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- <a href="class.boost.function.html" title="Class template function">boost::function</a> objects with
- <tt class="computeroutput">operator==</tt> or
- <tt class="computeroutput">operator!=</tt>?</p></td></tr><tr class="answer"><td align="left" valign="top"><b></b></td><td align="left" valign="top"><p>Comparison between <a href="class.boost.function.html" title="Class template function">boost::function</a>
- objects cannot be implemented "well", and therefore will not be
- implemented. The typical semantics requested for <tt class="computeroutput">f ==
- g</tt> given <a href="class.boost.function.html" title="Class template function">boost::function</a> objects
- <tt class="computeroutput">f</tt> and <tt class="computeroutput">g</tt> are:</p><div class="itemizedlist"><ul type="disc"><li>If <tt class="computeroutput">f</tt> and <tt class="computeroutput">g</tt>
- store function objects of the same type, use that type's
- <tt class="computeroutput">operator==</tt> to compare
- them.</li><li>If <tt class="computeroutput">f</tt> and <tt class="computeroutput">g</tt>
- store function objects of different types, return
- <tt class="computeroutput">false</tt>.</li></ul></div><p>The problem occurs when the type of the function objects
- stored by both <tt class="computeroutput">f</tt> and <tt class="computeroutput">g</tt> doesn't have an
- <tt class="computeroutput">operator==</tt>: we would like the expression <tt class="computeroutput">f ==
- g</tt> to fail to compile, as occurs with, e.g., the standard
- containers. However, this is not implementable for
- <a href="class.boost.function.html" title="Class template function">boost::function</a> because it necessarily
- "erases" some type information after it has been assigned a
- function object, so it cannot try to call
- <tt class="computeroutput">operator==</tt> later: it must either find a way to call
- <tt class="computeroutput">operator==</tt> now, or it will never be able to call it
- later. Note, for instance, what happens if you try to put a
- <tt class="computeroutput">float</tt> value into a
- <a href="class.boost.function.html" title="Class template function">boost::function</a> object: you will get an
- error at the assignment operator or constructor, not in
- <tt class="computeroutput">operator()</tt>, because the function-call expression
- must be bound in the constructor or assignment operator.</p><p>The most promising approach is to find a method of
- determining if <tt class="computeroutput">operator==</tt> can be called for a
- particular type, and then supporting it only when it is
- available; in other situations, an exception would be
- thrown. However, to date there is no known way to detect if an
- arbitrary operator expression <tt class="computeroutput">f == g</tt> is suitably
- defined. The best solution known has the following undesirable
- qualities:</p><div class="orderedlist"><ol type="1"><li>Fails at compile-time for objects where
- <tt class="computeroutput">operator==</tt> is not accessible (e.g., because it is
- <tt class="computeroutput">private</tt>).</li><li>Fails at compile-time if calling
- <tt class="computeroutput">operator==</tt> is ambiguous.</li><li>Appears to be correct if the
- <tt class="computeroutput">operator==</tt> declaration is correct, even though
- <tt class="computeroutput">operator==</tt> may not compile.</li></ol></div><p>All of these problems translate into failures in the
- <a href="class.boost.function.html" title="Class template function">boost::function</a> constructors or
- assignment operator, <span class="emphasis"><em>even if the user never invokes
- operator==</em></span>. We can't do that to users.</p><p>The other option is to place the burden on users that want
- to use <tt class="computeroutput">operator==</tt>, e.g., by providing an
- <tt class="computeroutput">is_equality_comparable</tt> trait they may
- specialize. This is a workable solution, but is dangerous in
- practice, because forgetting to specialize the trait will result
- in unexpected exceptions being thrown from
- <a href="class.boost.function.html" title="Class template function">boost::function</a>'s
- <tt class="computeroutput">operator==</tt>. This essentially negates the usefulness
- of <tt class="computeroutput">operator==</tt> in the context in which it is most
- desired: multitarget callbacks. The
- <a href="signals.html" title="Boost.Signals">Signals</a> library has a way around
- this.</p></td></tr><tr class="question"><td align="left" valign="top"><a name="id2517410"></a><a name="id2517411"></a><b>2. </b></td><td align="left" valign="top"><p>I see void pointers; is this [mess] type safe?</p></td></tr><tr class="answer"><td align="left" valign="top"><b></b></td><td align="left" valign="top"><p>Yes, <tt class="computeroutput">boost::function</tt> is type
- safe even though it uses void pointers and pointers to functions
- returning void and taking no arguments. Essentially, all type
- information is encoded in the functions that manage and invoke
- function pointers and function objects. Only these functions are
- instantiated with the exact type that is pointed to by the void
- pointer or pointer to void function. The reason that both are required
- is that one may cast between void pointers and object pointers safely
- or between different types of function pointers (provided you don't
- invoke a function pointer with the wrong type). </p></td></tr><tr class="question"><td align="left" valign="top"><a name="id2517434"></a><a name="id2517435"></a><b>3. </b></td><td align="left" valign="top"><p>Why are there workarounds for void returns? C++ allows them!</p></td></tr><tr class="answer"><td align="left" valign="top"><b></b></td><td align="left" valign="top"><p>Void returns are permitted by the C++ standard, as in this code snippet:
- </p><pre class="programlisting">void f();
- void g() { return f(); }</pre><p> This is a valid usage of <tt class="computeroutput">boost::function</tt> because void returns are not used. With void returns, we would attempting to compile ill-formed code similar to:
- </p><pre class="programlisting">int f();
- void g() { return f(); }</pre><p> In essence, not using void returns allows
- <tt class="computeroutput">boost::function</tt> to swallow a return value. This is
- consistent with allowing the user to assign and invoke functions and
- function objects with parameters that don't exactly match.</p></td></tr><tr class="question"><td align="left" valign="top"><a name="id2517475"></a><a name="id2517476"></a><b>4. </b></td><td align="left" valign="top"><p>Why (function) cloning?</p></td></tr><tr class="answer"><td align="left" valign="top"><b></b></td><td align="left" valign="top"><p>In November and December of 2000, the issue of cloning
- vs. reference counting was debated at length and it was decided
- that cloning gave more predictable semantics. I won't rehash the
- discussion here, but if it cloning is incorrect for a particular
- application a reference-counting allocator could be used.</p></td></tr></tbody></table></div></div><table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr><td align="left"><small>Last revised: October 12, 2003 at 16:02:26 GMT</small></td><td align="right"><small>Copyright © 2001-2003 Douglas Gregor</small></td></tr></table><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="class.boost.function.html"><img src="images/prev.png" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="function.html"><img src="images/up.png" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="function.misc.html"><img src="images/next.png" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">Class template function </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a></td><td width="40%" align="right" valign="top"> Miscellaneous Notes</td></tr></table></div></body></html>
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