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PODetecting the parameter types in a Spirit semantic action
 

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  1. PODetecting the parameter types in a Spirit semantic action
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    <p>General case: I can't figure out why my Spirit grammar/semantics actions aren't compiling. </p> <p>Sometimes, the compiler will complain about assignment or type incompatibilities and I have no clue what's wrong. The problem occurs in two main areas:</p> <ul> <li>predicting the type of synthesized attributes for a rule/expression <ul> <li>consequently, predicting what types of attributes can be legally defined as the exposed attribute for the rule (relying in builting conversions, fusion adaptors or Spirit customization points)</li> </ul></li> <li>matching the argument types for my semantic action so that <ul> <li>the compiler will be able to compile the function invocation</li> <li>the invocation will not invoke unnecessary implicit conversions in the process</li> </ul></li> </ul> <p>The compiler error is not exactly tractable, and either the documentation is wrong, or I misunderstood it. </p> <p><strong>Is there a way to find out <em>exactly</em> what Spirit passes into my semantic action, anyway?</strong></p> <h3>Example code:</h3> <pre><code>struct mybase { int a,b; }; struct myderived : mybase { int c,d; }; BOOST_FUSION_ADAPT_STRUCT(mybase, (int,a)(int,b)); BOOST_FUSION_ADAPT_STRUCT(myderived, (int,a)(int,b)(int,c)(int,d)); auto base_expr = int_ &gt;&gt; int_; // avoids assigning to struct attribute rule&lt;decltype(f), mybase() , space_type&gt; base_ = int_ &gt;&gt; int_; rule&lt;decltype(f), myderived(), space_type&gt; derived_ = base_ &gt;&gt; int_ &gt;&gt; int_; myderived data; bool ok = phrase_parse(f,l,derived_,space,data); </code></pre> <p>This code won't compile, with a huge amount of impenetrable errors.</p> <p>(<em>loosely adapted from a posting on the <a href="http://boost.2283326.n4.nabble.com/struct-derived-struct-fusion-adapted-and-the-sequence-operator-tc4259090.html" rel="noreferrer">spirit-general</a> list</em>)</p>
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    1. CO+1 <starred> exactly what I'm struggling with on a daily basis, as a beginning user...
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