cpp-useful-iterators/include/iterator/detail/capture_fn.h

//
//  capture.h
//  iterator
//
//  Created by Sam Jaffe on 9/23/25.
//  Copyright © 2025 Sam Jaffe. All rights reserved.
//

#pragma once

#include <functional>

namespace iterator::detail {
template <typename P, typename T> class CaptureFn {
private:
  using R = std::invoke_result_t<P, T>;

private:
  P fn_;
  void const * key_;
  mutable R cache_;

public:
  CaptureFn(P fn) : fn_(fn) {} // NOLINT

  auto const & operator()(T & arg) const {
    if (key_ != &arg) {
      new (&cache_) R(std::invoke(fn_, std::forward<T>(arg)));
    }
    return cache_;
  }
};
}