function_ref.hpp

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// Copyright (c) Sleipnir contributors
 
#pragma once
 
#include <functional>
#include <memory>
#include <type_traits>
#include <utility>
 
namespace slp {
 
template <class F>
class function_ref;
 
/**
 * An implementation of std::function_ref, a lightweight non-owning reference to
 * a callable.
 */
template <class R, class... Args>
class function_ref<R(Args...)> {
 public:
  constexpr function_ref() noexcept = delete;
 
  /**
   * Creates a `function_ref` which refers to the same callable as `rhs`.
   *
   * @param rhs Other `function_ref`.
   */
  constexpr function_ref(const function_ref<R(Args...)>& rhs) noexcept =
      default;
 
  /**
   * Constructs a `function_ref` referring to `f`.
   *
   * @tparam F Callable type.
   * @param f Callable to which to refer.
   */
  template <typename F>
    requires(!std::is_same_v<std::decay_t<F>, function_ref> &&
             std::is_invocable_r_v<R, F &&, Args...>)
  constexpr function_ref(F&& f) noexcept  // NOLINT(google-explicit-constructor)
      : obj_(const_cast<void*>(
            reinterpret_cast<const void*>(std::addressof(f)))) {
    callback_ = [](void* obj, Args... args) -> R {
      return std::invoke(
          *reinterpret_cast<typename std::add_pointer<F>::type>(obj),
          std::forward<Args>(args)...);
    };
  }
 
  /**
   * Makes `*this` refer to the same callable as `rhs`.
   *
   * @param rhs Other `function_ref`.
   * @return `*this`
   */
  constexpr function_ref<R(Args...)>& operator=(
      const function_ref<R(Args...)>& rhs) noexcept = default;
 
  /**
   * Makes `*this` refer to `f`.
   *
   * @param f Callable to which to refer.
   * @return `*this`
   */
  template <typename F>
    requires std::is_invocable_r_v<R, F&&, Args...>
  constexpr function_ref<R(Args...)>& operator=(F&& f) noexcept {
    obj_ = reinterpret_cast<void*>(std::addressof(f));
    callback_ = [](void* obj, Args... args) {
      return std::invoke(
          *reinterpret_cast<typename std::add_pointer<F>::type>(obj),
          std::forward<Args>(args)...);
    };
 
    return *this;
  }
 
  /**
   * Swaps the referred callables of `*this` and `rhs`.
   *
   * @param rhs Other `function_ref`.
   */
  constexpr void swap(function_ref<R(Args...)>& rhs) noexcept {
    std::swap(obj_, rhs.obj_);
    std::swap(callback_, rhs.callback_);
  }
 
  /**
   * Call the stored callable with the given arguments.
   *
   * @param args The arguments.
   * @return The return value of the callable.
   */
  R operator()(Args... args) const {
    return callback_(obj_, std::forward<Args>(args)...);
  }
 
 private:
  void* obj_ = nullptr;
  R (*callback_)(void*, Args...) = nullptr;
};
 
/**
 * Swaps the referred callables of `lhs` and `rhs`.
 */
template <typename R, typename... Args>
constexpr void swap(function_ref<R(Args...)>& lhs,
                    function_ref<R(Args...)>& rhs) noexcept {
  lhs.swap(rhs);
}
 
template <typename R, typename... Args>
function_ref(R (*)(Args...)) -> function_ref<R(Args...)>;
 
}  // namespace slp