once_function.h

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/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
 
#pragma once
 
#include <cstring>
#include <utility>
 
#include <dispenso/detail/once_callable_impl.h>
#include <dispenso/platform.h>
 
namespace dispenso {
 
#if DISPENSO_HAS_CONCEPTS
template <typename F>
concept OnceCallableFunc = std::invocable<F>;
#endif // DISPENSO_HAS_CONCEPTS
 
namespace detail {
template <typename Result>
class FutureBase;
template <typename Result>
class FutureImplBase;
} // namespace detail
 
class OnceFunction {
 public:
  OnceFunction()
#if defined DISPENSO_DEBUG
      : invoke_(nullptr)
#endif // DISPENSO_DEBUG
  {
  }
 
  template <typename F>
  DISPENSO_REQUIRES(OnceCallableFunc<F>)
  OnceFunction(F&& f) {
    auto callable = detail::createOnceCallable(std::forward<F>(f), buf_);
    invoke_ = callable.invoke;
  }
 
  OnceFunction(const OnceFunction& other) = delete;
 
  OnceFunction(OnceFunction&& other) noexcept {
    std::memcpy(static_cast<void*>(this), &other, sizeof(OnceFunction));
#if defined DISPENSO_DEBUG
    other.invoke_ = nullptr;
#endif // DISPENSO_DEBUG
  }
 
  OnceFunction& operator=(OnceFunction&& other) noexcept {
    if (this != &other) {
#if defined DISPENSO_DEBUG
      assert(
          invoke_ == nullptr &&
          "OnceFunction must be invoked or cleanupNotRun() before reassignment");
#endif // DISPENSO_DEBUG
      std::memcpy(static_cast<void*>(this), &other, sizeof(OnceFunction));
#if defined DISPENSO_DEBUG
      other.invoke_ = nullptr;
#endif // DISPENSO_DEBUG
    }
    return *this;
  }
 
  void cleanupNotRun() {
#if defined DISPENSO_DEBUG
    assert(invoke_ != nullptr && "Must not cleanup an invalid OnceFunction!");
    invoke_(buf_, false);
    invoke_ = nullptr;
#else
    invoke_(buf_, false);
#endif // DISPENSO_DEBUG
  }
 
  void operator()() const {
#if defined DISPENSO_DEBUG
    assert(invoke_ != nullptr && "Must not use OnceFunction more than once!");
#endif // DISPENSO_DEBUG
 
    invoke_(buf_, true);
 
#if defined DISPENSO_DEBUG
    invoke_ = nullptr;
#endif // DISPENSO_DEBUG
  }
 
 private:
  // 64 bytes total (one cache line): 56 bytes inline storage + 8 bytes invoke ptr.
  // Inline: buf_ holds the functor directly.
  // Spill: buf_[0..7] holds a void* to pool-allocated storage.
  // Moves are a 64-byte memcpy — no pointer fixup needed.
  alignas(64) mutable char buf_[detail::kOnceFunctionInlineSize];
  mutable void (*invoke_)(void*, bool);
 
  template <typename Result>
  friend class detail::FutureBase;
  template <typename Result>
  friend class detail::FutureImplBase;
};
 
} // namespace dispenso