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