<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>
<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">"Hinting" during insertion</A>
</UL>
<HR>
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">"Hinting" 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 "iterator p is a hint
+ pointing to where the insert should start to search," 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)
+ "<". 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 "greater" and "lesser"
+ 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>
<!-- ####################################################### -->
<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>
projects / platform / upstream / linaro-gcc.git / commitdiff