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< title > pointee and indirect_reference< / title >
< meta name = "author" content = "David Abrahams" / >
< meta name = "organization" content = "Boost Consulting" / >
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< meta name = "date" content = "2006-09-11" / >
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< meta name = "copyright" content = "Copyright David Abrahams 2004." / >
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< div class = "document" id = "pointee-and-indirect-reference" >
< h1 class = "title" > < tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > and < tt class = "docutils literal" > < span class = "pre" > indirect_reference< / span > < / tt > < / h1 >
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< table class = "docinfo" frame = "void" rules = "none" >
< col class = "docinfo-name" / >
< col class = "docinfo-content" / >
< tbody valign = "top" >
< tr > < th class = "docinfo-name" > Author:< / th >
< td > David Abrahams< / td > < / tr >
< tr > < th class = "docinfo-name" > Contact:< / th >
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< td > < a class = "first last reference external" href = "mailto:dave@boost-consulting.com" > dave@ boost-consulting.com< / a > < / td > < / tr >
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< tr > < th class = "docinfo-name" > Organization:< / th >
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< td > < a class = "first last reference external" href = "http://www.boost-consulting.com" > Boost Consulting< / a > < / td > < / tr >
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< tr > < th class = "docinfo-name" > Date:< / th >
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< td > 2006-09-11< / td > < / tr >
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< tr > < th class = "docinfo-name" > Copyright:< / th >
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< td > Copyright David Abrahams 2004.< / td > < / tr >
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< / tbody >
< / table >
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<!-- Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > abstract:< / th > < td class = "field-body" > Provides the capability to deduce the referent types of
pointers, smart pointers and iterators in generic code.< / td >
< / tr >
< / tbody >
< / table >
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< div class = "section" id = "overview" >
< h1 > Overview< / h1 >
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< p > Have you ever wanted to write a generic function that can operate
on any kind of dereferenceable object? If you have, you've
probably run into the problem of how to determine the type that the
object " points at" :< / p >
< pre class = "literal-block" >
template < class Dereferenceable>
void f(Dereferenceable p)
{
< em > what-goes-here?< / em > value = *p;
...
}
< / pre >
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< div class = "section" id = "pointee" >
< h2 > < tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > < / h2 >
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< p > It turns out to be impossible to come up with a fully-general
algorithm to do determine < em > what-goes-here< / em > directly, but it is
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possible to require that < tt class = "docutils literal" > < span class = "pre" > pointee< Dereferenceable> ::type< / span > < / tt > is
correct. Naturally, < tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > has the same difficulty: it can't
determine the appropriate < tt class = "docutils literal" > < span class = "pre" > ::type< / span > < / tt > reliably for all
< tt class = "docutils literal" > < span class = "pre" > Dereferenceable< / span > < / tt > s, but it makes very good guesses (it works
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for all pointers, standard and boost smart pointers, and
iterators), and when it guesses wrongly, it can be specialized as
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necessary:< / p >
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< pre class = "literal-block" >
namespace boost
{
template < class T>
struct pointee< third_party_lib::smart_pointer< T> >
{
typedef T type;
};
}
< / pre >
< / div >
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< div class = "section" id = "indirect-reference" >
< h2 > < tt class = "docutils literal" > < span class = "pre" > indirect_reference< / span > < / tt > < / h2 >
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< p > < tt class = "docutils literal" > < span class = "pre" > indirect_reference< T> ::type< / span > < / tt > is rather more specialized than
< tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > , and is meant to be used to forward the result of
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dereferencing an object of its argument type. Most dereferenceable
types just return a reference to their pointee, but some return
proxy references or return the pointee by value. When that
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information is needed, call on < tt class = "docutils literal" > < span class = "pre" > indirect_reference< / span > < / tt > .< / p >
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< p > Both of these templates are essential to the correct functioning of
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< a class = "reference external" href = "indirect_iterator.html" > < tt class = "docutils literal" > < span class = "pre" > indirect_iterator< / span > < / tt > < / a > .< / p >
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< / div >
< / div >
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< div class = "section" id = "reference" >
< h1 > Reference< / h1 >
< div class = "section" id = "id1" >
< h2 > < tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > < / h2 >
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<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
< pre class = "literal-block" >
template < class Dereferenceable>
struct pointee
{
typedef /* see below */ type;
};
< / pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
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< tr class = "field" > < th class = "field-name" > Requires:< / th > < td class = "field-body" > For an object < tt class = "docutils literal" > < span class = "pre" > x< / span > < / tt > of type < tt class = "docutils literal" > < span class = "pre" > Dereferenceable< / span > < / tt > , < tt class = "docutils literal" > < span class = "pre" > *x< / span > < / tt >
is well-formed. If < tt class = "docutils literal" > < span class = "pre" > ++x< / span > < / tt > is ill-formed it shall neither be
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ambiguous nor shall it violate access control, and
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< tt class = "docutils literal" > < span class = "pre" > Dereferenceable::element_type< / span > < / tt > shall be an accessible type.
Otherwise < tt class = "docutils literal" > < span class = "pre" > iterator_traits< Dereferenceable> ::value_type< / span > < / tt > shall
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be well formed. [Note: These requirements need not apply to
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explicit or partial specializations of < tt class = "docutils literal" > < span class = "pre" > pointee< / span > < / tt > ]< / td >
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< / tr >
< / tbody >
< / table >
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< p > < tt class = "docutils literal" > < span class = "pre" > type< / span > < / tt > is determined according to the following algorithm, where
< tt class = "docutils literal" > < span class = "pre" > x< / span > < / tt > is an object of type < tt class = "docutils literal" > < span class = "pre" > Dereferenceable< / span > < / tt > :< / p >
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< pre class = "literal-block" >
if ( ++x is ill-formed )
{
return ``Dereferenceable::element_type``
}
else if (``*x`` is a mutable reference to
std::iterator_traits< Dereferenceable> ::value_type)
{
return iterator_traits< Dereferenceable> ::value_type
}
else
{
return iterator_traits< Dereferenceable> ::value_type const
}
< / pre >
< / div >
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< div class = "section" id = "id2" >
< h2 > < tt class = "docutils literal" > < span class = "pre" > indirect_reference< / span > < / tt > < / h2 >
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<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
< pre class = "literal-block" >
template < class Dereferenceable>
struct indirect_reference
{
typedef /* see below */ type;
};
< / pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
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< tr class = "field" > < th class = "field-name" > Requires:< / th > < td class = "field-body" > For an object < tt class = "docutils literal" > < span class = "pre" > x< / span > < / tt > of type < tt class = "docutils literal" > < span class = "pre" > Dereferenceable< / span > < / tt > , < tt class = "docutils literal" > < span class = "pre" > *x< / span > < / tt >
is well-formed. If < tt class = "docutils literal" > < span class = "pre" > ++x< / span > < / tt > is ill-formed it shall neither be
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ambiguous nor shall it violate access control, and
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< tt class = "docutils literal" > < span class = "pre" > pointee< Dereferenceable> ::type& < / span > < / tt > shall be well-formed.
Otherwise < tt class = "docutils literal" > < span class = "pre" > iterator_traits< Dereferenceable> ::reference< / span > < / tt > shall
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be well formed. [Note: These requirements need not apply to
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explicit or partial specializations of < tt class = "docutils literal" > < span class = "pre" > indirect_reference< / span > < / tt > ]< / td >
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< / tr >
< / tbody >
< / table >
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< p > < tt class = "docutils literal" > < span class = "pre" > type< / span > < / tt > is determined according to the following algorithm, where
< tt class = "docutils literal" > < span class = "pre" > x< / span > < / tt > is an object of type < tt class = "docutils literal" > < span class = "pre" > Dereferenceable< / span > < / tt > :< / p >
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< pre class = "literal-block" >
if ( ++x is ill-formed )
return ``pointee< Dereferenceable> ::type& ``
else
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std::iterator_traits< Dereferenceable> ::reference
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< / pre >
< / div >
< / div >
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