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  73. Next: <a href="Restricted-Pointers.html#Restricted-Pointers" accesskey="n" rel="next">Restricted Pointers</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

  74. </div>

  75. <hr>

  76. <a name="When-is-a-Volatile-C_002b_002b-Object-Accessed_003f"></a>

  77. <h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>

  78. <a name="index-accessing-volatiles-1"></a>

  79. <a name="index-volatile-read-1"></a>

  80. <a name="index-volatile-write-1"></a>

  81. <a name="index-volatile-access-1"></a>

  82. 

  83. <p>The C++ standard differs from the C standard in its treatment of

  84. volatile objects.  It fails to specify what constitutes a volatile

  85. access, except to say that C++ should behave in a similar manner to C

  86. with respect to volatiles, where possible.  However, the different

  87. lvalueness of expressions between C and C++ complicate the behavior.

  88. 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.

  89. </p>

  90. <p>The C and C++ language specifications differ when an object is

  91. accessed in a void context:

  92. </p>

  93. <div class="smallexample">

  94. <pre class="smallexample">volatile int *src = <var>somevalue</var>;

  95. *src;

  96. </pre></div>

  97. 

  98. <p>The C++ standard specifies that such expressions do not undergo lvalue

  99. to rvalue conversion, and that the type of the dereferenced object may

  100. be incomplete.  The C++ standard does not specify explicitly that it

  101. is lvalue to rvalue conversion that is responsible for causing an

  102. access.  There is reason to believe that it is, because otherwise

  103. certain simple expressions become undefined.  However, because it

  104. would surprise most programmers, G++ treats dereferencing a pointer to

  105. volatile object of complete type as GCC would do for an equivalent

  106. type in C.  When the object has incomplete type, G++ issues a

  107. warning; if you wish to force an error, you must force a conversion to

  108. rvalue with, for instance, a static cast.

  109. </p>

  110. <p>When using a reference to volatile, G++ does not treat equivalent

  111. expressions as accesses to volatiles, but instead issues a warning that

  112. no volatile is accessed.  The rationale for this is that otherwise it

  113. becomes difficult to determine where volatile access occur, and not

  114. possible to ignore the return value from functions returning volatile

  115. references.  Again, if you wish to force a read, cast the reference to

  116. an rvalue.

  117. </p>

  118. <p>G++ implements the same behavior as GCC does when assigning to a

  119. volatile object&mdash;there is no reread of the assigned-to object, the

  120. assigned rvalue is reused.  Note that in C++ assignment expressions

  121. are lvalues, and if used as an lvalue, the volatile object is

  122. referred to.  For instance, <var>vref</var> refers to <var>vobj</var>, as

  123. expected, in the following example:

  124. </p>

  125. <div class="smallexample">

  126. <pre class="smallexample">volatile int vobj;

  127. volatile int &amp;vref = vobj = <var>something</var>;

  128. </pre></div>

  129. 

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