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Separate access and traversal for iterator_adaptor

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[SVN r20162]
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Dave Abrahams
2003-09-22 19:55:01 +00:00
parent 50fe1b062b
commit ebb09db44e
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doc/facade-and-adaptor.html
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@ -219,7 +219,7 @@ Lab</a>, University of Hanover <a class="last reference" href="http://www.ive.un
Railway Operation and Construction</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2003-09-22</td></tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body"><strong>This document is a revised version of the official</strong> N1476=03-0059</td>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">N1530=03-0113</td>
</tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved</td></tr>
@ -642,17 +642,18 @@ above. The <tt class="literal"><span class="pre">Base</span></tt> type need not
iterator. It need only support the operations used by the core
interface functions of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> that have not been
redefined in the user's derived class.</p>
<p>Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default to
<tt class="literal"><span class="pre">use_default</span></tt>. This allows the user to make use of a default
parameter even when the user wants to specify a parameter later in the
parameter list. Also, the defaults for the corresponding associated
types are fairly complicated, so metaprogramming is required to
compute them, and <tt class="literal"><span class="pre">use_default</span></tt> can help to simplify the
implementation. Finally, <tt class="literal"><span class="pre">use_default</span></tt> is not left unspecified
<p>Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default
to <tt class="literal"><span class="pre">use_default</span></tt> (or <tt class="literal"><span class="pre">use_default_access</span></tt>). This allows the
user to make use of a default parameter even when she wants to
specify a parameter later in the parameter list. Also, the
defaults for the corresponding associated types are somewhat
complicated, so metaprogramming is required to compute them, and
<tt class="literal"><span class="pre">use_default</span></tt> can help to simplify the implementation. Finally,
the identity of the <tt class="literal"><span class="pre">use_default</span></tt> type is not left unspecified
because specification helps to highlight that the <tt class="literal"><span class="pre">Reference</span></tt>
template parameter may not always be identical to the iterator's
<tt class="literal"><span class="pre">reference</span></tt> type, and will keep users making mistakes based on that
assumption.</p>
<tt class="literal"><span class="pre">reference</span></tt> type, and will keep users making mistakes based on
that assumption.</p>
</div>
<div class="section" id="specialized-adaptors">
<h2><a class="toc-backref" href="#id28" name="specialized-adaptors">Specialized Adaptors</a></h2>
@ -697,13 +698,14 @@ Standard compliant iterators).</p>
<h2><a class="toc-backref" href="#id30" name="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">Header <tt class="literal"><span class="pre">&lt;iterator_helper&gt;</span></tt> synopsis [lib.iterator.helper.synopsis]</a></h2>
<pre class="literal-block">
struct use_default;
const unsigned use_default_access = -1;
struct iterator_core_access { /* implementation detail */ };
template &lt;
class Derived
, class Value
, class AccessCategory
, unsigned AccessCategory
, class TraversalCategory
, class Reference = Value&amp;
, class Difference = ptrdiff_t
@ -714,7 +716,8 @@ template &lt;
class Derived
, class Base
, class Value = use_default
, class Category = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
&gt;
@ -723,7 +726,8 @@ class iterator_adaptor;
template &lt;
class Iterator
, class Value = use_default
, class Category = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
&gt;
@ -745,7 +749,8 @@ class filter_iterator;
template &lt;
class Incrementable
, class Category = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Difference = use_default
&gt;
class counting_iterator
@ -765,7 +770,7 @@ and associated types, to be supplied by a derived iterator class.</p>
template &lt;
class Derived
, class Value
, class AccessCategory
, unsigned AccessCategory
, class TraversalCategory
, class Reference = /* see <a class="reference" href="#iterator-facade-requirements">below</a> */
, class Difference = ptrdiff_t
@ -855,11 +860,21 @@ out of the overload set when the types are not interoperable.]</p>
title prevents an automatic link from being generated -->
<a class="target" id="iterator-facade-requirements" name="iterator-facade-requirements"></a><div class="section" id="id13">
<h4><a class="toc-backref" href="#id33" name="id13"><tt class="literal"><span class="pre">iterator_facade</span></tt> requirements</a></h4>
<p>Some of the constraints on template parameters to
<tt class="literal"><span class="pre">iterator_facade</span></tt> are expressed in terms of resulting nested
types and should be viewed in the context of their impact on
<tt class="literal"><span class="pre">iterator_traits&lt;Derived&gt;</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a class derived from
<tt class="literal"><span class="pre">iterator_facade</span></tt>.</p>
<p>The default for the <tt class="literal"><span class="pre">Reference</span></tt> parameter is <tt class="literal"><span class="pre">Value&amp;</span></tt> if the
access category for <tt class="literal"><span class="pre">iterator_facade</span></tt> is implicitly convertible to
<tt class="literal"><span class="pre">writable_iterator_tag</span></tt>, and <tt class="literal"><span class="pre">const</span> <span class="pre">Value&amp;</span></tt> otherwise.</p>
<p>The nested <tt class="literal"><span class="pre">::value_type</span></tt> type will be the same as
<tt class="literal"><span class="pre">remove_cv&lt;Value&gt;::type</span></tt>, so the <tt class="literal"><span class="pre">Value</span></tt> parameter must be
an (optionally <tt class="literal"><span class="pre">const</span></tt>-qualified) non-reference type.</p>
<p><tt class="literal"><span class="pre">AccessCategory</span></tt> must be an unsigned value which uses no more
bits than the greatest value of <tt class="literal"><span class="pre">iterator_access</span></tt>.</p>
<p>The nested <tt class="literal"><span class="pre">::reference</span></tt> will be the same as the <tt class="literal"><span class="pre">Reference</span></tt>
parameter; it must be a suitable reference type for the resulting
iterator. The default for the <tt class="literal"><span class="pre">Reference</span></tt> parameter is
<tt class="literal"><span class="pre">Value&amp;</span></tt>.</p>
<p>The following table describes the other requirements on the
<tt class="literal"><span class="pre">Derived</span></tt> parameter. Depending on the resulting iterator's
<tt class="literal"><span class="pre">iterator_category</span></tt>, a subset of the expressions listed in the table
@ -973,8 +988,8 @@ of type <tt class="literal"><span class="pre">X::pointer</span></tt> equal to:</
<p>Otherwise returns an object of unspecified type such that, given an
object <tt class="literal"><span class="pre">a</span></tt> of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">a-&gt;m</span></tt> is equivalent to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">*a,</span>
<span class="pre">w.m)</span></tt> for some temporary object <tt class="literal"><span class="pre">w</span></tt> of type <tt class="literal"><span class="pre">X::value_type</span></tt>.</p>
<p class="last">The type <tt class="literal"><span class="pre">X::pointer</span></tt> is <tt class="literal"><span class="pre">Value*</span></tt> if the access category for
<tt class="literal"><span class="pre">X</span></tt> is implicitly convertible to <tt class="literal"><span class="pre">writable_iterator_tag</span></tt>, and
<p class="last">The type <tt class="literal"><span class="pre">X::pointer</span></tt> is <tt class="literal"><span class="pre">Value*</span></tt> if
<tt class="literal"><span class="pre">is_writable_iterator&lt;X&gt;::value</span></tt> is <tt class="literal"><span class="pre">true</span></tt>, and
<tt class="literal"><span class="pre">Value</span> <span class="pre">const*</span></tt> otherwise.</p>
</td>
</tr>
@ -1107,11 +1122,13 @@ core interface functions of <tt class="literal"><span class="pre">iterator_facad
<div class="section" id="class-template-iterator-adaptor">
<h3><a class="toc-backref" href="#id36" name="class-template-iterator-adaptor">Class template <tt class="literal"><span class="pre">iterator_adaptor</span></tt></a></h3>
<pre class="literal-block">
bool
template &lt;
class Derived
, class Base
, class Value = use_default
, class Category = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
&gt;
@ -1163,54 +1180,47 @@ parameters specify the types for the member typedefs in
<tt class="literal"><span class="pre">iterator_facade</span></tt>. The following pseudo-code specifies the
traits types for <tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</p>
<pre class="literal-block">
if (Value == use_default)
value_type = iterator_traits&lt;Base&gt;::value_type;
else
value_type = remove_cv&lt;Value&gt;::type;
if (Reference == use_default) {
if (Value == use_default)
reference = iterator_traits&lt;Base&gt;::reference;
else
reference = Value&amp;;
} else
reference = Reference;
if (Distance == use_default)
difference_type = iterator_traits&lt;Base&gt;::difference_type;
if (Value != use_default)
value_type = remove_cv&lt;Value&gt;::type;
else
difference_type = Distance;
value_type = iterator_traits&lt;Base&gt;::value_type;
if (Category == use_default)
iterator_category = iterator_tag&lt;
access_category&lt; Base &gt;,
traversal_category&lt; Base &gt;
&gt;
else if (Category is convertible to a standard access tag)
iterator_category = iterator_tag&lt;
Category
else if (Category has a nested traversal type)
if (reference is not a reference-to-const)
Category::access
else
if (Category
else if (Category is convertable to a standard traversal tag)
...
if (Traversal != use_default)
traversal_category = Traversal
else
iterator_category = Category;
// Actually the above is wrong. See the use of
// access_category_tag and
// new_category_to_access/iter_category_to_access.
traversal_category = traversal_category&lt; Base &gt;::type
iterator_category = iterator_tag&lt;
access_category
, traversal_category
&gt;
if (Access != use_default)
{
access_category = Access
}
else
{
access_category
= access_category&lt; Base &gt;::value
if (is_const&lt;Value&gt;)
access_category &amp;= ~writable_iterator;
}
iterator_category = iterator_tag&lt;
access_category
, traversal_category
&gt;
if (Reference != use_default)
reference = Reference
else if (Value != use_default)
reference = Value&amp;
else
reference = iterator_traits&lt;Base&gt;::reference
</pre>
<!-- Replaced with new semantics - -thw
if (Category == use_default)
iterator_category = iterator_traits<Base>::iterator_category;
else
iterator_category = Category;
Fix this up!! -->
</div>
<div class="section" id="iterator-adaptor-public-operations">
<h3><a class="toc-backref" href="#id38" name="iterator-adaptor-public-operations"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h3>
@ -1362,7 +1372,8 @@ iterators over smart pointers, which the impl handles. -JGS -->
template &lt;
class Iterator
, class Value = use_default
, class Category = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
&gt;
@ -1374,12 +1385,12 @@ class indirect_iterator
indirect_iterator();
indirect_iterator(Iterator x);
template &lt;
class Iterator2, class Value2, class Category2
class Iterator2, class Value2, unsigned Access2, class Traversal2
, class Reference2, class Difference2
&gt;
indirect_iterator(
indirect_iterator&lt;
Iterator2, Value2, Category2, Reference2, Difference2
Iterator2, Value2, Access2, Traversal2, Reference2, Difference2
&gt; const&amp; y
, typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
);
@ -1408,9 +1419,10 @@ specialization of <tt class="literal"><span class="pre">iterator_traits</span></
iterator.</p>
<p>The <tt class="literal"><span class="pre">Reference</span></tt> parameter will be the <tt class="literal"><span class="pre">reference</span></tt> type of the
<tt class="literal"><span class="pre">indirect_iterator</span></tt>. The default is <tt class="literal"><span class="pre">Value&amp;</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">Category</span></tt> parameter is the <tt class="literal"><span class="pre">iterator_category</span></tt> type for the
<tt class="literal"><span class="pre">indirect_iterator</span></tt>. The default is
<tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::iterator_category</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">Access</span></tt> and <tt class="literal"><span class="pre">Traversal</span></tt> parameters are passed unchanged to
the corresponding parameters of the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> base
class, and the <tt class="literal"><span class="pre">Iterator</span></tt> parameter is passed unchanged as the
<tt class="literal"><span class="pre">Base</span></tt> parameter to the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class.</p>
<p>The indirect iterator will model the most refined standard traversal
concept that is modeled by the <tt class="literal"><span class="pre">Iterator</span></tt> type. The indirect
iterator will model the most refined standard access concept that is
@ -1442,12 +1454,12 @@ the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> subobject
</table>
<pre class="literal-block">
template &lt;
class Iterator2, class Value2, class Category2
class Iterator2, class Value2, unsigned Access, class Traversal
, class Reference2, class Difference2
&gt;
indirect_iterator(
indirect_iterator&lt;
Iterator2, Value2, Category2, Reference2, Difference2
Iterator2, Value2, Access, Traversal, Reference2, Difference2
&gt; const&amp; y
, typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
);
@ -1733,12 +1745,13 @@ class filter_iterator
</div>
<div class="section" id="filter-iterator-requirements">
<h3><a class="toc-backref" href="#id56" name="filter-iterator-requirements"><tt class="literal"><span class="pre">filter_iterator</span></tt> requirements</a></h3>
<p>The base <tt class="literal"><span class="pre">Iterator</span></tt> parameter must be a model of Readable Iterator
and Single Pass Iterator. The resulting <tt class="literal"><span class="pre">filter_iterator</span></tt> will be a
model of Forward Traversal Iterator if <tt class="literal"><span class="pre">Iterator</span></tt> is, otherwise the
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be a model of Single Pass Iterator. The
access category of the <tt class="literal"><span class="pre">filter_iterator</span></tt> will be the most refined
standard access category that is modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<p>The base <tt class="literal"><span class="pre">Iterator</span></tt> parameter must be a model of Readable
Iterator and Single Pass Iterator. The resulting
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be a model of Forward Traversal Iterator
if <tt class="literal"><span class="pre">Iterator</span></tt> is, otherwise the <tt class="literal"><span class="pre">filter_iterator</span></tt> will be a
model of Single Pass Iterator. The access category of the
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be the same as the access category of
<tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<!-- Thomas is going to try implementing filter_iterator so that
it will be bidirectional if the underlying iterator is. -JGS -->
<p>The <tt class="literal"><span class="pre">Predicate</span></tt> must be Assignable, Copy Constructible, and the
@ -1830,13 +1843,19 @@ the base <tt class="literal"><span class="pre">m_iterator</span></tt>, as per th
<div class="section" id="class-template-counting-iterator">
<h4><a class="toc-backref" href="#id59" name="class-template-counting-iterator">Class template <tt class="literal"><span class="pre">counting_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;class Incrementable, class Category = use_default, class Difference = use_default&gt;
template &lt;
class Incrementable
, unsigned Access = use_default_access
, class Traversal = use_default
, class Difference = use_default
&gt;
class counting_iterator
: public iterator_adaptor&lt;
counting_iterator&lt;Incrementable, Category, Difference&gt;
counting_iterator&lt;Incrementable, Access, Traversal, Difference&gt;
, Incrementable
, Incrementable
, /* see details for category */
, Access
, /* see details for traversal category */
, Incrementable const&amp;
, Incrementable const*
, /* distance = Difference or a signed integral type */&gt;
@ -1935,7 +1954,7 @@ template &lt;class UnaryFunction&gt;
class function_output_iterator {
public:
typedef iterator_tag&lt;
writable_iterator_tag
writable_iterator
, incrementable_traversal_tag
&gt; iterator_category;
typedef void value_type;
@ -2043,7 +2062,7 @@ LocalWords: OtherIncrementable Coplien -->
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