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diff --git a/libstdc++-v3/doc/xml/manual/iterators.xml b/libstdc++-v3/doc/xml/manual/iterators.xml new file mode 100644 index 000000000..bcfa30cd5 --- /dev/null +++ b/libstdc++-v3/doc/xml/manual/iterators.xml @@ -0,0 +1,185 @@ +<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" + xml:id="std.iterators" xreflabel="Iterators"> +<?dbhtml filename="iterators.html"?> + +<info><title> + Iterators + <indexterm><primary>Iterators</primary></indexterm> +</title> + <keywordset> + <keyword> + ISO C++ + </keyword> + <keyword> + library + </keyword> + </keywordset> +</info> + + + +<!-- Sect1 01 : Predefined --> +<section xml:id="std.iterators.predefined" xreflabel="Predefined"><info><title>Predefined</title></info> + + + <section xml:id="iterators.predefined.vs_pointers" xreflabel="Versus Pointers"><info><title>Iterators vs. Pointers</title></info> + + <para> + The following +FAQ <link linkend="faq.iterator_as_pod">entry</link> points out that +iterators are not implemented as pointers. They are a generalization +of pointers, but they are implemented in libstdc++ as separate +classes. + </para> + <para> + Keeping that simple fact in mind as you design your code will + prevent a whole lot of difficult-to-understand bugs. + </para> + <para> + You can think of it the other way 'round, even. Since iterators + are a generalization, that means + that <emphasis>pointers</emphasis> are + <emphasis>iterators</emphasis>, and that pointers can be used + whenever an iterator would be. All those functions in the + Algorithms sect1 of the Standard will work just as well on plain + arrays and their pointers. + </para> + <para> + That doesn't mean that when you pass in a pointer, it gets + wrapped into some special delegating iterator-to-pointer class + with a layer of overhead. (If you think that's the case + anywhere, you don't understand templates to begin with...) Oh, + no; if you pass in a pointer, then the compiler will instantiate + that template using T* as a type, and good old high-speed + pointer arithmetic as its operations, so the resulting code will + be doing exactly the same things as it would be doing if you had + hand-coded it yourself (for the 273rd time). + </para> + <para> + How much overhead <emphasis>is</emphasis> there when using an + iterator class? Very little. Most of the layering classes + contain nothing but typedefs, and typedefs are + "meta-information" that simply tell the compiler some + nicknames; they don't create code. That information gets passed + down through inheritance, so while the compiler has to do work + looking up all the names, your runtime code does not. (This has + been a prime concern from the beginning.) + </para> + + + </section> + + <section xml:id="iterators.predefined.end" xreflabel="end() Is One Past the End"><info><title>One Past the End</title></info> + + + <para>This starts off sounding complicated, but is actually very easy, + especially towards the end. Trust me. + </para> + <para>Beginners usually have a little trouble understand the whole + 'past-the-end' thing, until they remember their early algebra classes + (see, they <emphasis>told</emphasis> you that stuff would come in handy!) and + the concept of half-open ranges. + </para> + <para>First, some history, and a reminder of some of the funkier rules in + C and C++ for builtin arrays. The following rules have always been + true for both languages: + </para> + <orderedlist inheritnum="ignore" continuation="restarts"> + <listitem> + <para>You can point anywhere in the array, <emphasis>or to the first element + past the end of the array</emphasis>. A pointer that points to one + past the end of the array is guaranteed to be as unique as a + pointer to somewhere inside the array, so that you can compare + such pointers safely. + </para> + </listitem> + <listitem> + <para>You can only dereference a pointer that points into an array. + If your array pointer points outside the array -- even to just + one past the end -- and you dereference it, Bad Things happen. + </para> + </listitem> + <listitem> + <para>Strictly speaking, simply pointing anywhere else invokes + undefined behavior. Most programs won't puke until such a + pointer is actually dereferenced, but the standards leave that + up to the platform. + </para> + </listitem> + </orderedlist> + <para>The reason this past-the-end addressing was allowed is to make it + easy to write a loop to go over an entire array, e.g., + while (*d++ = *s++);. + </para> + <para>So, when you think of two pointers delimiting an array, don't think + of them as indexing 0 through n-1. Think of them as <emphasis>boundary + markers</emphasis>: + </para> + <programlisting> + + beginning end + | | + | | This is bad. Always having to + | | remember to add or subtract one. + | | Off-by-one bugs very common here. + V V + array of N elements + |---|---|--...--|---|---| + | 0 | 1 | ... |N-2|N-1| + |---|---|--...--|---|---| + + ^ ^ + | | + | | This is good. This is safe. This + | | is guaranteed to work. Just don't + | | dereference 'end'. + beginning end + + </programlisting> + <para>See? Everything between the boundary markers is chapter of the array. + Simple. + </para> + <para>Now think back to your junior-high school algebra course, when you + were learning how to draw graphs. Remember that a graph terminating + with a solid dot meant, "Everything up through this point," + and a graph terminating with an open dot meant, "Everything up + to, but not including, this point," respectively called closed + and open ranges? Remember how closed ranges were written with + brackets, <emphasis>[a,b]</emphasis>, and open ranges were written with parentheses, + <emphasis>(a,b)</emphasis>? + </para> + <para>The boundary markers for arrays describe a <emphasis>half-open range</emphasis>, + starting with (and including) the first element, and ending with (but + not including) the last element: <emphasis>[beginning,end)</emphasis>. See, I + told you it would be simple in the end. + </para> + <para>Iterators, and everything working with iterators, follows this same + time-honored tradition. A container's <code>begin()</code> method returns + an iterator referring to the first element, and its <code>end()</code> + method returns a past-the-end iterator, which is guaranteed to be + unique and comparable against any other iterator pointing into the + middle of the container. + </para> + <para>Container constructors, container methods, and algorithms, all take + pairs of iterators describing a range of values on which to operate. + All of these ranges are half-open ranges, so you pass the beginning + iterator as the starting parameter, and the one-past-the-end iterator + as the finishing parameter. + </para> + <para>This generalizes very well. You can operate on sub-ranges quite + easily this way; functions accepting a <emphasis>[first,last)</emphasis> range + don't know or care whether they are the boundaries of an entire {array, + sequence, container, whatever}, or whether they only enclose a few + elements from the center. This approach also makes zero-length + sequences very simple to recognize: if the two endpoints compare + equal, then the {array, sequence, container, whatever} is empty. + </para> + <para>Just don't dereference <code>end()</code>. + </para> + + </section> +</section> + +<!-- Sect1 02 : Stream --> + +</chapter> |