CODEWITHSUNDEEP
c++templates
maidamai
maidamai

Reputation: 712

use class template in its definition

In the following code, why no recursive instantiate

template <class T, T v>
struct integral_constant {
  static constexpr T value = v;
  using value_type         = T;
  using type               = integral_constant<T, v>;  // no recurse?
  constexpr operator value_type() const noexcept {
    return value;
  }
  constexpr value_type operator()() const noexcept {
    return value;
  }
};

int main() {
  integral_constant<long, 1>       l;
  integral_constant<long, 1>::type use_type;
  return 0;
}

compiler explorer cpp insights

I think this comment by pasbi has answered this question.

Upvotes: 0

Views: 43

Answers (1)

Alan Birtles
Alan Birtles

Reputation: 36389

Your code does result in infinite recursion, you can use integral_constant<long, 1>::type::type::type as many times as you like.

This isn't a problem though as the compiler only instantiates templates that you are using not all the templates that you could possibly use.

You would have an error if you tried to declare a member recursively:

template <class T, T v>
struct integral_constant {
  static constexpr T value = v;
  using value_type         = T;
  using type               = integral_constant<T, v>;
  type member;
  constexpr operator value_type() const noexcept {
    return value;
  }
  constexpr value_type operator()() const noexcept {
    return value;
  }
};

Upvotes: 1

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