[libc++] Implement std::move_only_function (P0288R9) #94670
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| Original file line number | Diff line number | Diff line change |
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| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #ifndef _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_H | ||
| #define _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_H | ||
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| #include <__config> | ||
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| #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) | ||
| # pragma GCC system_header | ||
| #endif | ||
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| #if _LIBCPP_STD_VER >= 23 && defined(_LIBCPP_ENABLE_EXPERIMENTAL) | ||
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| // move_only_function design: | ||
| // | ||
| // move_only_function has a small buffer with a size of `3 * sizeof(void*)` bytes. This buffer can only be used when the | ||
| // object that should be stored is trivially relocatable (currently only when it is trivially move constructible and | ||
| // trivially destructible). There is also a bool in the lower bits of the vptr stored which is set when the contained | ||
| // object is not trivially destructible. | ||
| // | ||
| // trivially relocatable: It would also be possible to store nothrow_move_constructible types, but that would mean | ||
| // that move_only_function itself would not be trivially relocatable anymore. The decision to keep move_only_function | ||
| // trivially relocatable was made because we expect move_only_function to be mostly used to store a functor. To only | ||
| // forward functors there is C++26's std::function_ref. | ||
| // | ||
| // buffer size: We did a survey of six implementations from various vendors. Three of them had a buffer size of 24 bytes | ||
| // on 64 bit systems. This also allows storing a std::string or std::vector inside the small buffer (once the compiler | ||
| // has full support of trivially_relocatable annotations). | ||
| // | ||
| // trivially-destructible bit: This allows us to keep the overall binary size smaller because we don't have to store | ||
| // a pointer to a noop function inside the vtable. It also avoids loading the vtable during destruction, potentially | ||
| // resulting in fewer cache misses. The downside is that calling the function now also requires setting the lower bits | ||
| // of the pointer to zero, but this is a very fast operation on modern CPUs. | ||
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| // | ||
| // interaction with copyable_function: When converting a copyable_function into a move_only_function we want to avoid | ||
| // wrapping the copyable_function inside the move_only_function to avoid a double indirection. Instead, we copy the | ||
| // small buffer and use copyable_function's vtable. | ||
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| // NOLINTBEGIN(readability-duplicate-include) | ||
| # define _LIBCPP_IN_MOVE_ONLY_FUNCTION_H | ||
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| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF & | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF && | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF & | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF && | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF & | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF && | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF & | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT true | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_CV const | ||
| # define _LIBCPP_MOVE_ONLY_FUNCTION_REF && | ||
| # include <__functional/move_only_function_impl.h> | ||
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| # undef _LIBCPP_IN_MOVE_ONLY_FUNCTION_H | ||
| // NOLINTEND(readability-duplicate-include) | ||
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| #endif // _LIBCPP_STD_VER >= 23 && defined(_LIBCPP_ENABLE_EXPERIMENTAL) | ||
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| #endif // _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_H |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,45 @@ | ||
| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #ifndef _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_COMMON_H | ||
| #define _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_COMMON_H | ||
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| #include <__config> | ||
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| #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) | ||
| # pragma GCC system_header | ||
| #endif | ||
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| _LIBCPP_BEGIN_NAMESPACE_STD | ||
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| template <class...> | ||
| class move_only_function; | ||
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| template <class> | ||
| inline constexpr bool __is_move_only_function_v = false; | ||
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| template <class... _Args> | ||
| inline constexpr bool __is_move_only_function_v<move_only_function<_Args...>> = true; | ||
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| template <class _BufferT, class _ReturnT, class... _ArgTypes> | ||
| struct _MoveOnlyFunctionTrivialVTable { | ||
| using _CallFunc = _ReturnT(_BufferT&, _ArgTypes...); | ||
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| _CallFunc* __call_; | ||
| }; | ||
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| template <class _BufferT, class _ReturnT, class... _ArgTypes> | ||
| struct _MoveOnlyFunctionNonTrivialVTable : _MoveOnlyFunctionTrivialVTable<_BufferT, _ReturnT, _ArgTypes...> { | ||
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There was a problem hiding this comment. We don't need to instantiate this type with That's alot of template bloat. There was a problem hiding this comment. Could you elaborate? I don't see how we can have the vtable function pointer without the template arguments. There was a problem hiding this comment. You either don't need the inheritance, or you don't need to give the "destruct" base class all the arguments, you only need that for the call. There was a problem hiding this comment. The call function pointer is in the base class though. I always want the call pointer and only sometimes the destructor pointer. |
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| using _DestroyFunc = void(_BufferT&) noexcept; | ||
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| _DestroyFunc* __destroy_; | ||
| }; | ||
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| _LIBCPP_END_NAMESPACE_STD | ||
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| #endif // _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_COMMON_H |
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