mfbt/MoveOnlyFunction.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
   5  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_MoveOnlyFunction_h
   8 #define mozilla_MoveOnlyFunction_h
   9
  10 // Use stl-like empty propagation to avoid issues with wrapping closures which
  11 // implicitly coerce to bool.
  12 #define FU2_WITH_LIMITED_EMPTY_PROPAGATION
  13
  14 #include "function2/function2.hpp"
  15
  16 namespace mozilla {
  17
  18 /// A type like `std::function`, but with support for move-only callable
  19 /// objects.
  20 ///
  21 /// A similar type is proposed to be added to the standard library as
  22 /// `std::move_only_function` in C++23.
  23 ///
  24 /// Unlike `std::function`, the function signature may be given const or
  25 /// reference qualifiers which will be applied to `operator()`. This can be used
  26 /// to declare const qualified or move-only functions.
  27 ///
  28 /// The implementation this definition depends on (function2) also has support
  29 /// for callables with overload sets, however support for this was not exposed
  30 /// to align better with the proposed `std::move_only_function`, which does not
  31 /// support overload sets.
  32 ///
  33 /// A custom typedef over `fu2::function_base` is used to control the size and
  34 /// alignment of the inline storage to store 2 aligned pointers, and ensure the
  35 /// type is compatible with `nsTArray`.
  36 template <typename Signature>
  37 using MoveOnlyFunction = fu2::function_base<
  38     /* IsOwning */ true,
  39     /* IsCopyable */ false,
  40     /* Capacity */ fu2::capacity_fixed<2 * sizeof(void*), alignof(void*)>,
  41     /* IsThrowing */ false,
  42     /* HasStrongExceptionGuarantee */ false,
  43     /* Signature */ Signature>;
  44
  45 }  // namespace mozilla
  46
  47 #endif  // mozilla_MoveOnlyFunction_h