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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_parser.tutorial.hello__whomever"></a><a class="link" href="hello__whomever.html" title="Hello, Whomever">Hello, Whomever</a>
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<p>
This is just about the most minimal example of using Boost.Parser that one
could write. We take a string from the command line, or <code class="computeroutput"><span class="string">"World"</span></code>
if none is given, and then we parse it:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">parser</span><span class="special">/</span><span class="identifier">parser</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">bp</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">input</span> <span class="special">=</span> <span class="string">"World"</span><span class="special">;</span>
<span class="keyword">if</span> <span class="special">(</span><span class="number">1</span> <span class="special">&lt;</span> <span class="identifier">argc</span><span class="special">)</span>
<span class="identifier">input</span> <span class="special">=</span> <span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">result</span><span class="special">;</span>
<span class="identifier">bp</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">input</span><span class="special">,</span> <span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span><span class="special">,</span> <span class="identifier">result</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Hello, "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</span> <span class="string">"!\n"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
The expression <code class="computeroutput"><span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span></code>
is a parser-expression. It uses one of the many parsers that Boost.Parser
provides: <code class="computeroutput"><a class="link" href="../../boost/parser/char_.html" title="Global char_">char_</a></code>.
Like all Boost.Parser parsers, it has certain operations defined on it. In
this case, <code class="computeroutput"><span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span></code>
is using an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> as the C++ version of a <a href="https://en.wikipedia.org/wiki/Kleene_star" target="_top">Kleene
star</a> operator. Since C++ has no postfix unary <code class="computeroutput"><span class="special">*</span></code>
operator, we have to use the one we have, so it is used as a prefix.
</p>
<p>
So, <code class="computeroutput"><span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span></code>
means "any number of characters". In other words, it really cannot
fail. Even an empty string will match it.
</p>
<p>
The parse operation is performed by calling the <code class="computeroutput"><a class="link" href="../../boost/parser/parse_id2.html" title="Function template parse">parse()</a></code>
function, passing the parser as one of the arguments:
</p>
<pre class="programlisting"><span class="identifier">bp</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">input</span><span class="special">,</span> <span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span><span class="special">,</span> <span class="identifier">result</span><span class="special">);</span>
</pre>
<p>
The arguments here are: <code class="computeroutput"><span class="identifier">input</span></code>,
the range to parse; <code class="computeroutput"><span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span></code>,
the parser used to do the parse; and <code class="computeroutput"><span class="identifier">result</span></code>,
an out-parameter into which to put the result of the parse. Don't get too
caught up on this method of getting the parse result out of <code class="computeroutput"><a class="link" href="../../boost/parser/parse_id2.html" title="Function template parse">parse()</a></code>; there are multiple ways
of doing so, and we'll cover all of them in subsequent sections.
</p>
<p>
Also, just ignore for now the fact that Boost.Parser somehow figured out
that the result type of the <code class="computeroutput"><span class="special">*</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">char_</span></code>
parser is a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>. There are clear rules for this
that we'll cover later.
</p>
<p>
The effects of this call to <code class="computeroutput"><a class="link" href="../../boost/parser/parse_id2.html" title="Function template parse">parse()</a></code>
is not very interesting — since the parser we gave it cannot ever
fail, and because we're placing the output in the same type as the input,
it just copies the contents of <code class="computeroutput"><span class="identifier">input</span></code>
to <code class="computeroutput"><span class="identifier">result</span></code>.
</p>
</div>
<div class="copyright-footer">Copyright © 2020 T. Zachary Laine<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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