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diff --git a/doc/html/function/faq.html b/doc/html/function/faq.html new file mode 100644 index 0000000000..7d981606f0 --- /dev/null +++ b/doc/html/function/faq.html @@ -0,0 +1,216 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> +<title>Frequently Asked Questions</title> +<link rel="stylesheet" href="../boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.68.1"> +<link rel="start" href="../index.html" title="The Boost C++ Libraries"> +<link rel="up" href="../function.html" title="Chapter 4. Boost.Function"> +<link rel="prev" href="../function_equal.html" title="Function template function_equal"> +<link rel="next" href="misc.html" title="Miscellaneous Notes"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"> +<td valign="top"><img alt="boost.png (6897 bytes)" width="277" height="86" src="../../../boost.png"></td> +<td align="center"><a href="../../../index.htm">Home</a></td> +<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> +<td align="center"><a href="../../../people/people.htm">People</a></td> +<td align="center"><a href="../../../more/faq.htm">FAQ</a></td> +<td align="center"><a href="../../../more/index.htm">More</a></td> +</table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="../function_equal.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../function.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="misc.html"><img src="../images/next.png" alt="Next"></a> +</div> +<div class="section" lang="en"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="function.faq"></a>Frequently Asked Questions</h3></div></div></div> +<div class="qandaset"> +<dl> +<dt>1. <a href="faq.html#id1214641">Why can't I compare + boost::function objects with + operator== or + operator!=?</a> +</dt> +<dt>2. <a href="faq.html#id1214973">I see void pointers; is this [mess] type safe?</a> +</dt> +<dt>3. <a href="faq.html#id1214987">Why are there workarounds for void returns? C++ allows them!</a> +</dt> +<dt>4. <a href="faq.html#id1215029">Why (function) cloning?</a> +</dt> +<dt>5. <a href="faq.html#id1215043">How much overhead does a call through boost::function incur?</a> +</dt> +</dl> +<table border="0" summary="Q and A Set"> +<col align="left" width="1%"> +<tbody> +<tr class="question"> +<td align="left" valign="top"> +<a name="id1214641"></a><a name="id1214642"></a><b>1.</b> +</td> +<td align="left" valign="top"><p>Why can't I compare + <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> objects with + <code class="computeroutput">operator==</code> or + <code class="computeroutput">operator!=</code>?</p></td> +</tr> +<tr class="answer"> +<td align="left" valign="top"><b></b></td> +<td align="left" valign="top"> +<p>Comparison between <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> + objects cannot be implemented "well", and therefore will not be + implemented. The typical semantics requested for <code class="computeroutput">f == + g</code> given <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> objects + <code class="computeroutput">f</code> and <code class="computeroutput">g</code> are:</p> +<div class="itemizedlist"><ul type="disc"> +<li>If <code class="computeroutput">f</code> and <code class="computeroutput">g</code> + store function objects of the same type, use that type's + <code class="computeroutput">operator==</code> to compare + them.</li> +<li>If <code class="computeroutput">f</code> and <code class="computeroutput">g</code> + store function objects of different types, return + <code class="computeroutput">false</code>.</li> +</ul></div> +<p>The problem occurs when the type of the function objects + stored by both <code class="computeroutput">f</code> and <code class="computeroutput">g</code> doesn't have an + <code class="computeroutput">operator==</code>: we would like the expression <code class="computeroutput">f == + g</code> to fail to compile, as occurs with, e.g., the standard + containers. However, this is not implementable for + <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> because it necessarily + "erases" some type information after it has been assigned a + function object, so it cannot try to call + <code class="computeroutput">operator==</code> later: it must either find a way to call + <code class="computeroutput">operator==</code> now, or it will never be able to call it + later. Note, for instance, what happens if you try to put a + <code class="computeroutput">float</code> value into a + <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> object: you will get an + error at the assignment operator or constructor, not in + <code class="computeroutput">operator()</code>, 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 <code class="computeroutput">operator==</code> 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 <code class="computeroutput">f == g</code> 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 + <code class="computeroutput">operator==</code> is not accessible (e.g., because it is + <code class="computeroutput">private</code>).</li> +<li>Fails at compile-time if calling + <code class="computeroutput">operator==</code> is ambiguous.</li> +<li>Appears to be correct if the + <code class="computeroutput">operator==</code> declaration is correct, even though + <code class="computeroutput">operator==</code> may not compile.</li> +</ol></div> +<p>All of these problems translate into failures in the + <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> 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 <code class="computeroutput">operator==</code>, e.g., by providing an + <code class="computeroutput">is_equality_comparable</code> 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 + <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code>'s + <code class="computeroutput">operator==</code>. This essentially negates the usefulness + of <code class="computeroutput">operator==</code> in the context in which it is most + desired: multitarget callbacks. The + <a href="../signals.html" title="Chapter 9. Boost.Signals">Signals</a> library has a way around + this.</p> +</td> +</tr> +<tr class="question"> +<td align="left" valign="top"> +<a name="id1214973"></a><a name="id1214974"></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, <code class="computeroutput">boost::function</code> 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="id1214987"></a><a name="id1214988"></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 <code class="computeroutput">boost::function</code> 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 +<code class="computeroutput">boost::function</code> 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="id1215029"></a><a name="id1215030"></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> +<tr class="question"> +<td align="left" valign="top"> +<a name="id1215043"></a><a name="id1215044"></a><b>5.</b> +</td> +<td align="left" valign="top"><p>How much overhead does a call through <code class="computeroutput"><a href="../boost/function.html" title="Class template function">boost::function</a></code> incur?</p></td> +</tr> +<tr class="answer"> +<td align="left" valign="top"><b></b></td> +<td align="left" valign="top"> +<p>The cost of <code class="computeroutput">boost::function</code> can be reasonably + consistently measured at around 20ns +/- 10 ns on a modern >2GHz + platform versus directly inlining the code.</p> +<p>However, the performance of your application may benefit + from or be disadvantaged by <code class="computeroutput">boost::function</code> + depending on how your C++ optimiser optimises. Similar to a + standard function pointer, differences of order of 10% have been + noted to the benefit or disadvantage of using + <code class="computeroutput">boost::function</code> to call a function that contains a + tight loop depending on your compilation circumstances.</p> +<p>[Answer provided by Matt Hurd. See <a href="http://article.gmane.org/gmane.comp.lib.boost.devel/33278" target="_top">http://article.gmane.org/gmane.comp.lib.boost.devel/33278</a>]</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><p>Last revised: February 18, 2004 at 06:37:13 GMT</p></small></td> +<td align="right"><small>Copyright © 2001-2004 Douglas Gregor</small></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="../function_equal.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../function.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="misc.html"><img src="../images/next.png" alt="Next"></a> +</div> +</body> +</html> |