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howto.html: Describe implementation of insertion with hints. 

2001-08-24  Phil Edwards  <pme@sources.redhat.com>

	* docs/html/23_containers/howto.html:  Describe implementation of
	insertion with hints.

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Phil Edwards 2001-08-24 20:34:34 +00:00
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2001-08-24 Phil Edwards <pme@sources.redhat.com>
* docs/html/23_containers/howto.html: Describe implementation of
insertion with hints.
2001-08-24 Kenny Simpson <kenny.simpson@gs.com>
libstdc++/3740

82
libstdc++-v3/docs/html/23_containers/howto.html
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<META NAME="GENERATOR" CONTENT="vi and eight fingers">
<TITLE>libstdc++-v3 HOWTO: Chapter 23</TITLE>
<LINK REL=StyleSheet HREF="../lib3styles.css">
<!-- $Id: howto.html,v 1.5 2001/05/31 02:45:02 ljrittle Exp $ -->
<!-- $Id: howto.html,v 1.6 2001/06/08 03:53:35 ljrittle Exp $ -->
</HEAD>
<BODY>
@ -25,6 +25,7 @@
<LI><A HREF="#1">Making code unaware of the container/array difference</A>
<LI><A HREF="#2">Variable-sized bitmasks</A>
<LI><A HREF="#3">Containers and multithreading</A>
<LI><A HREF="#4">&quot;Hinting&quot; during insertion</A>
</UL>
<HR>
@ -245,12 +246,85 @@
a catch-all general template solution would probably be more trouble
than it's worth.
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<HR>
<H2><A NAME="4">&quot;Hinting&quot; during insertion</A></H2>
<P>Section [23.1.2], Table 69, of the C++ standard lists this function
for all of the associative containers (map, set, etc):
<PRE>
a.insert(p,t);</PRE>
where 'p' is an iterator into the container 'a', and 't' is the item
to insert. The standard says that &quot;iterator p is a hint
pointing to where the insert should start to search,&quot; but
specifies nothing more. (LWG Issue #233, currently in review,
addresses this topic, but I will ignore it here because it is not yet
finalized.)
</P>
<P>Here we'll describe how the hinting works in the libstdc++-v3
implementation, and what you need to do in order to take advantage of
it. (Insertions can change from logarithmic complexity to amortized
constant time, if the hint is properly used.) Also, since the current
implementation is based on the SGI STL one, these points may hold true
for other library implementations also, since the HP/SGI code is used
in a lot of places.
</P>
<P>In the following text, the phrases <EM>greater than</EM> and <EM>less
than</EM> refer to the results of the strict weak ordering imposed on
the container by its comparison object, which defaults to (basically)
&quot;&lt;&quot;. Using those phrases is semantically sloppy, but I
didn't want to get bogged down in syntax. I assume that if you are
intelligent enough to use your own comparison objects, you are also
intelligent enough to assign &quot;greater&quot; and &quot;lesser&quot;
their new meanings in the next paragraph. *grin*
</P>
<P>If the <TT>hint</TT> parameter ('p' above) is equivalent to:
<UL>
<LI><TT>begin()</TT>, then the item being inserted should have a key
less than all the other keys in the container. The item will
be inserted at the beginning of the container, becoming the new
entry at <TT>begin()</TT>.
<LI><TT>end()</TT>, then the item being inserted should have a key
greater than all the other keys in the container. The item will
be inserted at the end of the container, becoming the new entry
at <TT>end()</TT>.
<LI>neither <TT>begin()</TT> nor <TT>end()</TT>, then: Let <TT>h</TT>
be the entry in the container pointed to by <TT>hint</TT>, that
is, <TT>h = *hint</TT>. Then the item being inserted should have
a key less than that of <TT>h</TT>, and greater than that of the
item preceeding <TT>h</TT>. The new item will be inserted
between <TT>h</TT> and <TT>h</TT>'s predecessor.
</UL>
</P>
<P>If the conditions are not met, then the hint is not used, and the
insertion proceeds as if you had called <TT> a.insert(t) </TT>
instead. (<STRONG>Note </STRONG> that GCC releases prior to 3.0.2
had a bug in the case with <TT>hint == begin()</TT>. You should not
use a hint argument in those releases.)
(Was it just with map or with all the rbtree-using containers? Still need
to check that.)
</P>
<P>This behavior goes well with other container's <TT>insert()</TT>
functions which take an iterator: if used, the new item will be
inserted before the iterator passed as an argument, same as the other
containers. The exception
(in a sense) is with a hint of <TT>end()</TT>: the new item will
actually be inserted after <TT>end()</TT>, but it also becomes the
new <TT>end()</TT>.
</P>
<P><STRONG>Note </STRONG> also that the hint in this implementation is a
one-shot. The insertion-with-hint routines check the immediately
surrounding entries to ensure that the new item would in fact belong
there. If the hint does not point to the correct place, then no
further local searching is done; the search begins from scratch in
logarithmic time. (Further local searching would only increase the
time required when the hint is too far off.)
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<!-- ####################################################### -->
@ -259,7 +333,7 @@
<P CLASS="fineprint"><EM>
Comments and suggestions are welcome, and may be sent to
<A HREF="mailto:libstdc++@gcc.gnu.org">the mailing list</A>.
<BR> $Id: howto.html,v 1.5 2001/05/31 02:45:02 ljrittle Exp $
<BR> $Id: howto.html,v 1.6 2001/06/08 03:53:35 ljrittle Exp $
</EM></P>