// 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,
// except types of `std::reference_wrapper` which are 'unwound' and stored by
// reference.
//
// 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 string& a, const string& b) { LOG(INFO) << 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!
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_
