C_002b_002b-Volatiles.html 6.5 KB

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  68. <a name="When-is-a-Volatile-C_002b_002b-Object-Accessed_003f"></a>
  69. <h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>
  70. <a name="index-accessing-volatiles-1"></a>
  71. <a name="index-volatile-read-1"></a>
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  74. <p>The C++ standard differs from the C standard in its treatment of
  75. volatile objects. It fails to specify what constitutes a volatile
  76. access, except to say that C++ should behave in a similar manner to C
  77. with respect to volatiles, where possible. However, the different
  78. lvalueness of expressions between C and C++ complicate the behavior.
  79. G++ behaves the same as GCC for volatile access, See <a href="C-Extensions.html#C-Extensions">Volatiles</a>, for a description of GCC&rsquo;s behavior.
  80. </p>
  81. <p>The C and C++ language specifications differ when an object is
  82. accessed in a void context:
  83. </p>
  84. <div class="smallexample">
  85. <pre class="smallexample">volatile int *src = <var>somevalue</var>;
  86. *src;
  87. </pre></div>
  88. <p>The C++ standard specifies that such expressions do not undergo lvalue
  89. to rvalue conversion, and that the type of the dereferenced object may
  90. be incomplete. The C++ standard does not specify explicitly that it
  91. is lvalue to rvalue conversion that is responsible for causing an
  92. access. There is reason to believe that it is, because otherwise
  93. certain simple expressions become undefined. However, because it
  94. would surprise most programmers, G++ treats dereferencing a pointer to
  95. volatile object of complete type as GCC would do for an equivalent
  96. type in C. When the object has incomplete type, G++ issues a
  97. warning; if you wish to force an error, you must force a conversion to
  98. rvalue with, for instance, a static cast.
  99. </p>
  100. <p>When using a reference to volatile, G++ does not treat equivalent
  101. expressions as accesses to volatiles, but instead issues a warning that
  102. no volatile is accessed. The rationale for this is that otherwise it
  103. becomes difficult to determine where volatile access occur, and not
  104. possible to ignore the return value from functions returning volatile
  105. references. Again, if you wish to force a read, cast the reference to
  106. an rvalue.
  107. </p>
  108. <p>G++ implements the same behavior as GCC does when assigning to a
  109. volatile object&mdash;there is no reread of the assigned-to object, the
  110. assigned rvalue is reused. Note that in C++ assignment expressions
  111. are lvalues, and if used as an lvalue, the volatile object is
  112. referred to. For instance, <var>vref</var> refers to <var>vobj</var>, as
  113. expected, in the following example:
  114. </p>
  115. <div class="smallexample">
  116. <pre class="smallexample">volatile int vobj;
  117. volatile int &amp;vref = vobj = <var>something</var>;
  118. </pre></div>
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