1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
|
/**
* Copyright 2018 MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* 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 GNU Affero General 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::move(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::move(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
|
