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127 lines
5.2 KiB
127 lines
5.2 KiB
/*
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* Copyright (C) 2021 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <stdint.h>
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#include <type_traits>
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#include <utility>
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namespace android::base {
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//
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// function_ref<> - a class that stores a reference to a callable object,
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// similar to string_view for strings.
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//
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// We need to pass around lots of callbacks. The standard way of doing it
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// is via std::function<> class, and it usually works OK. But there are some
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// noticeable drawbacks:
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//
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// 1. std::function<> in most STLs performs heap allocation for all callables
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// bigger than a single poiner to a function.
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// 2. std::function<> goes through at least two pointers + a vptr call to call
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// the stored function.
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// 3. std::function<> copies the passed object inside at least once; this also
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// means it can't work with non-copyable functors.
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//
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// function_ref is an alternative way of passing functors around. Instead of
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// storing a copy of the functor inside, it follows the path of string_view and
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// merely captures a pointer to the object to call. This allows for a simple,
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// fast and lightweight wrapper design; it also dictates the limitations:
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//
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// 1. function_ref<> stores a pointer to outside functor. That functor _must_
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// outlive the ref.
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// 2. function_ref<> has two calls through a function pointer in its call
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// operator. That's still better than std::function<>, but slower compared
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// to a raw function pointer call with a "void* opaque" context parameter.
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//
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// Limitation #1 dictates the best use case: a function parameter type for some
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// generic callback which doesn't get stored inside an object field but only
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// gets called in this call. E.g.:
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//
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// void someLongOperation(function_ref<void(int progress)> onProgress) {
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// firstStep(onProgress);
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// ...
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// onProgress(50);
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// ...
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// lastStep(onProgress);
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// onProgress(100);
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// }
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//
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// In this code std::function<> is an overkill as the whole use of |onProgresss|
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// callback is scoped and easy to track. An alternative design - making it a
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// template parameter (template <class Callback> ... (Callback onProgress))
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// forces one to put someLongOperation() + some private functions into the
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// header. function_ref<> is the choice then.
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//
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// NOTE: Beware of passing temporary functions via function_ref<>! Temporaries
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// live until the end of full expression (usually till the next semicolon), and
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// having a function_ref<> that refers to a dangling pointer is a bug that's
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// hard to debug. E.g.:
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// function_ref<...> v = [](){}; // this is fine
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// function_ref<...> v = std::function<...>([](){}); // this will kill you
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//
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// NOTE2: function_ref<> should not have an empty state, but it doesn't have a
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// runtime check against that. Don't construct it from a null function!
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template <class Signature>
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class function_ref;
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template <class Ret, class... Args>
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class function_ref<Ret(Args...)> final {
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public:
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constexpr function_ref() noexcept = delete;
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constexpr function_ref(const function_ref& other) noexcept = default;
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constexpr function_ref& operator=(const function_ref&) noexcept = default;
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template <class Callable, class = std::enable_if_t<
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std::is_invocable_r<Ret, Callable, Args...>::value &&
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!std::is_same_v<function_ref, std::remove_reference_t<Callable>>>>
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function_ref(Callable&& c) noexcept
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: mTypeErasedFunction([](const function_ref* self, Args... args) -> Ret {
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// Generate a lambda that remembers the type of the passed
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// |Callable|.
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return (*reinterpret_cast<std::remove_reference_t<Callable>*>(self->mCallable))(
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std::forward<Args>(args)...);
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}),
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mCallable(reinterpret_cast<intptr_t>(&c)) {}
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template <class Callable, class = std::enable_if_t<
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std::is_invocable_r<Ret, Callable, Args...>::value &&
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!std::is_same_v<function_ref, std::remove_reference_t<Callable>>>>
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function_ref& operator=(Callable&& c) noexcept {
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mTypeErasedFunction = [](const function_ref* self, Args... args) -> Ret {
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// Generate a lambda that remembers the type of the passed
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// |Callable|.
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return (*reinterpret_cast<std::remove_reference_t<Callable>*>(self->mCallable))(
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std::forward<Args>(args)...);
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};
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mCallable = reinterpret_cast<intptr_t>(&c);
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return *this;
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}
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Ret operator()(Args... args) const {
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return mTypeErasedFunction(this, std::forward<Args>(args)...);
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}
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private:
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using TypeErasedFunc = Ret(const function_ref*, Args...);
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TypeErasedFunc* mTypeErasedFunction;
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intptr_t mCallable;
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};
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} // namespace android::base
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