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Import of BoostBook documentation for any, array, bind, function, ref, and

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signals libraries.


[SVN r17866]
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Douglas Gregor
2003-03-12 23:27:25 +00:00
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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<section id="function.faq" last-revision="$Date$">
<title>Frequently Asked Questions</title>
<qandaset>
<qandaentry>
<question><para>I see void pointers; is this [mess] type safe?</para></question>
<answer>
<para>Yes, <computeroutput>boost::function</computeroutput> 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). </para>
</answer>
</qandaentry>
<qandaentry>
<question><para>Why are there workarounds for void returns? C++ allows them!</para></question>
<answer><para>Void returns are permitted by the C++ standard, as in this code snippet:
<programlisting>void f();
void g() { return f(); }</programlisting>
</para>
<para> This is a valid usage of <computeroutput>boost::function</computeroutput> because void returns are not used. With void returns, we would attempting to compile ill-formed code similar to:
<programlisting>int f();
void g() { return f(); }</programlisting>
</para>
<para> In essence, not using void returns allows
<computeroutput>boost::function</computeroutput> 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.</para>
</answer>
</qandaentry>
<qandaentry>
<question><para>Why (function) cloning?</para></question>
<answer>
<para>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.</para>
</answer>
</qandaentry>
</qandaset>
</section>