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114 lines
3.9 KiB
C++
114 lines
3.9 KiB
C++
/*
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* Copyright 2016 Facebook, Inc.
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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 <folly/ApplyTuple.h>
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namespace folly {
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namespace detail {
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namespace partial {
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// helper type to make sure that the templated constructor in Partial does
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// not accidentally act as copy or move constructor
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struct PartialConstructFromCallable {};
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template <typename F, typename Tuple>
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class Partial {
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private:
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F f_;
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Tuple stored_args_;
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public:
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template <typename Callable, typename... Args>
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Partial(PartialConstructFromCallable, Callable&& callable, Args&&... args)
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: f_(std::forward<Callable>(callable)),
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stored_args_(std::forward<Args>(args)...) {}
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// full auto doesn't work here due to
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// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70983 :(
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template <typename... CArgs>
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auto operator()(CArgs&&... cargs) & -> decltype(applyTuple(
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static_cast<F&>(f_),
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static_cast<Tuple&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...))) {
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return applyTuple(
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static_cast<F&>(f_),
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static_cast<Tuple&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...));
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}
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template <typename... CArgs>
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auto operator()(CArgs&&... cargs) const& -> decltype(applyTuple(
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static_cast<F const&>(f_),
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static_cast<Tuple const&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...))) {
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return applyTuple(
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static_cast<F const&>(f_),
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static_cast<Tuple const&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...));
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}
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template <typename... CArgs>
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auto operator()(CArgs&&... cargs) && -> decltype(applyTuple(
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static_cast<F&&>(f_),
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static_cast<Tuple&&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...))) {
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return applyTuple(
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static_cast<F&&>(f_),
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static_cast<Tuple&&>(stored_args_),
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std::forward_as_tuple(std::forward<CArgs>(cargs)...));
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}
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};
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} // namespace partial
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} // namespace detail
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/**
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* Partially applies arguments to a callable
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*
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* `partial` takes a callable and zero or more additional arguments and returns
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* a callable object, which when called with zero or more arguments, will invoke
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* the original callable with the additional arguments passed to `partial`,
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* followed by those passed to the call.
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*
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* E.g. `partial(Foo, 1, 2)(3)` is equivalent to `Foo(1, 2, 3)`.
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*
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* `partial` can be used to bind a class method to an instance:
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* `partial(&Foo::method, foo_pointer)` returns a callable object that can be
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* invoked in the same way as `foo_pointer->method`. In case the first
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* argument in a call to `partial` is a member pointer, the second argument
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* can be a reference, pointer or any object that can be dereferenced to
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* an object of type Foo (like `std::shared_ptr` or `std::unique_ptr`).
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*
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* `partial` is similar to `std::bind`, but you don't have to use placeholders
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* to have arguments passed on. Any number of arguments passed to the object
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* returned by `partial` when called will be added to those passed to `partial`
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* and passed to the original callable.
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*/
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template <typename F, typename... Args>
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auto partial(F&& f, Args&&... args) -> detail::partial::Partial<
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typename std::decay<F>::type,
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std::tuple<typename std::decay<Args>::type...>> {
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return {detail::partial::PartialConstructFromCallable{},
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std::forward<F>(f),
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std::forward<Args>(args)...};
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}
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} // namespace folly
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