asserts.h

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#ifndef FCPP_VALIDATIONS_H
#define FCPP_VALIDATIONS_H
 
#include <exception>
#include <optional>
#include <sstream>
#include <stdexcept>
#include <string>
#include <utility>
 
namespace fcpp::asserts {

class invariant {
private:
  // Only engaged when the condition is violated.
  std::optional<std::ostringstream> message_;
 
  explicit invariant(bool condition) {
    if (!condition) {
      message_.emplace();
    }
  }
 
  // https://herbsutter.com/2014/05/03/reader-qa-how-can-i-prevent-a-type-from-being-instantiated-on-the-stack/
  invariant() = delete;
  invariant(const invariant &) = delete;
  invariant &operator=(const invariant &) = delete;
 
public:
  ~invariant() noexcept(false) { // NOLINT(bugprone-exception-escape)
    // Throwing while another exception unwinds the stack would terminate.
    if (message_ && std::uncaught_exceptions() == 0) {
      // Living dangerously, guaranteed not to live on the stack though. See
      // above link.
      throw std::invalid_argument(message_->str());
    }
  }

  template <typename Text>
  friend invariant &&operator<<(invariant &&item, const Text &text) {
    if (item.message_) {
      *item.message_ << text;
    }
    return std::move(item);
  }

  static invariant eval(bool condition) { return invariant(condition); }
};
 
} // namespace fcpp::asserts
 
#endif // FCPP_VALIDATIONS_H