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Diffstat (limited to 'abseil-cpp/absl/functional/any_invocable.h')
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diff --git a/abseil-cpp/absl/functional/any_invocable.h b/abseil-cpp/absl/functional/any_invocable.h new file mode 100644 index 0000000..68d8825 --- /dev/null +++ b/abseil-cpp/absl/functional/any_invocable.h @@ -0,0 +1,324 @@ +// Copyright 2022 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. +// +// ----------------------------------------------------------------------------- +// File: any_invocable.h +// ----------------------------------------------------------------------------- +// +// This header file defines an `absl::AnyInvocable` type that assumes ownership +// and wraps an object of an invocable type. (Invocable types adhere to the +// concept specified in https://en.cppreference.com/w/cpp/concepts/invocable.) +// +// In general, prefer `absl::AnyInvocable` when you need a type-erased +// function parameter that needs to take ownership of the type. +// +// NOTE: `absl::AnyInvocable` is similar to the C++23 `std::move_only_function` +// abstraction, but has a slightly different API and is not designed to be a +// drop-in replacement or C++11-compatible backfill of that type. +// +// Credits to Matt Calabrese (https://github.com/mattcalabrese) for the original +// implementation. + +#ifndef ABSL_FUNCTIONAL_ANY_INVOCABLE_H_ +#define ABSL_FUNCTIONAL_ANY_INVOCABLE_H_ + +#include <cstddef> +#include <initializer_list> +#include <type_traits> +#include <utility> + +#include "absl/base/config.h" +#include "absl/functional/internal/any_invocable.h" +#include "absl/meta/type_traits.h" +#include "absl/utility/utility.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN + +// absl::AnyInvocable +// +// `absl::AnyInvocable` is a functional wrapper type, like `std::function`, that +// assumes ownership of an invocable object. Unlike `std::function`, an +// `absl::AnyInvocable` is more type-safe and provides the following additional +// benefits: +// +// * Properly adheres to const correctness of the underlying type +// * Is move-only so avoids concurrency problems with copied invocables and +// unnecessary copies in general. +// * Supports reference qualifiers allowing it to perform unique actions (noted +// below). +// +// `absl::AnyInvocable` is a template, and an `absl::AnyInvocable` instantiation +// may wrap any invocable object with a compatible function signature, e.g. +// having arguments and return types convertible to types matching the +// `absl::AnyInvocable` signature, and also matching any stated reference +// qualifiers, as long as that type is moveable. It therefore provides broad +// type erasure for functional objects. +// +// An `absl::AnyInvocable` is typically used as a type-erased function parameter +// for accepting various functional objects: +// +// // Define a function taking an AnyInvocable parameter. +// void my_func(absl::AnyInvocable<int()> f) { +// ... +// }; +// +// // That function can accept any invocable type: +// +// // Accept a function reference. We don't need to move a reference. +// int func1() { return 0; }; +// my_func(func1); +// +// // Accept a lambda. We use std::move here because otherwise my_func would +// // copy the lambda. +// auto lambda = []() { return 0; }; +// my_func(std::move(lambda)); +// +// // Accept a function pointer. We don't need to move a function pointer. +// func2 = &func1; +// my_func(func2); +// +// // Accept an std::function by moving it. Note that the lambda is copyable +// // (satisfying std::function requirements) and moveable (satisfying +// // absl::AnyInvocable requirements). +// std::function<int()> func6 = []() { return 0; }; +// my_func(std::move(func6)); +// +// `AnyInvocable` also properly respects `const` qualifiers, reference +// qualifiers, and the `noexcept` specification (only in C++ 17 and beyond) as +// part of the user-specified function type (e.g. +// `AnyInvocable<void()&& const noexcept>`). These qualifiers will be applied to +// the `AnyInvocable` object's `operator()`, and the underlying invocable must +// be compatible with those qualifiers. +// +// Comparison of const and non-const function types: +// +// // Store a closure inside of `func` with the function type `int()`. +// // Note that we have made `func` itself `const`. +// const AnyInvocable<int()> func = [](){ return 0; }; +// +// func(); // Compile-error: the passed type `int()` isn't `const`. +// +// // Store a closure inside of `const_func` with the function type +// // `int() const`. +// // Note that we have also made `const_func` itself `const`. +// const AnyInvocable<int() const> const_func = [](){ return 0; }; +// +// const_func(); // Fine: `int() const` is `const`. +// +// In the above example, the call `func()` would have compiled if +// `std::function` were used even though the types are not const compatible. +// This is a bug, and using `absl::AnyInvocable` properly detects that bug. +// +// In addition to affecting the signature of `operator()`, the `const` and +// reference qualifiers of the function type also appropriately constrain which +// kinds of invocable objects you are allowed to place into the `AnyInvocable` +// instance. If you specify a function type that is const-qualified, then +// anything that you attempt to put into the `AnyInvocable` must be callable on +// a `const` instance of that type. +// +// Constraint example: +// +// // Fine because the lambda is callable when `const`. +// AnyInvocable<int() const> func = [=](){ return 0; }; +// +// // This is a compile-error because the lambda isn't callable when `const`. +// AnyInvocable<int() const> error = [=]() mutable { return 0; }; +// +// An `&&` qualifier can be used to express that an `absl::AnyInvocable` +// instance should be invoked at most once: +// +// // Invokes `continuation` with the logical result of an operation when +// // that operation completes (common in asynchronous code). +// void CallOnCompletion(AnyInvocable<void(int)&&> continuation) { +// int result_of_foo = foo(); +// +// // `std::move` is required because the `operator()` of `continuation` is +// // rvalue-reference qualified. +// std::move(continuation)(result_of_foo); +// } +// +// Attempting to call `absl::AnyInvocable` multiple times in such a case +// results in undefined behavior. +template <class Sig> +class AnyInvocable : private internal_any_invocable::Impl<Sig> { + private: + static_assert( + std::is_function<Sig>::value, + "The template argument of AnyInvocable must be a function type."); + + using Impl = internal_any_invocable::Impl<Sig>; + + public: + // The return type of Sig + using result_type = typename Impl::result_type; + + // Constructors + + // Constructs the `AnyInvocable` in an empty state. + AnyInvocable() noexcept = default; + AnyInvocable(std::nullptr_t) noexcept {} // NOLINT + + // Constructs the `AnyInvocable` from an existing `AnyInvocable` by a move. + // Note that `f` is not guaranteed to be empty after move-construction, + // although it may be. + AnyInvocable(AnyInvocable&& /*f*/) noexcept = default; + + // Constructs an `AnyInvocable` from an invocable object. + // + // Upon construction, `*this` is only empty if `f` is a function pointer or + // member pointer type and is null, or if `f` is an `AnyInvocable` that is + // empty. + template <class F, typename = absl::enable_if_t< + internal_any_invocable::CanConvert<Sig, F>::value>> + AnyInvocable(F&& f) // NOLINT + : Impl(internal_any_invocable::ConversionConstruct(), + std::forward<F>(f)) {} + + // Constructs an `AnyInvocable` that holds an invocable object of type `T`, + // which is constructed in-place from the given arguments. + // + // Example: + // + // AnyInvocable<int(int)> func( + // absl::in_place_type<PossiblyImmovableType>, arg1, arg2); + // + template <class T, class... Args, + typename = absl::enable_if_t< + internal_any_invocable::CanEmplace<Sig, T, Args...>::value>> + explicit AnyInvocable(absl::in_place_type_t<T>, Args&&... args) + : Impl(absl::in_place_type<absl::decay_t<T>>, + std::forward<Args>(args)...) { + static_assert(std::is_same<T, absl::decay_t<T>>::value, + "The explicit template argument of in_place_type is required " + "to be an unqualified object type."); + } + + // Overload of the above constructor to support list-initialization. + template <class T, class U, class... Args, + typename = absl::enable_if_t<internal_any_invocable::CanEmplace< + Sig, T, std::initializer_list<U>&, Args...>::value>> + explicit AnyInvocable(absl::in_place_type_t<T>, + std::initializer_list<U> ilist, Args&&... args) + : Impl(absl::in_place_type<absl::decay_t<T>>, ilist, + std::forward<Args>(args)...) { + static_assert(std::is_same<T, absl::decay_t<T>>::value, + "The explicit template argument of in_place_type is required " + "to be an unqualified object type."); + } + + // Assignment Operators + + // Assigns an `AnyInvocable` through move-assignment. + // Note that `f` is not guaranteed to be empty after move-assignment + // although it may be. + AnyInvocable& operator=(AnyInvocable&& /*f*/) noexcept = default; + + // Assigns an `AnyInvocable` from a nullptr, clearing the `AnyInvocable`. If + // not empty, destroys the target, putting `*this` into an empty state. + AnyInvocable& operator=(std::nullptr_t) noexcept { + this->Clear(); + return *this; + } + + // Assigns an `AnyInvocable` from an existing `AnyInvocable` instance. + // + // Upon assignment, `*this` is only empty if `f` is a function pointer or + // member pointer type and is null, or if `f` is an `AnyInvocable` that is + // empty. + template <class F, typename = absl::enable_if_t< + internal_any_invocable::CanAssign<Sig, F>::value>> + AnyInvocable& operator=(F&& f) { + *this = AnyInvocable(std::forward<F>(f)); + return *this; + } + + // Assigns an `AnyInvocable` from a reference to an invocable object. + // Upon assignment, stores a reference to the invocable object in the + // `AnyInvocable` instance. + template < + class F, + typename = absl::enable_if_t< + internal_any_invocable::CanAssignReferenceWrapper<Sig, F>::value>> + AnyInvocable& operator=(std::reference_wrapper<F> f) noexcept { + *this = AnyInvocable(f); + return *this; + } + + // Destructor + + // If not empty, destroys the target. + ~AnyInvocable() = default; + + // absl::AnyInvocable::swap() + // + // Exchanges the targets of `*this` and `other`. + void swap(AnyInvocable& other) noexcept { std::swap(*this, other); } + + // absl::AnyInvocable::operator bool() + // + // Returns `true` if `*this` is not empty. + // + // WARNING: An `AnyInvocable` that wraps an empty `std::function` is not + // itself empty. This behavior is consistent with the standard equivalent + // `std::move_only_function`. + // + // In other words: + // std::function<void()> f; // empty + // absl::AnyInvocable<void()> a = std::move(f); // not empty + explicit operator bool() const noexcept { return this->HasValue(); } + + // Invokes the target object of `*this`. `*this` must not be empty. + // + // Note: The signature of this function call operator is the same as the + // template parameter `Sig`. + using Impl::operator(); + + // Equality operators + + // Returns `true` if `*this` is empty. + friend bool operator==(const AnyInvocable& f, std::nullptr_t) noexcept { + return !f.HasValue(); + } + + // Returns `true` if `*this` is empty. + friend bool operator==(std::nullptr_t, const AnyInvocable& f) noexcept { + return !f.HasValue(); + } + + // Returns `false` if `*this` is empty. + friend bool operator!=(const AnyInvocable& f, std::nullptr_t) noexcept { + return f.HasValue(); + } + + // Returns `false` if `*this` is empty. + friend bool operator!=(std::nullptr_t, const AnyInvocable& f) noexcept { + return f.HasValue(); + } + + // swap() + // + // Exchanges the targets of `f1` and `f2`. + friend void swap(AnyInvocable& f1, AnyInvocable& f2) noexcept { f1.swap(f2); } + + private: + // Friending other instantiations is necessary for conversions. + template <bool /*SigIsNoexcept*/, class /*ReturnType*/, class... /*P*/> + friend class internal_any_invocable::CoreImpl; +}; + +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_FUNCTIONAL_ANY_INVOCABLE_H_ |