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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <functional>
#include <type_traits>
#include "mongo/stdx/type_traits.h"
#include "mongo/util/assert_util.h"
namespace mongo {
template <typename Function>
class unique_function;
/**
* A `unique_function` is a move-only, type-erased functor object similar to `std::function`.
* It is useful in situations where a functor cannot be wrapped in `std::function` objects because
* it is incapable of being copied. Often this happens with C++14 or later lambdas which capture a
* `std::unique_ptr` by move. The interface of `unique_function` is nearly identical to
* `std::function`, except that it is not copyable.
*/
template <typename RetType, typename... Args>
class unique_function<RetType(Args...)> {
private:
// `TagTypeBase` is used as a base for the `TagType` type, to prevent it from being an
// aggregate.
struct TagTypeBase {
protected:
TagTypeBase() = default;
};
// `TagType` is used as a placeholder type in parameter lists for `enable_if` clauses. They
// have to be real parameters, not template parameters, due to MSVC limitations.
class TagType : TagTypeBase {
TagType() = default;
friend unique_function;
};
public:
using result_type = RetType;
~unique_function() noexcept = default;
unique_function() = default;
unique_function(const unique_function&) = delete;
unique_function& operator=(const unique_function&) = delete;
unique_function(unique_function&&) noexcept = default;
unique_function& operator=(unique_function&&) noexcept = default;
void swap(unique_function& that) noexcept {
using std::swap;
swap(this->impl, that.impl);
}
friend void swap(unique_function& a, unique_function& b) noexcept {
a.swap(b);
}
// TODO: Look into creating a mechanism based upon a unique_ptr to `void *`-like state, and a
// `void *` accepting function object. This will permit reusing the core impl object when
// converting between related function types, such as
// `int (std::string)` -> `void (const char *)`
template <typename Functor>
/* implicit */
unique_function(
Functor&& functor,
// The remaining arguments here are only for SFINAE purposes to enable this ctor when our
// requirements are met. They must be concrete parameters not template parameters to work
// around bugs in some compilers that we presently use. We may be able to revisit this
// design after toolchain upgrades for C++17.
std::enable_if_t<stdx::is_invocable_r<RetType, Functor, Args...>::value, TagType> =
makeTag(),
std::enable_if_t<std::is_move_constructible<Functor>::value, TagType> = makeTag(),
std::enable_if_t<!std::is_same<Functor, unique_function>::value, TagType> = makeTag())
: impl(makeImpl(std::forward<Functor>(functor))) {}
unique_function(std::nullptr_t) noexcept {}
RetType operator()(Args... args) const {
invariant(static_cast<bool>(*this));
return impl->call(std::forward<Args>(args)...);
}
explicit operator bool() const noexcept {
return static_cast<bool>(this->impl);
}
// Needed to make `std::is_convertible<mongo::unique_function<...>, std::function<...>>` be
// `std::false_type`. `mongo::unique_function` objects are not convertible to any kind of
// `std::function` object, since the latter requires a copy constructor, which the former does
// not provide. If you see a compiler error which references this line, you have tried to
// assign a `unique_function` object to a `std::function` object which is impossible -- please
// check your variables and function signatures.
//
// NOTE: This is not quite able to disable all `std::function` conversions on MSVC, at this
// time.
template <typename Signature>
operator std::function<Signature>() const = delete;
private:
// The `TagType` type cannot be constructed as a default function-parameter in Clang. So we use
// a static member function that initializes that default parameter.
static TagType makeTag() {
return {};
}
struct Impl {
virtual ~Impl() noexcept = default;
virtual RetType call(Args&&... args) = 0;
};
// These overload helpers are needed to squelch problems in the `T ()` -> `void ()` case.
template <typename Functor>
static void callRegularVoid(const std::true_type isVoid, Functor& f, Args&&... args) {
// The result of this call is not cast to void, to help preserve detection of
// `[[nodiscard]]` violations.
f(std::forward<Args>(args)...);
}
template <typename Functor>
static RetType callRegularVoid(const std::false_type isNotVoid, Functor& f, Args&&... args) {
return f(std::forward<Args>(args)...);
}
template <typename Functor>
static auto makeImpl(Functor&& functor) {
struct SpecificImpl : Impl {
explicit SpecificImpl(Functor&& func) : f(std::forward<Functor>(func)) {}
RetType call(Args&&... args) override {
return callRegularVoid(std::is_void<RetType>(), f, std::forward<Args>(args)...);
}
std::decay_t<Functor> f;
};
return std::make_unique<SpecificImpl>(std::forward<Functor>(functor));
}
std::unique_ptr<Impl> impl;
};
template <typename Signature>
bool operator==(const unique_function<Signature>& lhs, std::nullptr_t) noexcept {
return !lhs;
}
template <typename Signature>
bool operator!=(const unique_function<Signature>& lhs, std::nullptr_t) noexcept {
return static_cast<bool>(lhs);
}
template <typename Signature>
bool operator==(std::nullptr_t, const unique_function<Signature>& rhs) noexcept {
return !rhs;
}
template <typename Signature>
bool operator!=(std::nullptr_t, const unique_function<Signature>& rhs) noexcept {
return static_cast<bool>(rhs);
}
} // namespace mongo
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