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<a name="C++-Volatiles"></a>
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<a name="C_002b_002b-Volatiles"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Restricted-Pointers.html#Restricted-Pointers">Restricted Pointers</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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<h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>
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<p><a name="index-accessing-volatiles-4199"></a><a name="index-volatile-read-4200"></a><a name="index-volatile-write-4201"></a><a name="index-volatile-access-4202"></a>
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The C++ standard differs from the C standard in its treatment of
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volatile objects. It fails to specify what constitutes a volatile
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access, except to say that C++ should behave in a similar manner to C
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with respect to volatiles, where possible. However, the different
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lvalueness of expressions between C and C++ complicate the behavior.
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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.
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<p>The C and C++ language specifications differ when an object is
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accessed in a void context:
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<pre class="smallexample"> volatile int *src = <var>somevalue</var>;
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*src;
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</pre>
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<p>The C++ standard specifies that such expressions do not undergo lvalue
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to rvalue conversion, and that the type of the dereferenced object may
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be incomplete. The C++ standard does not specify explicitly that it
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is lvalue to rvalue conversion that is responsible for causing an
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access. There is reason to believe that it is, because otherwise
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certain simple expressions become undefined. However, because it
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would surprise most programmers, G++ treats dereferencing a pointer to
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volatile object of complete type as GCC would do for an equivalent
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type in C. When the object has incomplete type, G++ issues a
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warning; if you wish to force an error, you must force a conversion to
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rvalue with, for instance, a static cast.
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<p>When using a reference to volatile, G++ does not treat equivalent
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expressions as accesses to volatiles, but instead issues a warning that
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no volatile is accessed. The rationale for this is that otherwise it
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becomes difficult to determine where volatile access occur, and not
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possible to ignore the return value from functions returning volatile
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references. Again, if you wish to force a read, cast the reference to
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an rvalue.
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<p>G++ implements the same behavior as GCC does when assigning to a
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volatile object—there is no reread of the assigned-to object, the
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assigned rvalue is reused. Note that in C++ assignment expressions
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are lvalues, and if used as an lvalue, the volatile object is
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referred to. For instance, <var>vref</var> refers to <var>vobj</var>, as
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expected, in the following example:
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<pre class="smallexample"> volatile int vobj;
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volatile int &vref = vobj = <var>something</var>;
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</pre>
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