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
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
|
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef mozilla_NotNull_h
#define mozilla_NotNull_h
// It's often unclear if a particular pointer, be it raw (T*) or smart
// (RefPtr<T>, nsCOMPtr<T>, etc.) can be null. This leads to missing null
// checks (which can cause crashes) and unnecessary null checks (which clutter
// the code).
//
// C++ has a built-in alternative that avoids these problems: references. This
// module defines another alternative, NotNull, which can be used in cases
// where references are not suitable.
//
// In the comments below we use the word "handle" to cover all varieties of
// pointers and references.
//
// References
// ----------
// References are always non-null. (You can do |T& r = *p;| where |p| is null,
// but that's undefined behaviour. C++ doesn't provide any built-in, ironclad
// guarantee of non-nullness.)
//
// A reference works well when you need a temporary handle to an existing
// single object, e.g. for passing a handle to a function, or as a local handle
// within another object. (In Rust parlance, this is a "borrow".)
//
// A reference is less appropriate in the following cases.
//
// - As a primary handle to an object. E.g. code such as this is possible but
// strange: |T& t = *new T(); ...; delete &t;|
//
// - As a handle to an array. It's common for |T*| to refer to either a single
// |T| or an array of |T|, but |T&| cannot refer to an array of |T| because
// you can't index off a reference (at least, not without first converting it
// to a pointer).
//
// - When the handle identity is meaningful, e.g. if you have a hashtable of
// handles, because you have to use |&| on the reference to convert it to a
// pointer.
//
// - Some people don't like using non-const references as function parameters,
// because it is not clear at the call site that the argument might be
// modified.
//
// - When you need "smart" behaviour. E.g. we lack reference equivalents to
// RefPtr and nsCOMPtr.
//
// - When interfacing with code that uses pointers a lot, sometimes using a
// reference just feels like an odd fit.
//
// Furthermore, a reference is impossible in the following cases.
//
// - When the handle is rebound to another object. References don't allow this.
//
// - When the handle has type |void|. |void&| is not allowed.
//
// NotNull is an alternative that can be used in any of the above cases except
// for the last one, where the handle type is |void|. See below.
#include "mozilla/Assertions.h"
namespace mozilla {
// NotNull can be used to wrap a "base" pointer (raw or smart) to indicate it
// is not null. Some examples:
//
// - NotNull<char*>
// - NotNull<RefPtr<Event>>
// - NotNull<nsCOMPtr<Event>>
//
// NotNull has the following notable properties.
//
// - It has zero space overhead.
//
// - It must be initialized explicitly. There is no default initialization.
//
// - It auto-converts to the base pointer type.
//
// - It does not auto-convert from a base pointer. Implicit conversion from a
// less-constrained type (e.g. T*) to a more-constrained type (e.g.
// NotNull<T*>) is dangerous. Creation and assignment from a base pointer can
// only be done with WrapNotNull(), which makes them impossible to overlook,
// both when writing and reading code.
//
// - When initialized (or assigned) it is checked, and if it is null we abort.
// This guarantees that it cannot be null.
//
// - |operator bool()| is deleted. This means you cannot check a NotNull in a
// boolean context, which eliminates the possibility of unnecessary null
// checks.
//
// NotNull currently doesn't work with UniquePtr. See
// https://github.com/Microsoft/GSL/issues/89 for some discussion.
//
template <typename T>
class NotNull
{
template <typename U> friend NotNull<U> WrapNotNull(U aBasePtr);
T mBasePtr;
// This constructor is only used by WrapNotNull().
template <typename U>
explicit NotNull(U aBasePtr) : mBasePtr(aBasePtr) {}
public:
// Disallow default construction.
NotNull() = delete;
// Construct/assign from another NotNull with a compatible base pointer type.
template <typename U>
MOZ_IMPLICIT NotNull(const NotNull<U>& aOther) : mBasePtr(aOther.get()) {}
// Default copy/move construction and assignment.
NotNull(const NotNull<T>&) = default;
NotNull<T>& operator=(const NotNull<T>&) = default;
NotNull(NotNull<T>&&) = default;
NotNull<T>& operator=(NotNull<T>&&) = default;
// Disallow null checks, which are unnecessary for this type.
explicit operator bool() const = delete;
// Explicit conversion to a base pointer. Use only to resolve ambiguity or to
// get a castable pointer.
const T& get() const { return mBasePtr; }
// Implicit conversion to a base pointer. Preferable to get().
operator const T&() const { return get(); }
// Dereference operators.
const T& operator->() const { return get(); }
decltype(*mBasePtr) operator*() const { return *mBasePtr; }
};
template <typename T>
NotNull<T>
WrapNotNull(const T aBasePtr)
{
NotNull<T> notNull(aBasePtr);
MOZ_RELEASE_ASSERT(aBasePtr);
return notNull;
}
// Compare two NotNulls.
template <typename T, typename U>
inline bool
operator==(const NotNull<T>& aLhs, const NotNull<U>& aRhs)
{
return aLhs.get() == aRhs.get();
}
template <typename T, typename U>
inline bool
operator!=(const NotNull<T>& aLhs, const NotNull<U>& aRhs)
{
return aLhs.get() != aRhs.get();
}
// Compare a NotNull to a base pointer.
template <typename T, typename U>
inline bool
operator==(const NotNull<T>& aLhs, const U& aRhs)
{
return aLhs.get() == aRhs;
}
template <typename T, typename U>
inline bool
operator!=(const NotNull<T>& aLhs, const U& aRhs)
{
return aLhs.get() != aRhs;
}
// Compare a base pointer to a NotNull.
template <typename T, typename U>
inline bool
operator==(const T& aLhs, const NotNull<U>& aRhs)
{
return aLhs == aRhs.get();
}
template <typename T, typename U>
inline bool
operator!=(const T& aLhs, const NotNull<U>& aRhs)
{
return aLhs != aRhs.get();
}
// Disallow comparing a NotNull to a nullptr.
template <typename T>
bool
operator==(const NotNull<T>&, decltype(nullptr)) = delete;
template <typename T>
bool
operator!=(const NotNull<T>&, decltype(nullptr)) = delete;
// Disallow comparing a nullptr to a NotNull.
template <typename T>
bool
operator==(decltype(nullptr), const NotNull<T>&) = delete;
template <typename T>
bool
operator!=(decltype(nullptr), const NotNull<T>&) = delete;
} // namespace mozilla
#endif /* mozilla_NotNull_h */
|
