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<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Position
Iterator</b></font> </td>
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<p>Often, when writing a parser that is able to detect errors in the format of
the input stream, we want it to communicate to the user where the error happened
within that input. The most classic is when writing a compiler or interpreter
that detects syntactical errors in the parsed program, indicating the line number
and maybe even the position within the line where the error was found.</p>
<p> The class position_iterator is a tool provided within Spirit that allows parser
writers to easily implement this functionality. The concept is quite simple:
this class is an iterator wrapper that keeps track of the current position within
the file, including current file, line and column. It requires a single template
parameter, which should be the type of the iterator that is to be wrapped.</p>
<p> In order to use it, you'll need to add the following include:</p>
<pre>
<code><span class=preprocessor>#include </span><span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>spirit</span><span class=special>/</span><span class=identifier>iterator</span><span class=special>/</span><span class=identifier>position_iterator</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span></code></pre>
<p> Or include all the iterators in Spirit:</p>
<pre>
<code><span class=preprocessor>#include </span><span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>spirit</span><span class=special>/</span><span class=identifier>iterator</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span></code></pre>
<p> To construct the wrapper, it needs both the begin and end iterators of the
input sequence, and the file name of the input sequence. Optionally, you can
also specify the starting line and column numbers, which default to 1. Default
construction, with no parameters, creates a generic end-of-sequence iterator,
in a similar manner as it's done in the stream operators of the standard C++
library.</p>
<p> The wrapped iterator must belong to the input or forward iterator category,
and the position_iterator just inherits that category.</p>
<p> For example, to create begin and end positional iterators from an input C-
string, you'd use:</p>
<pre>
<code><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>inputstring </span><span class=special>= </span><span class=string>"..."</span><span class=special>;
</span><span class=keyword>typedef </span><span class=identifier>position_iterator</span><span class=special><</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*> </span><span class=identifier>iterator_t</span><span class=special>;
</span><span class=identifier>iterator_t </span><span class=identifier>begin</span><span class=special>(</span><span class=identifier>inputstring</span><span class=special>, </span><span class=identifier>inputstring</span><span class=special>+</span><span class=identifier>strlen</span><span class=special>(</span><span class=identifier>inputstring</span><span class=special>));
</span><span class=identifier>iterator_t </span><span class=identifier>end</span><span class=special>;</span></code></pre>
<a name="operations"></a>
<h2>Operations</h2>
<pre>
<code><span class=keyword>void </span><span class=identifier>set_position</span><span class=special>(</span><span class=identifier>file_position </span><span class=keyword>const</span><span class=special>&);</span></code></pre>
<p> Call this function when you need to change the current position stored in
the iterator. For example, if parsing C-style #include directives, the included
file's input must be marked by restarting the file and column to 1 and 1 and
the name to the new file's name.<br>
</p>
<pre>
<code><span class=identifier>file_position </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>get_position</span><span class=special>() </span><span class=keyword>const</span><span class=special>;</span></code></pre>
<p> Call this function to retrieve the current position.</p>
<pre>
<code><span class=keyword>void </span><span class=identifier>set_tabchars</span><span class=special>(</span><span class=keyword>int</span><span class=special>);</span></code></pre>
<p> Call this to set the number of tabs per character. This value is necessary
to correctly track the column number.<br>
</p>
<p> <a name="file_position"></a> </p>
<h2>file_position</h2>
<p> file_position is a structure that holds the position within a file. Its fields
are:</p>
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<td class="table_title" colspan="2">file_position fields</td>
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<td class="table_cells" width="26%"><code><span class=identifier>std</span><span class=special>::</span><span class=identifier>string
</span><span class=identifier>file</span><span class=special>;</span></code></td>
<td class="table_cells" width="74%">Name of the file. Hopefully a full pathname</td>
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<td class="table_cells" width="26%"><code><span class=keyword>int</span><span class=identifier>
line</span><span class=special>;</span></code></td>
<td class="table_cells" width="74%">Line number within the file. By default,
the first line is number 1</td>
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<td class="table_cells" width="26%"><code><span class=keyword>int </span><span class=identifier>column</span><span class=special>;</span></code></td>
<td class="table_cells" width="74%">Column position within the file. The first
column is 1</td>
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<p class="copyright">Copyright © 2002 Juan Carlos Arevalo-Baeza<br>
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