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

/**
 * https://quuxplusone.github.io/blog/2019/02/06/arrow-proxy/
 */
#pragma once

namespace iterator::detail {
template <typename Reference> struct arrow_proxy {
  arrow_proxy(Reference r) : r(std::move(r)) {}
  Reference r;
  Reference * operator->() { return std::addressof(r); }
};

template <typename R> arrow_proxy(R r) -> arrow_proxy<R>;

template <typename Value> struct value_proxy {
  value_proxy operator=(Value && value) {
    this->value = std::move(value);
    return *this;
  }

  operator Value &() const { return value; }
  Value & get() const { return value; }

  Value value;
};

template <typename Value> struct value_proxy<Value &> {
  value_proxy operator=(Value & value) {
    this->value = &value;
    return *this;
  }

  operator Value &() const { return *value; }
  Value & get() const { return *value; }

  Value * value;
};

struct ignore_proxy {
  template <typename Value> decltype(auto) operator=(Value && value) {
    return std::forward<Value>(value);
  }
};

}