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<title>Bound member functions - Using the GNU Compiler Collection (GCC)</title>
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<a name="Bound-member-functions"></a>
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Next: <a rel="next" accesskey="n" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>,
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Previous: <a rel="previous" accesskey="p" href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</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">7.6 Extracting the Function Pointer from a Bound Pointer to Member Function</h3>
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<p><a name="index-pmf-4225"></a><a name="index-pointer-to-member-function-4226"></a><a name="index-bound-pointer-to-member-function-4227"></a>
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In C++, pointer to member functions (PMFs) are implemented using a wide
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pointer of sorts to handle all the possible call mechanisms; the PMF
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needs to store information about how to adjust the ‘<samp><span class="samp">this</span></samp>’ pointer,
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and if the function pointed to is virtual, where to find the vtable, and
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where in the vtable to look for the member function. If you are using
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PMFs in an inner loop, you should really reconsider that decision. If
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that is not an option, you can extract the pointer to the function that
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would be called for a given object/PMF pair and call it directly inside
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the inner loop, to save a bit of time.
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<p>Note that you still pay the penalty for the call through a
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function pointer; on most modern architectures, such a call defeats the
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branch prediction features of the CPU. This is also true of normal
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virtual function calls.
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<p>The syntax for this extension is
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<pre class="smallexample"> extern A a;
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extern int (A::*fp)();
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typedef int (*fptr)(A *);
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fptr p = (fptr)(a.*fp);
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</pre>
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<p>For PMF constants (i.e. expressions of the form ‘<samp><span class="samp">&Klasse::Member</span></samp>’),
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no object is needed to obtain the address of the function. They can be
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converted to function pointers directly:
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<pre class="smallexample"> fptr p1 = (fptr)(&A::foo);
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</pre>
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<p><a name="index-Wno_002dpmf_002dconversions-4228"></a>You must specify <samp><span class="option">-Wno-pmf-conversions</span></samp> to use this extension.
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