Add funcptr FAQ

[SVN r17974]

doc/v2/faq.html

@@ -29,6 +29,10 @@
     <hr>
 
     <dl class="page-index">
+
+      <dt><a href="#funcptr">How can I wrap a function which takes a
+      function pointer as an argument?</a><dd>
+
       <dt><a href="#dangling">I'm getting the "attempt to return dangling
       reference" error. What am I doing wrong?</a></dt>
 
@@ -56,6 +60,57 @@
     </dl>
     <hr>
 
+    <h2><a name="funcptr">How can I wrap a function which takes a
+      function pointer as an argument?</a></h2>
+
+    If what you're trying to do is something like this:
+<pre>
+typedef boost::function&lt;void (string s) &gt; funcptr;
+
+void foo(funcptr fp)
+{
+    fp(&quot;hello,world!&quot;);
+}
+
+BOOST_PYTHON_MODULE(test)
+{
+    def(&quot;foo&quot;,foo) ;
+}
+</pre>
+
+And then:
+
+<pre>
+&gt;&gt;&gt; def hello(s):
+...    print s 
+...
+&gt;&gt;&gt; foo(hello)
+hello, world!
+</pre>
+
+    The short answer is: &quot;you can't&quot;.  This is not a
+    Boost.Python limitation so much as a limitation of C++.  The
+    problem is that a Python function is actually data, and the only
+    way of associating data with a C++ function pointer is to store it
+    in a static variable of the function.  The problem with that is
+    that you can only associate one piece of data with every C++
+    function, and we have no way of compiling a new C++ function
+    on-the-fly for every Python function you decide to pass
+    to <code>foo</code>.  In other words, this could work if the C++
+    function is always going to invoke the <em>same</em> Python
+    function, but you probably don't want that.
+
+    <p>If you have the luxury of changing the C++ code you're
+    wrapping, pass it an <code>object</code> instead and call that;
+    the overloaded function call operator will invoke the Python
+    function you pass it behind the <code>object</code>.
+
+    <p>For more perspective on the issue, see <a
+    href="http://aspn.activestate.com/ASPN/Mail/Message/1554837">this
+    posting</a>.
+
+    <hr>
+
     <h2><a name="dangling">I'm getting the "attempt to return dangling
     reference" error. What am I doing wrong?</a></h2>
     That exception is protecting you from causing a nasty crash. It usually
@@ -492,7 +547,7 @@ void b_insert(B&amp; b, std::auto_ptr&lt;A&gt; a)
 
     <p>Revised 
     <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
-     23 January, 2003 
+     18 March, 2003 
     <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
     </p>