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| <h3 class="section">7.6 Extracting the function pointer from a bound pointer to member function</h3> |
| |
| <p><a name="index-pmf-3185"></a><a name="index-pointer-to-member-function-3186"></a><a name="index-bound-pointer-to-member-function-3187"></a> |
| In C++, pointer to member functions (PMFs) are implemented using a wide |
| pointer of sorts to handle all the possible call mechanisms; the PMF |
| needs to store information about how to adjust the ‘<samp><span class="samp">this</span></samp>’ pointer, |
| and if the function pointed to is virtual, where to find the vtable, and |
| where in the vtable to look for the member function. If you are using |
| PMFs in an inner loop, you should really reconsider that decision. If |
| that is not an option, you can extract the pointer to the function that |
| would be called for a given object/PMF pair and call it directly inside |
| the inner loop, to save a bit of time. |
| |
| <p>Note that you will still be paying the penalty for the call through a |
| function pointer; on most modern architectures, such a call defeats the |
| branch prediction features of the CPU. This is also true of normal |
| virtual function calls. |
| |
| <p>The syntax for this extension is |
| |
| <pre class="smallexample"> extern A a; |
| extern int (A::*fp)(); |
| typedef int (*fptr)(A *); |
| |
| fptr p = (fptr)(a.*fp); |
| </pre> |
| <p>For PMF constants (i.e. expressions of the form ‘<samp><span class="samp">&Klasse::Member</span></samp>’), |
| no object is needed to obtain the address of the function. They can be |
| converted to function pointers directly: |
| |
| <pre class="smallexample"> fptr p1 = (fptr)(&A::foo); |
| </pre> |
| <p><a name="index-Wno_002dpmf_002dconversions-3188"></a>You must specify <samp><span class="option">-Wno-pmf-conversions</span></samp> to use this extension. |
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