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193 lines
7.3 KiB
193 lines
7.3 KiB
// Copyright 2018 The Abseil Authors. |
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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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// https://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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// ----------------------------------------------------------------------------- |
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// File: bind_front.h |
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// ----------------------------------------------------------------------------- |
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// |
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// `absl::bind_front()` returns a functor by binding a number of arguments to |
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// the front of a provided (usually more generic) functor. Unlike `std::bind`, |
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// it does not require the use of argument placeholders. The simpler syntax of |
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// `absl::bind_front()` allows you to avoid known misuses with `std::bind()`. |
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// |
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// `absl::bind_front()` is meant as a drop-in replacement for C++20's upcoming |
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// `std::bind_front()`, which similarly resolves these issues with |
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// `std::bind()`. Both `bind_front()` alternatives, unlike `std::bind()`, allow |
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// partial function application. (See |
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// https://en.wikipedia.org/wiki/Partial_application). |
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#ifndef ABSL_FUNCTIONAL_BIND_FRONT_H_ |
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#define ABSL_FUNCTIONAL_BIND_FRONT_H_ |
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#if defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L |
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#include <functional> // For std::bind_front. |
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#endif // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L |
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#include "absl/functional/internal/front_binder.h" |
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#include "absl/utility/utility.h" |
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namespace absl { |
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ABSL_NAMESPACE_BEGIN |
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// bind_front() |
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// |
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// Binds the first N arguments of an invocable object and stores them by value. |
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// |
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// Like `std::bind()`, `absl::bind_front()` is implicitly convertible to |
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// `std::function`. In particular, it may be used as a simpler replacement for |
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// `std::bind()` in most cases, as it does not require placeholders to be |
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// specified. More importantly, it provides more reliable correctness guarantees |
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// than `std::bind()`; while `std::bind()` will silently ignore passing more |
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// parameters than expected, for example, `absl::bind_front()` will report such |
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// mis-uses as errors. In C++20, `absl::bind_front` is replaced by |
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// `std::bind_front`. |
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// |
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// absl::bind_front(a...) can be seen as storing the results of |
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// std::make_tuple(a...). |
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// |
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// Example: Binding a free function. |
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// |
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// int Minus(int a, int b) { return a - b; } |
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// |
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// assert(absl::bind_front(Minus)(3, 2) == 3 - 2); |
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// assert(absl::bind_front(Minus, 3)(2) == 3 - 2); |
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// assert(absl::bind_front(Minus, 3, 2)() == 3 - 2); |
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// |
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// Example: Binding a member function. |
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// |
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// struct Math { |
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// int Double(int a) const { return 2 * a; } |
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// }; |
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// |
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// Math math; |
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// |
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// assert(absl::bind_front(&Math::Double)(&math, 3) == 2 * 3); |
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// // Stores a pointer to math inside the functor. |
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// assert(absl::bind_front(&Math::Double, &math)(3) == 2 * 3); |
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// // Stores a copy of math inside the functor. |
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// assert(absl::bind_front(&Math::Double, math)(3) == 2 * 3); |
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// // Stores std::unique_ptr<Math> inside the functor. |
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// assert(absl::bind_front(&Math::Double, |
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// std::unique_ptr<Math>(new Math))(3) == 2 * 3); |
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// |
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// Example: Using `absl::bind_front()`, instead of `std::bind()`, with |
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// `std::function`. |
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// |
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// class FileReader { |
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// public: |
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// void ReadFileAsync(const std::string& filename, std::string* content, |
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// const std::function<void()>& done) { |
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// // Calls Executor::Schedule(std::function<void()>). |
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// Executor::DefaultExecutor()->Schedule( |
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// absl::bind_front(&FileReader::BlockingRead, this, |
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// filename, content, done)); |
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// } |
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// |
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// private: |
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// void BlockingRead(const std::string& filename, std::string* content, |
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// const std::function<void()>& done) { |
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// CHECK_OK(file::GetContents(filename, content, {})); |
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// done(); |
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// } |
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// }; |
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// |
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// `absl::bind_front()` stores bound arguments explicitly using the type passed |
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// rather than implicitly based on the type accepted by its functor. |
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// |
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// Example: Binding arguments explicitly. |
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// |
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// void LogStringView(absl::string_view sv) { |
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// LOG(INFO) << sv; |
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// } |
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// |
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// Executor* e = Executor::DefaultExecutor(); |
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// std::string s = "hello"; |
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// absl::string_view sv = s; |
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// |
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// // absl::bind_front(LogStringView, arg) makes a copy of arg and stores it. |
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// e->Schedule(absl::bind_front(LogStringView, sv)); // ERROR: dangling |
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// // string_view. |
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// |
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// e->Schedule(absl::bind_front(LogStringView, s)); // OK: stores a copy of |
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// // s. |
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// |
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// To store some of the arguments passed to `absl::bind_front()` by reference, |
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// use std::ref()` and `std::cref()`. |
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// |
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// Example: Storing some of the bound arguments by reference. |
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// |
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// class Service { |
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// public: |
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// void Serve(const Request& req, std::function<void()>* done) { |
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// // The request protocol buffer won't be deleted until done is called. |
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// // It's safe to store a reference to it inside the functor. |
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// Executor::DefaultExecutor()->Schedule( |
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// absl::bind_front(&Service::BlockingServe, this, std::cref(req), |
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// done)); |
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// } |
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// |
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// private: |
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// void BlockingServe(const Request& req, std::function<void()>* done); |
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// }; |
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// |
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// Example: Storing bound arguments by reference. |
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// |
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// void Print(const std::string& a, const std::string& b) { |
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// std::cerr << a << b; |
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// } |
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// |
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// std::string hi = "Hello, "; |
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// std::vector<std::string> names = {"Chuk", "Gek"}; |
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// // Doesn't copy hi. |
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// for_each(names.begin(), names.end(), |
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// absl::bind_front(Print, std::ref(hi))); |
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// |
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// // DO NOT DO THIS: the functor may outlive "hi", resulting in |
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// // dangling references. |
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// foo->DoInFuture(absl::bind_front(Print, std::ref(hi), "Guest")); // BAD! |
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// auto f = absl::bind_front(Print, std::ref(hi), "Guest"); // BAD! |
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// |
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// Example: Storing reference-like types. |
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// |
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// void Print(absl::string_view a, const std::string& b) { |
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// std::cerr << a << b; |
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// } |
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// |
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// std::string hi = "Hello, "; |
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// // Copies "hi". |
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// absl::bind_front(Print, hi)("Chuk"); |
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// |
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// // Compile error: std::reference_wrapper<const string> is not implicitly |
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// // convertible to string_view. |
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// // absl::bind_front(Print, std::cref(hi))("Chuk"); |
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// |
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// // Doesn't copy "hi". |
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// absl::bind_front(Print, absl::string_view(hi))("Chuk"); |
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// |
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#if defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L |
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using std::bind_front; |
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#else // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L |
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template <class F, class... BoundArgs> |
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constexpr functional_internal::bind_front_t<F, BoundArgs...> bind_front( |
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F&& func, BoundArgs&&... args) { |
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return functional_internal::bind_front_t<F, BoundArgs...>( |
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absl::in_place, absl::forward<F>(func), |
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absl::forward<BoundArgs>(args)...); |
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} |
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#endif // defined(__cpp_lib_bind_front) && __cpp_lib_bind_front >= 201907L |
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ABSL_NAMESPACE_END |
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} // namespace absl |
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#endif // ABSL_FUNCTIONAL_BIND_FRONT_H_
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