Mercurial > hgrepos > Python2 > PyMuPDF

annotate mupdf-source/thirdparty/harfbuzz/docs/usermanual-object-model.xml @ 2:b50eed0cc0ef upstream

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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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1 <?xml version="1.0"?>
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2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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3 "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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4 <!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'">
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5 <!ENTITY version SYSTEM "version.xml">
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6 ]>
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7 <chapter id="object-model">
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8 <title>The HarfBuzz object model</title>
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9 <section id="object-model-intro">
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10 <title>An overview of data types in HarfBuzz</title>
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11 <para>
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12 HarfBuzz features two kinds of data types: non-opaque,
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13 pass-by-value types and opaque, heap-allocated types. This kind
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14 of separation is common in C libraries that have to provide
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15 API/ABI compatibility (almost) indefinitely.
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16 </para>
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17 <para>
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18 <emphasis>Value types:</emphasis> The non-opaque, pass-by-value
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19 types include integer types, enums, and small structs. Exposing
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20 a struct in the public API makes it impossible to expand the
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21 struct in the future. As such, exposing structs is reserved for
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22 cases where it’s extremely inefficient to do otherwise.
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23 </para>
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24 <para>
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25 In HarfBuzz, several structs, like <literal>hb_glyph_info_t</literal> and
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26 <literal>hb_glyph_position_t</literal>, fall into that efficiency-sensitive
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27 category and are non-opaque.
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28 </para>
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29 <para>
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30 For all non-opaque structs where future extensibility may be
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31 necessary, reserved members are included to hold space for
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32 possible future members. As such, it’s important to provide
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33 <function>equal()</function>, and <function>hash()</function>
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34 methods for such structs, allowing users of the API do
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35 effectively deal with the type without having to
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36 adapt their code to future changes.
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37 </para>
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38 <para>
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39 Important value types provided by HarfBuzz include the structs
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40 for working with Unicode code points, glyphs, and tags for font
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41 tables and features, as well as the enums for many Unicode and
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42 OpenType properties.
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43 </para>
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44 </section>
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45
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46 <section id="object-model-object-types">
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47 <title>Objects in HarfBuzz</title>
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48 <para>
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49 <emphasis>Object types:</emphasis> Opaque struct types are used
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50 for what HarfBuzz loosely calls "objects." This doesn’t have
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51 much to do with the terminology from object-oriented programming
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52 (OOP), although some of the concepts are similar.
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53 </para>
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54 <para>
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55 In HarfBuzz, all object types provide certain
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56 lifecycle-management APIs. Objects are reference-counted, and
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57 constructed with various <function>create()</function> methods, referenced via
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58 <function>reference()</function> and dereferenced using
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59 <function>destroy()</function>.
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60 </para>
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61 <para>
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62 For example,
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63 the <literal>hb_buffer_t</literal> object has
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64 <function>hb_buffer_create()</function> as its constructor,
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65 <function>hb_buffer_reference()</function> to reference, and
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66 <function>hb_buffer_destroy()</function> to dereference.
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67 </para>
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68 <para>
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69 After construction, each object's properties are accessible only
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70 through the setter and getter functions described in the API
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71 Reference manual.
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72 </para>
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73 <para>
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74 Note that many object types can be marked as read-only or immutable,
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75 facilitating their use in multi-threaded environments.
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76 </para>
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77 <para>
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78 Key object types provided by HarfBuzz include:
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79 </para>
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80 <itemizedlist spacing="compact">
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81 <listitem>
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82 <para>
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83 <emphasis>blobs</emphasis>, which act as low-level wrappers around binary
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84 data. Blobs are typically used to hold the contents of a
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85 binary font file.
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86 </para>
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87 </listitem>
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88 <listitem>
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89 <para>
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90 <emphasis>faces</emphasis>, which represent typefaces from a
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91 font file, but without specific parameters (such as size) set.
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92 </para>
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93 </listitem>
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94 <listitem>
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95 <para>
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96 <emphasis>fonts</emphasis>, which represent instances of a
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97 face with all of their parameters specified.
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98 </para>
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99 </listitem>
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100 <listitem>
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101 <para>
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102 <emphasis>buffers</emphasis>, which hold Unicode code points
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103 for characters (before shaping) and the shaped glyph output
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104 (after shaping).
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105 </para>
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106 </listitem>
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107 <listitem>
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108 <para>
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109 <emphasis>shape plans</emphasis>, which store the settings
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110 that HarfBuzz will use when shaping a particular text
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111 segment. Shape plans are not generally used by client
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112 programs directly, but as we will see in a later chapter,
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113 they are still valuable to understand.
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114 </para>
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115 </listitem>
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116 </itemizedlist>
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117
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118 </section>
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119
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120
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121
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122 <section id="object-model-lifecycle">
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123 <title>Object lifecycle management</title>
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124 <para>
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125 Each object type in HarfBuzz provides a
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126 <function>create()</function> method. Some object types provide
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127 additional variants of <function>create()</function> to handle
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128 special cases or to speed up common tasks; those variants are
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129 documented in the API reference. For example,
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130 <function>hb_blob_create_from_file()</function> constructs a new
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131 blob directly from the contents of a file.
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132 </para>
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133 <para>
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134 All objects are created with an initial reference count of
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135 <literal>1</literal>. Client programs can increase the reference
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136 count on an object by calling its
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137 <function>reference()</function> method. Whenever a client
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138 program is finished with an object, it should call its
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139 corresponding <function>destroy()</function> method. The destroy
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140 method will decrease the reference count on the object and,
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141 whenever the reference count reaches zero, it will also destroy
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142 the object and free all of the associated memory.
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143 </para>
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144 <para>
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145 All of HarfBuzz's object-lifecycle-management APIs are
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146 thread-safe (unless you compiled HarfBuzz from source with the
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147 <literal>HB_NO_MT</literal> configuration flag), even when the
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148 object as a whole is not thread-safe.
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149 It is also permissible to <function>reference()</function> or to
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150 <function>destroy()</function> the <literal>NULL</literal>
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151 value.
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152 </para>
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153 <para>
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154 Some objects are thread-safe after they have been constructed
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155 and set up. The general pattern is to
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156 <function>create()</function> the object, make a few
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157 <function>set_*()</function> calls to set up the
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158 object, and then use it without further modification.
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159 </para>
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160 <para>
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161 To ensure that such an object is not modified, client programs
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162 can explicitly mark an object as immutable. HarfBuzz provides
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163 <function>make_immutable()</function> methods to mark an object
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164 as immutable and <function>is_immutable()</function> methods to
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165 test whether or not an object is immutable. Attempts to use
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166 setter functions on immutable objects will fail silently; see the API
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167 Reference manual for specifics.
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168 </para>
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169 <para>
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170 Note also that there are no "make mutable" methods. If client
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171 programs need to alter an object previously marked as immutable,
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172 they will need to make a duplicate of the original.
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173 </para>
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174 <para>
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175 Finally, object constructors (and, indeed, as much of the
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176 shaping API as possible) will never return
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177 <literal>NULL</literal>. Instead, if there is an allocation
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178 error, each constructor will return an “empty” object
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179 singleton.
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180 </para>
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181 <para>
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182 These empty-object singletons are inert and safe (although
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183 typically useless) to pass around. This design choice avoids
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184 having to check for <literal>NULL</literal> pointers all
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185 throughout the code.
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186 </para>
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187 <para>
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188 In addition, this “empty” object singleton can also be accessed
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189 using the <function>get_empty()</function> method of the object
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190 type in question.
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191 </para>
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192 </section>
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193
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194
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195 <section id="object-model-user-data">
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196 <title>User data</title>
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197 <para>
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198 To better integrate with client programs, HarfBuzz's objects
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199 offer a "user data" mechanism that can be used to attach
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200 arbitrary data to the object. User-data attachment can be
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201 useful for tying the lifecycles of various pieces of data
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202 together, or for creating language bindings.
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203 </para>
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204 <para>
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205 Each object type has a <function>set_user_data()</function>
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206 method and a <function>get_user_data()</function> method. The
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207 <function>set_user_data()</function> methods take a client-provided
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208 <literal>key</literal> and a pointer,
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209 <literal>user_data</literal>, pointing to the data itself. Once
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210 the key-data pair has been attached to the object, the
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211 <function>get_user_data()</function> method can be called with
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212 the key, returning the <function>user_data</function> pointer.
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213 </para>
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214 <para>
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215 The <function>set_user_data()</function> methods also support an
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216 optional <function>destroy</function> callback. Client programs
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217 can set the <function>destroy</function> callback and receive
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218 notification from HarfBuzz whenever the object is destructed.
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219 </para>
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220 <para>
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221 Finally, each <function>set_user_data()</function> method allows
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222 the client program to set a <literal>replace</literal> Boolean
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223 indicating whether or not the function call should replace any
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224 existing <literal>user_data</literal>
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225 associated with the specified key.
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226 </para>
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227 </section>
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228
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229
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230
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231 <section id="object-model-blobs">
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232 <title>Blobs</title>
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233 <para>
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234 While most of HarfBuzz's object types are specific to the
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235 shaping process, <emphasis>blobs</emphasis> are somewhat
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236 different.
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237 </para>
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238 <para>
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239 Blobs are an abstraction designed to negotiate lifecycle and
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240 permissions for raw pieces of data. For example, when you load
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241 the raw font data into memory and want to pass it to HarfBuzz,
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242 you do so in a <literal>hb_blob_t</literal> wrapper.
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243 </para>
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244 <para>
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245 This allows you to take advantage of HarfBuzz's
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246 reference-counting and <function>destroy</function>
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247 callbacks. If you allocated the memory for the data using
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248 <function>malloc()</function>, you would create the blob using
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249 </para>
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250 <programlisting language="C">
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251 hb_blob_create (data, length, HB_MEMORY_MODE_WRITABLE, data, free)
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252 </programlisting>
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253 <para>
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254 That way, HarfBuzz will call <function>free()</function> on the
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255 allocated memory whenever the blob drops its last reference and
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256 is deconstructed. Consequently, the user code can stop worrying
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257 about freeing memory and let the reference-counting machinery
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258 take care of that.
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259 </para>
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260 <para>
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261 Most of the time, blobs are read-only, facilitating their use in
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262 immutable objects.
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263 </para>
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264 </section>
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265
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266 </chapter>