summaryrefslogtreecommitdiff
path: root/xpcom/string/nsStringBuffer.h
blob: 432289bf6f1b9b527f066c99f4900545943c567e (plain)
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
/* -*- 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 nsStringBuffer_h__
#define nsStringBuffer_h__

#include "mozilla/Atomics.h"
#include "mozilla/MemoryReporting.h"

template<class T> struct already_AddRefed;

/**
 * This structure precedes the string buffers "we" allocate.  It may be the
 * case that nsTAString::mData does not point to one of these special
 * buffers.  The mFlags member variable distinguishes the buffer type.
 *
 * When this header is in use, it enables reference counting, and capacity
 * tracking.  NOTE: A string buffer can be modified only if its reference
 * count is 1.
 */
class nsStringBuffer
{
private:
  friend class CheckStaticAtomSizes;

  mozilla::Atomic<int32_t> mRefCount;
  uint32_t mStorageSize;

public:

  /**
   * Allocates a new string buffer, with given size in bytes and a
   * reference count of one.  When the string buffer is no longer needed,
   * it should be released via Release.
   *
   * It is up to the caller to set the bytes corresponding to the string
   * buffer by calling the Data method to fetch the raw data pointer.  Care
   * must be taken to properly null terminate the character array.  The
   * storage size can be greater than the length of the actual string
   * (i.e., it is not required that the null terminator appear in the last
   * storage unit of the string buffer's data).
   *
   * @return new string buffer or null if out of memory.
   */
  static already_AddRefed<nsStringBuffer> Alloc(size_t aStorageSize);

  /**
   * Resizes the given string buffer to the specified storage size.  This
   * method must not be called on a readonly string buffer.  Use this API
   * carefully!!
   *
   * This method behaves like the ANSI-C realloc function.  (i.e., If the
   * allocation fails, null will be returned and the given string buffer
   * will remain unmodified.)
   *
   * @see IsReadonly
   */
  static nsStringBuffer* Realloc(nsStringBuffer* aBuf, size_t aStorageSize);

  /**
   * Increment the reference count on this string buffer.
   */
  void NS_FASTCALL AddRef();

  /**
   * Decrement the reference count on this string buffer.  The string
   * buffer will be destroyed when its reference count reaches zero.
   */
  void NS_FASTCALL Release();

  /**
   * This method returns the string buffer corresponding to the given data
   * pointer.  The data pointer must have been returned previously by a
   * call to the nsStringBuffer::Data method.
   */
  static nsStringBuffer* FromData(void* aData)
  {
    return reinterpret_cast<nsStringBuffer*>(aData) - 1;
  }

  /**
   * This method returns the data pointer for this string buffer.
   */
  void* Data() const
  {
    return const_cast<char*>(reinterpret_cast<const char*>(this + 1));
  }

  /**
   * This function returns the storage size of a string buffer in bytes.
   * This value is the same value that was originally passed to Alloc (or
   * Realloc).
   */
  uint32_t StorageSize() const
  {
    return mStorageSize;
  }

  /**
   * If this method returns false, then the caller can be sure that their
   * reference to the string buffer is the only reference to the string
   * buffer, and therefore it has exclusive access to the string buffer and
   * associated data.  However, if this function returns true, then other
   * consumers may rely on the data in this buffer being immutable and
   * other threads may access this buffer simultaneously.
   */
  bool IsReadonly() const
  {
    return mRefCount > 1;
  }

  /**
   * The FromString methods return a string buffer for the given string
   * object or null if the string object does not have a string buffer.
   * The reference count of the string buffer is NOT incremented by these
   * methods.  If the caller wishes to hold onto the returned value, then
   * the returned string buffer must have its reference count incremented
   * via a call to the AddRef method.
   */
  static nsStringBuffer* FromString(const nsAString& aStr);
  static nsStringBuffer* FromString(const nsACString& aStr);

  /**
   * The ToString methods assign this string buffer to a given string
   * object.  If the string object does not support sharable string
   * buffers, then its value will be set to a copy of the given string
   * buffer.  Otherwise, these methods increment the reference count of the
   * given string buffer.  It is important to specify the length (in
   * storage units) of the string contained in the string buffer since the
   * length of the string may be less than its storage size.  The string
   * must have a null terminator at the offset specified by |len|.
   *
   * NOTE: storage size is measured in bytes even for wide strings;
   *       however, string length is always measured in storage units
   *       (2-byte units for wide strings).
   */
  void ToString(uint32_t aLen, nsAString& aStr, bool aMoveOwnership = false);
  void ToString(uint32_t aLen, nsACString& aStr, bool aMoveOwnership = false);

  /**
   * This measures the size only if the StringBuffer is unshared.
   */
  size_t SizeOfIncludingThisIfUnshared(mozilla::MallocSizeOf aMallocSizeOf) const;

  /**
   * This measures the size regardless of whether the StringBuffer is
   * unshared.
   *
   * WARNING: Only use this if you really know what you are doing, because
   * it can easily lead to double-counting strings.  If you do use them,
   * please explain clearly in a comment why it's safe and won't lead to
   * double-counting.
   */
  size_t SizeOfIncludingThisEvenIfShared(mozilla::MallocSizeOf aMallocSizeOf) const;
};

#endif /* !defined(nsStringBuffer_h__ */