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    <p>First, blow-by-blow. See below for a out-of-the-box answer.</p> <blockquote> <p><strong>Question 1</strong>: Why do I have to add a semantic action to the rule sign above? Isn't char convertible to std::string?</p> </blockquote> <p>Erm, no char is not convertible to string. See below for other options.</p> <blockquote> <p><strong>Question 2</strong>: Why does compilation fail when I try to merge the last two rules like this:</p> <pre><code>rule&lt;Iterator, std::string()&gt; floating = -sign &gt;&gt; (mantissa &gt;&gt; -(exp | suffix) | +digit &gt;&gt; (exp | suffix)); </code></pre> </blockquote> <p>This is due to the rules for atomic attribute assignment. The parser exposes something like </p> <pre><code>vector2&lt;optional&lt;string&gt;, variant&lt; vector2&lt;string, optional&lt;string&gt; &gt;, vector2&lt;std::vector&lt;char&gt;, optional&lt;string&gt; &gt; &gt; </code></pre> <p>or similar (see <a href="http://www.boost.org/doc/libs/1_48_0/libs/spirit/doc/html/spirit/qi/reference/operator.html" rel="nofollow noreferrer">the documentation for the parsers</a>, I typed this in the browser from memory). This is, obviously, not assignable to string. Use <code>qi::as&lt;&gt;</code> to coerce atomic assignment. For convenience ***there is <code>qi::as_string</code>:</p> <pre><code>floating = qi::as_string [ -sign &gt;&gt; (mantissa &gt;&gt; -(exp | suffix) | +digit &gt;&gt; (exp | suffix)) ] </code></pre> <blockquote> <p><strong>Question 3</strong>: Let's say I want to let the attribute of floating be double and write a semantic action to do the conversion from string to double. How can I refer to the entire string matched by the rule from inside the semantic action?</p> </blockquote> <p>You could use <code>qi::as_string</code> again, but the most appropriate would seem to be to use <code>qi::raw</code>:</p> <pre><code>floating = qi::raw [ -sign &gt;&gt; (mantissa &gt;&gt; -(exp | suffix) | +digit &gt;&gt; (exp | suffix)) ] [ _val = parse_float(_1, _2) ]; </code></pre> <p>This parser <em>directive</em> exposes a pair of source iterators, so you can use it to refer to the exact input sequence matched.</p> <blockquote> <p><strong>Question 4</strong>: In the rule floating of Question 2, what does the placeholder _2 refer to and what is its type?</p> </blockquote> <p>In general, to detect attribute types - that is, when the documentation has you confused or you want to double check your understanding of it - see the answers here:</p> <ul> <li><a href="https://stackoverflow.com/questions/9404189/detecting-the-parameter-types-in-a-spirit-semantic-action/9405265#9405265">Detecting the parameter types in a Spirit semantic action</a></li> </ul> <hr> <h2><strong>Out-of-the-box</strong></h2> <p>Have you looked at using Qi's <a href="http://www.boost.org/doc/libs/1_48_0/libs/spirit/doc/html/spirit/qi/reference/numeric/real.html#spirit.qi.reference.numeric.real._code__phrase_role__identifier__realpolicies__phrase___code_" rel="nofollow noreferrer">builtin <code>real_parser&lt;&gt;</code> template</a>, which can be comprehensively customized. It sure looks like you'd want to use that instead of doing custom parsing in your semantic action. </p> <p>The real_<code>parser</code> template with <em>policies</em> is both fast and very flexible and robust. See also the recent answer <a href="https://stackoverflow.com/questions/11420263/is-it-possible-to-read-infinity-or-nan-values-using-input-streams/11420901#11420901">Is it possible to read infinity or NaN values using input streams?</a>.</p> <p>For models of RealPolicies the following expressions must be valid: </p> <pre><code>Expression | Semantics ===========================+============================================================================= RP::allow_leading_dot | Allow leading dot. RP::allow_trailing_dot | Allow trailing dot. RP::expect_dot | Require a dot. RP::parse_sign(f, l) | Parse the prefix sign (e.g. '-'). Return true if successful, otherwise false. RP::parse_n(f, l, n) | Parse the integer at the left of the decimal point. Return true if successful, otherwise false. If successful, place the result into n. RP::parse_dot(f, l) | Parse the decimal point. Return true if successful, otherwise false. RP::parse_frac_n(f, l, n) | Parse the fraction after the decimal point. Return true if successful, otherwise false. If successful, place the result into n. RP::parse_exp(f, l) | Parse the exponent prefix (e.g. 'e'). Return true if successful, otherwise false. RP::parse_exp_n(f, l, n) | Parse the actual exponent. Return true if successful, otherwise false. If successful, place the result into n. RP::parse_nan(f, l, n) | Parse a NaN. Return true if successful, otherwise false. If successful, place the result into n. RP::parse_inf(f, l, n) | Parse an Inf. Return true if successful, otherwise false. If successful, place the result into n </code></pre> <p>See <strong><a href="http://www.boost.org/doc/libs/1_48_0/libs/spirit/doc/html/spirit/qi/reference/numeric/real.html#spirit.qi.reference.numeric.real.example" rel="nofollow noreferrer">the example</a></strong> for a compelling idea of how you'd use it.</p>
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    1. POQuestions about Spirit.Qi sequence operator and semantic actions
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    1. COThanks Seth. That's quite comprehensive! One question I still have though. You said this type is obviously not assignable to a string: `vector2<optional<string>, variant< vector2<string, optional<string> >, vector2<std::vector<char>, optional<string> > >` So what makes the type: `variant< vector2<string, optional<string> >, vector2<std::vector<char>, optional<string> >` assignable to a string? In other words, what makes two types compatible or incompatible?
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      1. USHaitham Gad
    2. COSpirit has _customization points_ for attribute handling (extraction, transformation, assignment) and these are documented [in the advanced section of the documentation](http://www.boost.org/doc/libs/1_48_0/libs/spirit/doc/html/spirit/advanced/customize.html)
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