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 <a name="C_002b_002b-Volatiles"></a>
 <div class="header">
 <p>
 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>
 </div>
 <hr>
 <a name="When-is-a-Volatile-C_002b_002b-Object-Accessed_003f"></a>
 <h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>
 <a name="index-accessing-volatiles-1"></a>
 <a name="index-volatile-read-1"></a>
 <a name="index-volatile-write-1"></a>
 <a name="index-volatile-access-1"></a>

 <p>The C++ standard differs from the C standard in its treatment of
 volatile objects.  It fails to specify what constitutes a volatile
 access, except to say that C++ should behave in a similar manner to C
 with respect to volatiles, where possible.  However, the different
 lvalueness of expressions between C and C++ complicate the behavior.
 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.
 </p>
 <p>The C and C++ language specifications differ when an object is
 accessed in a void context:
 </p>
 <div class="smallexample">
 <pre class="smallexample">volatile int *src = <var>somevalue</var>;
 *src;
 </pre></div>

 <p>The C++ standard specifies that such expressions do not undergo lvalue
 to rvalue conversion, and that the type of the dereferenced object may
 be incomplete.  The C++ standard does not specify explicitly that it
 is lvalue to rvalue conversion that is responsible for causing an
 access.  There is reason to believe that it is, because otherwise
 certain simple expressions become undefined.  However, because it
 would surprise most programmers, G++ treats dereferencing a pointer to
 volatile object of complete type as GCC would do for an equivalent
 type in C.  When the object has incomplete type, G++ issues a
 warning; if you wish to force an error, you must force a conversion to
 rvalue with, for instance, a static cast.
 </p>
 <p>When using a reference to volatile, G++ does not treat equivalent
 expressions as accesses to volatiles, but instead issues a warning that
 no volatile is accessed.  The rationale for this is that otherwise it
 becomes difficult to determine where volatile access occur, and not
 possible to ignore the return value from functions returning volatile
 references.  Again, if you wish to force a read, cast the reference to
 an rvalue.
 </p>
 <p>G++ implements the same behavior as GCC does when assigning to a
 volatile object&mdash;there is no reread of the assigned-to object, the
 assigned rvalue is reused.  Note that in C++ assignment expressions
 are lvalues, and if used as an lvalue, the volatile object is
 referred to.  For instance, <var>vref</var> refers to <var>vobj</var>, as
 expected, in the following example:
 </p>
 <div class="smallexample">
 <pre class="smallexample">volatile int vobj;
 volatile int &amp;vref = vobj = <var>something</var>;
 </pre></div>

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 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>
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	<a name="C_002b_002b-Volatiles"></a>
	<div class="header">
	<p>
	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>   [<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>
	</div>
	<hr>
	<a name="When-is-a-Volatile-C_002b_002b-Object-Accessed_003f"></a>
	<h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>
	<a name="index-accessing-volatiles-1"></a>
	<a name="index-volatile-read-1"></a>
	<a name="index-volatile-write-1"></a>
	<a name="index-volatile-access-1"></a>

	<p>The C++ standard differs from the C standard in its treatment of
	volatile objects. It fails to specify what constitutes a volatile
	access, except to say that C++ should behave in a similar manner to C
	with respect to volatiles, where possible. However, the different
	lvalueness of expressions between C and C++ complicate the behavior.
	G++ behaves the same as GCC for volatile access, See <a href="C-Extensions.html#C-Extensions">Volatiles</a>, for a description of GCC’s behavior.
	</p>
	<p>The C and C++ language specifications differ when an object is
	accessed in a void context:
	</p>
	<div class="smallexample">
	<pre class="smallexample">volatile int *src = <var>somevalue</var>;
	*src;
	</pre></div>

	<p>The C++ standard specifies that such expressions do not undergo lvalue
	to rvalue conversion, and that the type of the dereferenced object may
	be incomplete. The C++ standard does not specify explicitly that it
	is lvalue to rvalue conversion that is responsible for causing an
	access. There is reason to believe that it is, because otherwise
	certain simple expressions become undefined. However, because it
	would surprise most programmers, G++ treats dereferencing a pointer to
	volatile object of complete type as GCC would do for an equivalent
	type in C. When the object has incomplete type, G++ issues a
	warning; if you wish to force an error, you must force a conversion to
	rvalue with, for instance, a static cast.
	</p>
	<p>When using a reference to volatile, G++ does not treat equivalent
	expressions as accesses to volatiles, but instead issues a warning that
	no volatile is accessed. The rationale for this is that otherwise it
	becomes difficult to determine where volatile access occur, and not
	possible to ignore the return value from functions returning volatile
	references. Again, if you wish to force a read, cast the reference to
	an rvalue.
	</p>
	<p>G++ implements the same behavior as GCC does when assigning to a
	volatile object—there is no reread of the assigned-to object, the
	assigned rvalue is reused. Note that in C++ assignment expressions
	are lvalues, and if used as an lvalue, the volatile object is
	referred to. For instance, <var>vref</var> refers to <var>vobj</var>, as
	expected, in the following example:
	</p>
	<div class="smallexample">
	<pre class="smallexample">volatile int vobj;
	volatile int &vref = vobj = <var>something</var>;
	</pre></div>

	<hr>
	<div class="header">
	<p>
	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>   [<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>
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