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POTraversing boost::variant types with Visitor that takes template
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  1. POTraversing boost::variant types with Visitor that takes template
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    copied!<p>I've got a persistence class like:</p> <pre><code>class Writer: private boost::noncopyable { template&lt;typename T&gt; struct Record { std::vector&lt;T&gt; _queued; // waiting to be persisted hsize_t _fileRows; // on disk DataSet _ds; ... }; template&lt;typename T&gt; void write(Writer::Record&lt;T&gt;&amp; rcrd) { ... } ... </code></pre> <p>That is used to persist Types such as:</p> <pre><code>struct A { sockaddr_in udpAddr; ... } struct B { uint8_t id; ... } struct C { ... } ... </code></pre> <p>I can't change the APIs above and I want to perform bulk operations on these heterogeneous types. I'm using <a href="http://www.boost.org/doc/libs/1_53_0/doc/html/variant.html" rel="nofollow">boost::variant</a> with partial success, following <a href="http://www.boost.org/doc/libs/1_53_0/doc/html/variant/tutorial.html" rel="nofollow">their own tutorial</a>:</p> <pre><code>typedef boost::variant&lt; Record&lt;A&gt;, Record&lt;B&gt;, Record&lt;C&gt;, ...&gt; _types; std::vector&lt;_types&gt; _records; struct CloseRecordVisitor : public boost::static_visitor&lt;&gt; { template &lt;typename T&gt; void operator()(T&amp; operand) const { assert(operand._queued.empty()); operand._ds.close(); } }; // This seems to work -template argument is substituted with Record&lt;A&gt;, Record&lt;B&gt;,... struct WriteRecordVisitor : public boost::static_visitor&lt;&gt; { template &lt;typename T&gt; void operator()(T&amp; operand) const { Writer::write(operand); } }; // This never compiles </code></pre> <p>Then the bulk operations across all the (many) heterogeneous types was meant to do simply:</p> <pre><code>CloseRecordVisitor _closeRecordVisitor; std::for_each(_records.begin(), _records.end(), boost::apply_visitor(_closeRecordVisitor)); WriteRecordVisitor _writeRecordVisitor; std::for_each(_records.begin(), _records.end(), boost::apply_visitor(_writeRecordVisitor)); </code></pre> <p>WriteRecordVisitor doesn't compile. Error is </p> <pre><code>No matching function to call ...template substitution failed. Cannot convert 'operand' (type Writer::Record&lt;A&gt;) to type 'Writer::Record&lt;Record&lt;A&gt;&gt;&amp;' </code></pre> <p>Which evidently looks wrong but I can't figure out what's causing it.</p> <p>I'd like to either have the WriteRecordVisitor approach working or be able to iterate through the vector (obtaining boost::variant&lt;...&gt;) and somehow (template again, likely) use boost::get to pass each appropriate element (Record&lt;A&gt;, Record&lt;B&gt;, ...) to Writer::write(Record&lt;T&gt;).</p> <p>I'd also like to avoid defining a visitor operator for each possible heterogeneous type, as that would defeat the original goal of simplifying using an heterogeneous container in the 1st place.</p> <p>I'm using gcc 4.7.2 on Linux 3.5.4 Fedora 17. Any help is appreciated -I did read all the other posts on boost::variant before posting.</p>
 

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