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POQuestions about Spirit.Qi sequence operator and semantic actions
 

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  1. POQuestions about Spirit.Qi sequence operator and semantic actions
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    <p>I have some questions about the sequence operator and semantic actions in Spirit Qi.</p> <p>I'm trying to define a grammar rule for a floating point number that accepts metric prefixes (u, m, k, M, etc.) as well as the normal exponent form.</p> <pre><code> rule&lt;Iterator, std::string()&gt; sign = char_("+-") [ _val = _1 ]; rule&lt;Iterator, std::string()&gt; exp = char_("eE") &gt;&gt; -sign &gt;&gt; +digit; rule&lt;Iterator, std::string()&gt; suffix = char_("yzafpnumkKMGTPEZY") [ _val = _1 ]; rule&lt;Iterator, std::string()&gt; mantissa = ((*digit &gt;&gt; char_('.') &gt;&gt; +digit) | (+digit &gt;&gt; char_('.') &gt;&gt; *digit)); rule&lt;Iterator, std::string()&gt; unsigned_floating = (mantissa &gt;&gt; -(exp | suffix) | +digit &gt;&gt; (exp | suffix)); rule&lt;Iterator, std::string()&gt; floating = -sign &gt;&gt; unsigned_floating; </code></pre> <p><strong>Question 1:</strong> Why do I have to add a semantic action to the rule <code>sign</code> above? Isn't <code>char</code> convertible to <code>std::string</code>?</p> <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> <p><strong>Question 3:</strong> Let's say I want to let the attribute of <code>floating</code> be <code>double</code> 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> <p><strong>Question 4:</strong> In the rule <code>floating</code> of Question 2, what does the placeholder <code>_2</code> refer to and what is its type?</p> <p><strong>Edit:</strong></p> <p>I guess the last question needs some clarification:</p> <p>What does the placeholder _2 refer to in the semantic action of the following rule, and what's its type?</p> <pre><code> rule&lt;Iterator, std::string()&gt; floating = (-sign &gt;&gt; (mantissa &gt;&gt; -(exp | suffix) | +digit &gt;&gt; (exp | suffix))) [ _2 ]; </code></pre> <p>Thanks!</p>
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