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<H1>distance</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
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<TD Align=left><Img src = "algorithms.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Categories</b>: algorithms, iterators</TD>
<TD Align=right VAlign=top><b>Component type</b>: function</TD>
</TR>
</Table>
<h3>Prototype</h3>
<tt>Distance</tt> is an overloaded name; there are actually two <tt>distance</tt>
functions.
<pre>
template <class InputIterator>
inline <A href="iterator_traits.html">iterator_traits</A><InputIterator>::difference_type
distance(InputIterator first, InputIterator last);
template <class <A href="InputIterator.html">InputIterator</A>, class Distance>
void distance(InputIterator first, InputIterator last, Distance& n);
</pre>
<h3>Description</h3>
Finds the distance between <tt>first</tt> and <tt>last</tt>, <i>i.e.</i> the number of
times that <tt>first</tt> must be incremented until it is equal to
<tt>last</tt>. <A href="#1">[1]</A> The first version of <tt>distance</tt>, which takes two arguments,
simply returns that distance; the second version, which takes
three arguments and which has a return type of <tt>void</tt>, increments
<tt>n</tt> by that distance.
<P>
The second version of <tt>distance</tt> was the one defined in the original
STL, and the first version is the one defined in the
draft C++ standard; the definition was changed because the
older interface was clumsy and error-prone. The older interface
required the use of a temporary variable, and it has semantics
that are somewhat nonintuitive: it increments <tt>n</tt> by the distance
from <tt>first</tt> to <tt>last</tt>, rather than storing that distance in <tt>n</tt>. <A href="#2">[2]</A>
<P>
Both interfaces are currently supported <A href="#3">[3]</A>, for
reasons of backward compatibility, but eventually the older
version will be removed.
<h3>Definition</h3>
Defined in <A href="algobase.h">algobase.h</A>
<h3>Requirements on types</h3>
For the first version:
<UL>
<LI>
<tt>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A>.
</UL>
For the second version:
<UL>
<LI>
<tt>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A>.
<LI>
<tt>Distance</tt> is an integral type that is able to represent a distance
between iterators of type <tt>InputIterator</tt>.
</UL>
<h3>Preconditions</h3>
<UL>
<LI>
<tt>[first, last)</tt> is a <i>valid range</i>, as defined in the
<A href="InputIterator.html">Input Iterator</A> requirements.
</UL>
<h3>Complexity</h3>
Constant time if <tt>InputIterator</tt> is a model of
<A href="RandomAccessIterator.html">random access iterator</A>, otherwise linear time.
<h3>Example</h3>
<pre>
int main() {
<A href="List.html">list</A><int> L;
L.push_back(0);
L.push_back(1);
assert(distance(L.begin(), L.end()) == L.size());
}
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
This is the reason that <tt>distance</tt> is not defined for
<A href="OutputIterator.html">output iterators</A>: it is impossible to compare two <A href="OutputIterator.html" tppabs="http://www.sgi.com/Technology/STL/OutputIterator.shtml">output iterators</A>
for equality.
<P><A name="2">[2]</A>
Forgetting to initialize <tt>n</tt> to 0 is a common mistake.
<P><A name="3">[3]</A>
The new <tt>distance</tt> interface uses the <tt><A href="iterator_traits.html">iterator_traits</A></tt> class, which
relies on a C++ feature known as <i>partial specialization</i>. Many of
today's compilers don't implement the complete standard; in
particular, many compilers do not support partial specialization. If
your compiler does not support partial specialization, then you will
not be able to use the newer version of <tt>distance</tt>, or any other STL
components that involve <tt><A href="iterator_traits.html">iterator_traits</A></tt>.
<h3>See also</h3>
<tt><A href="distance_type.html">distance_type</A></tt>, <tt><A href="advance.html" tppabs="http://www.sgi.com/Technology/STL/advance.shtml">advance</A></tt>,
<A href="InputIterator.html">Input iterator</A>, <A href="RandomAccessIterator.html" tppabs="http://www.sgi.com/Technology/STL/RandomAccessIterator.shtml">Random access iterator</A>, <A href="iterator_tags.html" tppabs="http://www.sgi.com/Technology/STL/iterator_tags.shtml">Iterator tags</A>,
<tt><A href="iterator_traits.html">iterator_traits</A></tt>, <A href="Iterators.html" tppabs="http://www.sgi.com/Technology/STL/Iterators.shtml">Iterator overview</A>.
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