c++ - Traversing boost::variant types with Visitor that takes template -

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i've got persistence class like:

class writer: private boost::noncopyable {     template<typename t>     struct record {         std::vector<t> _queued; // waiting persisted         hsize_t _filerows; // on disk         dataset _ds;         ...     };      template<typename t>     void write(writer::record<t>& rcrd) { ... }     ...

that used persist types such as:

struct {     sockaddr_in udpaddr;     ... }  struct b {     uint8_t id;     ... }  struct c { ... } ...

i can't change apis above , want perform bulk operations on these heterogeneous types. i'm using boost::variant partial success, following their own tutorial:

typedef boost::variant< record<a>, record<b>, record<c>, ...> _types; std::vector<_types> _records;  struct closerecordvisitor : public boost::static_visitor<> {     template <typename t>     void operator()(t& operand) const {         assert(operand._queued.empty());         operand._ds.close();     } }; // seems work -template argument substituted record<a>, record<b>,...  struct writerecordvisitor : public boost::static_visitor<> {     template <typename t>     void operator()(t& operand) const {         writer::write(operand);     } }; // never compiles

then bulk operations across (many) heterogeneous types meant simply:

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));

writerecordvisitor doesn't compile. error 

no matching function call ...template substitution failed. cannot convert 'operand' (type writer::record<a>) type 'writer::record<record<a>>&'

which evidently looks wrong can't figure out what's causing it.

i'd either have writerecordvisitor approach working or able iterate through vector (obtaining boost::variant<...>) , somehow (template again, likely) use boost::get pass each appropriate element (record<a>, record<b>, ...) writer::write(record<t>).

i'd avoid defining visitor operator each possible heterogeneous type, defeat original goal of simplifying using heterogeneous container in 1st place.

i'm using gcc 4.7.2 on linux 3.5.4 fedora 17. appreciated -i did read other posts on boost::variant before posting.


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