type = next();
};

int main() {

  std::cout << "Foo1 " << Foo::Type << std::endl;
  std::cout << "Foo2 " << Foo2::Type << std::endl;
  return 0;
}

Thanks guys :) But it's too risky for use it in my main library which will be use in every project.

PS: I won't close this right now if there is another answer. Because yes it's ugly.

Upvotes: 3

Related Questions