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‎09_iterators.html‎

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and for all.</p>
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<p>Let me tell you a little about them.
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Anybody who programs in C++ is forced to use vector
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Anybody who programs in C++ is forced to use <code>std::vector</code>
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and those have iterators.
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But, there are lots of people who do not quite understand what they are,
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partially because iterators are called iterators.
@@ -157,12 +157,8 @@ <h2>History of iterators</h2>
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But, many people at <a href="https://en.wikipedia.org/wiki/Carnegie_Mellon_University">CMU</a> got tenure because of it.
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It has some interesting ideas, including the idea of a generator.
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For those of you who know <a href="https://en.wikipedia.org/wiki/Python_(programming_language)">Python</a>, it is like an iterator in Python<sup id="fnref:1"><a href="#fn:1" rel="footnote">1</a></sup>.
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Barbara Liskov said, wouldn&rsquo;t it be nice to write:</p>
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<pre><code>for x in thing
163-
</code></pre>
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<p>and iterators allow you to do that<sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>.
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Barbara Liskov said, &ldquo;wouldn&rsquo;t it be nice to write something like: <code>for x in thing</code>&rdquo;.
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Iterators allow you to do that<sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>.
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It is like a generator in Alphard.
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It is a procedure which returns multiple values, one by one.
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It&rsquo;s a procedural abstraction.
@@ -173,12 +169,12 @@ <h2>History of iterators</h2>
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<p>At the same time, I was working on how to do algorithms and I
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introduced the notion of position.
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A better name is coordinate, the name which Paul and I use
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in our book &ldquo;Elements of Programming&rdquo;<supid="fnref:3"><ahref="#fn:3" rel="footnote">3</a></sup>.
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in our book &ldquo;Elements of Programming&rdquo;.
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A coordinate is some way of indicating where in the data structure you are.
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It is not a control structure, it&rsquo;s just the pointer into a data structure,
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or generalized notion of a coordinate.
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It is something which allows me to navigate through the data structure in a
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natural way.</p>
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natural way<supid="fnref:3"><ahref="#fn:3" rel="footnote">3</a></sup>.</p>
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<p>Eventually I started talking to C++ guys and showed them coordinates
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and they said, &ldquo;we call it iterator&rdquo;.
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Maybe I was stupid,
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but I wasn&rsquo;t ignorant.</p>
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<a name="List-pool-iterators"></a>
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<h2>List pool iterators</h2>
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<a name="Affiliated-types-for-iterators"></a>
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<h2>Affiliated types for iterators</h2>
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<p>We always need to distinguish between how we type
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code in C++ and what the notion is behind it.
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gives me a pointer type, I want a way to find out what type
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it points to.</p>
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<p>To do this we need this notion of <em>type functions</em> which accept one type
223+
<p>To do this we need this notion of <strong>type functions</strong> which accept one type
228224
and return a different type.
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This problem of needing to obtain affiliated types is not specific to C and C++.
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It appears in Java and Python as well.
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In spite of Python&rsquo;s <a href="https://en.wikipedia.org/wiki/Duck_typing">duck typing</a> there&rsquo;s <em>still</em> a connection
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between types, even if they are duck types.</p>
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<p>So we need this notion of <em>type functions</em>,
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<p>So we need this notion of type functions,
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but C doesn&rsquo;t let us do this,
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and neither does C++.
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Instead of type functions we are going to solve
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this problem for iterators by using typedefs.</p>
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this problem for iterators by using <code>typedef</code>s.</p>
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<p>For an iterator, there are <a href="https://en.cppreference.com/w/cpp/iterator/iterator_traits">5 types</a> that we always need to define.
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Three of them are primary, two are secondary.
@@ -257,32 +253,12 @@ <h2>List pool iterators</h2>
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258254
<p> In C++ (without concepts) we use tag types to designate the iterator categories.
259255
Every iterator uses a tag type to denote which theory it supports.
260-
The tag lets you do compile time dispatch<sup id="fnref:5"><a href="#fn:5" rel="footnote">5</a></sup>.</p>
261-
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<p> What category is the iterator for <code>list_pool</code> from last chapter?
263-
We need <code>ForwardIterator</code>, because there is no way in a singly linked list
264-
to go backwards.</p></li>
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The tag lets you do compile time dispatch<sup id="fnref:5"><a href="#fn:5" rel="footnote">5</a></sup>.</p></li>
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</ol>
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<p>Let&rsquo;s define these types in the <code>list_pool</code> iterator.</p>
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<pre><code>#include &lt;iterator&gt;
271-
272-
// class list_pool {
273-
// ...
274-
275-
struct iterator {
276-
typedef list_pool::value_type value_type;
277-
typedef list_pool::list_type difference_type;
278-
typedef std::forward_iterator_tag iterator_category;
279-
};
280-
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// };
282-
</code></pre>
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<a name="Iterator-reference-types"></a>
285-
<h3>Iterator reference types</h3>
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<a name="Historical-artifacts"></a>
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<h3>Historical artifacts</h3>
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<p>The fourth and fifth types to be defined are required only for historical reasons.
288264
Microsoft said they were going to vote against STL unless it accommodated
@@ -317,15 +293,33 @@ <h3>Iterator reference types</h3>
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<p>&emsp;4. <code>reference</code>: the type of a reference to the value. <br/>
318294
&emsp;5. <code>pointer</code>: the type of a pointer to the value.</p>
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320-
<p>Here is our implementation:</p>
321-
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<pre><code>typedef value_type&amp; reference;
323-
typedef value_type* pointer;
324-
</code></pre>
325-
326296
<p>It&rsquo;s not particularly harmful, but it obfuscates things
327297
and it provides &ldquo;language experts&rdquo; with steady employment.</p>
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299+
<a name="List-pool-iterators"></a>
300+
<h2>List pool iterators</h2>
301+
302+
<p>Let&rsquo;s define these types in the <code>list_pool</code> iterator.
303+
What category is the iterator for <code>list_pool</code> from last chapter?
304+
We need <code>ForwardIterator</code>, because there is no way in a singly linked list
305+
to go backwards.</p>
306+
307+
<pre><code>#include &lt;iterator&gt;
308+
309+
// class list_pool {
310+
// ...
311+
312+
struct iterator {
313+
typedef list_pool::value_type value_type;
314+
typedef list_pool::list_type difference_type;
315+
typedef std::forward_iterator_tag iterator_category;
316+
typedef value_type&amp; reference;
317+
typedef value_type* pointer;
318+
};
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// };
321+
</code></pre>
322+
329323
<a name="Constructors"></a>
330324
<h3>Constructors</h3>
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532526
but it establishes a total ordering.
533527
A total ordering does not have to be topologically induced by the traversal.</p>
534528

535-
<p><strong>Exercise:</strong> extend the iterator to linked iterator
529+
<p><strong>Exercise:</strong> Extend the iterator to linked iterator
536530
so we can assign the next on a node.
537531
Specifically we want to be able to modify the successor of
538532
an iterator.</p>
@@ -581,9 +575,13 @@ <h2>Code</h2>
581575
<li id="fn:2">
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See this <a href="http://web.mit.edu/ghudson/info/iterators">brief description of CLU iterators</a>.<a href="#fnref:2" rev="footnote">&#8617;</a></li>
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<li id="fn:3">
584-
See chapter 7 of &ldquo;Elements of Programming&rdquo; on coordinate structures.<a href="#fnref:3" rev="footnote">&#8617;</a></li>
578+
See chapter 7 of &ldquo;Elements of Programming&rdquo; on coordinate structures.
579+
An interesting discussion on the general idea of &ldquo;coordinatisation&rdquo;
580+
is found in chapter 1 of &ldquo;Basic Notions of Algebra&rdquo; by Shafarevich.<a href="#fnref:3" rev="footnote">&#8617;</a></li>
585581
<li id="fn:4">
586-
<a href="https://www.boost.org/">Boost</a> is a popular collection of C++ libraries generally accepted as the next tool to reach for beyond the standard library.
582+
<a href="https://www.boost.org/">Boost</a> is a popular collection of C++ libraries, covering a wide range of uses,
583+
generally accepted as the next tool to reach for beyond the standard library.
584+
Many standard library features, such as smart pointers, were initially developed in Boost.
587585
Alex speaks positively of some parts (see <a href="http://stepanovpapers.com/siekforeword.pdf">his foreword</a> for &ldquo;The Boost Graph Library&rdquo;), but others he is more critical of.<a href="#fnref:4" rev="footnote">&#8617;</a></li>
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<li id="fn:5">
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Some algorithms can be implemented more efficiently for certain

‎09_iterators.md‎

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@@ -10,7 +10,7 @@ other algorithms and learn to write iterators right once
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and for all.
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1212
Let me tell you a little about them.
13-
Anybody who programs in C++ is forced to use vector
13+
Anybody who programs in C++ is forced to use `std::vector`
1414
and those have iterators.
1515
But, there are lots of people who do not quite understand what they are,
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partially because iterators are called iterators.
@@ -27,11 +27,8 @@ It's a mythical language. It was never implemented.
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But, many people at [CMU][cmu] got tenure because of it.
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It has some interesting ideas, including the idea of a generator.
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For those of you who know [Python][python], it is like an iterator in Python[^python-iterator].
30-
Barbara Liskov said, wouldn't it be nice to write:
31-
32-
for x in thing
33-
34-
and iterators allow you to do that[^clu-iterators].
30+
Barbara Liskov said, "wouldn't it be nice to write something like: `for x in thing`".
31+
Iterators allow you to do that[^clu-iterators].
3532
It is like a generator in Alphard.
3633
It is a procedure which returns multiple values, one by one.
3734
It's a procedural abstraction.
@@ -42,12 +39,12 @@ It was a generalization of a control structure.
4239
At the same time, I was working on how to do algorithms and I
4340
introduced the notion of position.
4441
A better name is coordinate, the name which Paul and I use
45-
in our book "Elements of Programming"[^eop].
42+
in our book "Elements of Programming".
4643
A coordinate is some way of indicating where in the data structure you are.
4744
It is not a control structure, it's just the pointer into a data structure,
4845
or generalized notion of a coordinate.
4946
It is something which allows me to navigate through the data structure in a
50-
natural way.
47+
natural way[^coordinate-references].
5148

5249
Eventually I started talking to C++ guys and showed them coordinates
5350
and they said, "we call it iterator".
@@ -78,9 +75,13 @@ It wasn't out of ignorance.
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Maybe I was stupid,
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but I wasn't ignorant.
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81-
[^eop]: See chapter 7 of "Elements of Programming" on coordinate structures.
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[^coordinate-references]: See chapter 7 of "Elements of Programming" on coordinate structures.
79+
An interesting discussion on the general idea of "coordinatisation"
80+
is found in chapter 1 of "Basic Notions of Algebra" by Shafarevich.
8281

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[^boost]: [Boost][boost] is a popular collection of C++ libraries generally accepted as the next tool to reach for beyond the standard library.
82+
[^boost]: [Boost][boost] is a popular collection of C++ libraries, covering a wide range of uses,
83+
generally accepted as the next tool to reach for beyond the standard library.
84+
Many standard library features, such as smart pointers, were initially developed in Boost.
8485
Alex speaks positively of some parts (see [his foreword][alex-graph-foreword] for "The Boost Graph Library"), but others he is more critical of.
8586

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@@ -125,7 +126,8 @@ but I wasn't ignorant.
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[snyder]: https://dblp.org/pid/04/4444.html
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[alex-graph-foreword]: http://stepanovpapers.com/siekforeword.pdf
127128

128-
## List pool iterators
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130+
## Affiliated types for iterators
129131

130132
We always need to distinguish between how we type
131133
code in C++ and what the notion is behind it.
@@ -139,18 +141,18 @@ way to obtain `int` from `int*`. That is, if somebody
139141
gives me a pointer type, I want a way to find out what type
140142
it points to.
141143

142-
To do this we need this notion of *type functions* which accept one type
144+
To do this we need this notion of **type functions** which accept one type
143145
and return a different type.
144146
This problem of needing to obtain affiliated types is not specific to C and C++.
145147
It appears in Java and Python as well.
146148
In spite of Python's [duck typing][duck] there's *still* a connection
147149
between types, even if they are duck types.
148150

149-
So we need this notion of *type functions*,
151+
So we need this notion of type functions,
150152
but C doesn't let us do this,
151153
and neither does C++.
152154
Instead of type functions we are going to solve
153-
this problem for iterators by using typedefs.
155+
this problem for iterators by using `typedef`s.
154156

155157
For an iterator, there are [5 types][cpp-iterator-traits] that we always need to define.
156158
Three of them are primary, two are secondary.
@@ -175,40 +177,7 @@ Let's start with the primary:
175177
Every iterator uses a tag type to denote which theory it supports.
176178
The tag lets you do compile time dispatch[^compile-time-dispatch].
177179

178-
What category is the iterator for `list_pool` from last chapter?
179-
We need `ForwardIterator`, because there is no way in a singly linked list
180-
to go backwards.
181-
182-
Let's define these types in the `list_pool` iterator.
183-
184-
#include <iterator>
185-
186-
// class list_pool {
187-
// ...
188-
189-
struct iterator {
190-
typedef list_pool::value_type value_type;
191-
typedef list_pool::list_type difference_type;
192-
typedef std::forward_iterator_tag iterator_category;
193-
};
194-
195-
// };
196-
197-
198-
[^compile-time-dispatch]: Some algorithms can be implemented more efficiently for certain
199-
iterator categories. For example [`std::distance`][std-distance] can be
200-
implemented as a constant time algorithm for `RandomAccessIterators` but
201-
only a linear time algorithm for other iterator categories. The
202-
`iterator_category` tag allows the appropriate algorithm to be selected at
203-
compile time. This technique is known as [tag disptach][tag-dispatch].
204-
205-
[duck]: https://en.wikipedia.org/wiki/Duck_typing
206-
[ptrdiff]: https://en.cppreference.com/w/c/types/ptrdiff_t
207-
[cpp-iterator-traits]: https://en.cppreference.com/w/cpp/iterator/iterator_traits
208-
[std-distance]: https://en.cppreference.com/w/cpp/iterator/distance
209-
[tag-dispatch]: https://quuxplusone.github.io/blog/2021/06/07/tag-dispatch-and-concept-overloading
210-
211-
### Iterator reference types
180+
### Historical artifacts
212181

213182
The fourth and fifth types to be defined are required only for historical reasons.
214183
Microsoft said they were going to vote against STL unless it accommodated
@@ -243,22 +212,51 @@ So now we need to provide it.
243212
&emsp;4. `reference`: the type of a reference to the value. <br/>
244213
&emsp;5. `pointer`: the type of a pointer to the value.
245214

246-
Here is our implementation:
247-
248-
typedef value_type& reference;
249-
typedef value_type* pointer;
250-
251215
It's not particularly harmful, but it obfuscates things
252216
and it provides "language experts" with steady employment.
253217

254218
[^ms-dos-pointers]: See ["A look back at memory models in 16-bit MS-DOS"][far-ptr-article]
255219
for a brief overview of these various pointer types.
256220
The more general concept behind them is [memory segmentation][memory-segmentation].
257221

222+
[^compile-time-dispatch]: Some algorithms can be implemented more efficiently for certain
223+
iterator categories. For example [`std::distance`][std-distance] can be
224+
implemented as a constant time algorithm for `RandomAccessIterators` but
225+
only a linear time algorithm for other iterator categories. The
226+
`iterator_category` tag allows the appropriate algorithm to be selected at
227+
compile time. This technique is known as [tag disptach][tag-dispatch].
228+
229+
[duck]: https://en.wikipedia.org/wiki/Duck_typing
230+
[ptrdiff]: https://en.cppreference.com/w/c/types/ptrdiff_t
231+
[cpp-iterator-traits]: https://en.cppreference.com/w/cpp/iterator/iterator_traits
232+
[std-distance]: https://en.cppreference.com/w/cpp/iterator/distance
233+
[tag-dispatch]: https://quuxplusone.github.io/blog/2021/06/07/tag-dispatch-and-concept-overloading
258234
[lvalue]: https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue
259235
[far-ptr-article]: https://devblogs.microsoft.com/oldnewthing/20200728-00/?p=104012
260236
[memory-segmentation]: https://en.wikipedia.org/wiki/Memory_segmentation
261237

238+
## List pool iterators
239+
240+
Let's define these types in the `list_pool` iterator.
241+
What category is the iterator for `list_pool` from last chapter?
242+
We need `ForwardIterator`, because there is no way in a singly linked list
243+
to go backwards.
244+
245+
#include <iterator>
246+
247+
// class list_pool {
248+
// ...
249+
250+
struct iterator {
251+
typedef list_pool::value_type value_type;
252+
typedef list_pool::list_type difference_type;
253+
typedef std::forward_iterator_tag iterator_category;
254+
typedef value_type& reference;
255+
typedef value_type* pointer;
256+
};
257+
258+
// };
259+
262260
### Constructors
263261

264262
Let's write constructors for our iterator:
@@ -476,8 +474,7 @@ A total ordering does not have to be topologically induced by the traversal.
476474

477475
[lex]: https://en.wikipedia.org/wiki/Lexicographic_order
478476

479-
480-
**Exercise:** extend the iterator to linked iterator
477+
**Exercise:** Extend the iterator to linked iterator
481478
so we can assign the next on a node.
482479
Specifically we want to be able to modify the successor of
483480
an iterator.

‎index.html‎

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330330
<li><a href="09_iterators.html#L9.-Iterators">9. Iterators</a>
331331
<ul>
332332
<li><a href="09_iterators.html#History-of-iterators">History of iterators</a></li>
333+
<li><a href="09_iterators.html#Affiliated-types-for-iterators">Affiliated types for iterators</a>
334+
<ul>
335+
<li><a href="09_iterators.html#Historical-artifacts">Historical artifacts</a></li>
336+
</ul>
337+
</li>
333338
<li><a href="09_iterators.html#List-pool-iterators">List pool iterators</a>
334339
<ul>
335-
<li><a href="09_iterators.html#Iterator-reference-types">Iterator reference types</a></li>
336340
<li><a href="09_iterators.html#Constructors">Constructors</a></li>
337341
<li><a href="09_iterators.html#Dereference">Dereference</a></li>
338342
<li><a href="09_iterators.html#Pre-2d-increment-2c--post-2d-increment">Pre-increment, post-increment</a></li>

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