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POGrammar parsing with Spirit::Qi fails
 

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  1. POGrammar parsing with Spirit::Qi fails
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    <p>I'm new to Spirit::Qi and I'm trying to write a simple Wavefront Obj parser. I've followed the tutorials from the Boost::Spirit documentation site (<a href="http://www.boost.org/doc/libs/1_54_0/libs/spirit/doc/html/spirit/qi/tutorials.html" rel="nofollow">link</a>) and I got most of the inline rules working. I've started experimenting with grammars, but I cannot seem to get them working. After a while I did get it to compile, but the parsing fails. I really don't know what I am doing wrong.</p> <p>To start out simple, I've created a simple text file containing the following:</p> <pre><code>v -1.5701 33.8087 0.3592 v -24.0119 0.0050 21.7439 v 20.8717 0.0050 21.7439 v 20.8717 0.0050 -21.0255 v -24.0119 0.0050 -21.0255 v -1.5701 0.0050 0.3592 </code></pre> <p>Just to be sure: Reading the input file works fine.</p> <p>I've written a small function that should parse the input string, but for some reason it fails:</p> <pre><code>bool Model::parseObj( std::string &amp;data, std::vector&lt;float&gt; &amp;v ) { struct objGram : qi::grammar&lt;std::string::const_iterator, float()&gt; { objGram() : objGram::base_type(vertex) { vertex = 'v' &gt;&gt; qi::float_ &gt;&gt; qi::float_ &gt;&gt; qi::float_; } qi::rule&lt;std::string::const_iterator, float()&gt; vertex; }; objGram grammar; return qi::phrase_parse( data.cbegin(), data.cend(), grammar, iso8859::space, v ); } </code></pre> <p>qi::phrase_parse keeps returning false and the std::vector v is still empty at the end...</p> <p>Any suggestions?</p> <p>EDIT:</p> <p>After adding adding space skippers (is that the correct name?), only the first 'v' is added to the std::vector encoded as a float (118.0f), but the actual numbers aren't added. My guess is that my rule isn't correct. I want to only add the numbers and skip the v's.</p> <p>Here is my modified function:</p> <pre><code>bool Model::parseObj( std::string &amp;data, std::vector&lt;float&gt; &amp;v ) { struct objGram : qi::grammar&lt;std::string::const_iterator, float(), iso8859::space_type&gt; { objGram() : objGram::base_type(vertex) { vertex = qi::char_('v') &gt;&gt; qi::float_ &gt;&gt; qi::float_ &gt;&gt; qi::float_; } qi::rule&lt;std::string::const_iterator, float(), iso8859::space_type&gt; vertex; } objGrammar; return qi::phrase_parse( data.cbegin(), data.cend(), objGrammar, iso8859::space, v ); } </code></pre>
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      1. VTUpMod
    1. CO1) If you provide complete code you have a chance of more attention 2) Making a quick glance ... 118 is ascii code for your 'v' so it propagates to the output by mistake. If you want to avoid it you have two choices ( maybe more ): a) qi::lit('v') >> qi::float_ >> qi::float_ >> qi::float_ b) qi::omit[ qi::char_('v') ] >> qi::float_ >> qi::float_ >> qi::float_
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      1. USG. Civardi
    2. CO@G.Civardi Ah. You found the question around the same time I did. You missed the culprit, but I linked to your answer on the float precision question :) Cheers
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    3. CO@sehe no problem and many thanks :-) Cheers
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      1. USG. Civardi
 

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