c++ - Template specialization with compile time constant -

i trying build specialization template class compile time constant.

the template class looks this:

template<class tnativeitem, class tcomitem = void,          vartype _vartype = _atl_automationtype<tcomitem>::type> class inoutcomarray { private:     ccomsafearray<tcomitem, _vartype> _safearray;     // ... public:     inoutcomarray(         tnativeitem* items, size_t length,         std::function<tcomitem(const tnativeitem&)> converttocom,         std::function<tnativeitem(const tcomitem&)> convertfromcom)         : _safearray(length)     {         // ...     }      // ... }; 

usage example:

inoutcomarray<bool, variant_bool, vt_bool>(     items, length, booltovariant_bool, variant_booltobool)); 

however, there exist types don't require conversion , wanted provide short hand version this:

inoutcomarray<long>(items, length); 

i tried implement this:

template<class titem, vartype _vartype = _atl_automationtype<titem>::type> class inoutcomarray<titem, void, _vartype>     : public inoutcomarray<titem, titem, _vartype> { public:     inoutcomarray(titem* items, size_t length)         : inoutcomarray<titem, titem, _vartype>(               items, length, noconverter<titem>, noconverter<titem>)     {      } }; 

however, following error:

'_vartype' : default template arguments not allowed on partial specialization

is there way around that?

you first define default arguments void , _atl_automationtype<tcomitem>::type, when 1 argument x given, want inoutcomarray<x> inoutcomarray<x, void, _atl_automationtype<void>::type>.

your partial specialization contradicts this: inoutcomarray<x> shall inoutcomarray<x, x, _atl_automationtype<x>::type>.

depending on thik more second argument, (i.e. void or same first argument), make second argument defaulted first 1 in first place:

template<class tnativeitem, class tcomitem = tnativeitem,      vartype _vartype = _atl_automationtype<tcomitem>::type> 

that way behavior of partial specialization covered, except additional constructor. can achieved using default arguments constructor:

template<class tnativeitem, class tcomitem = tnativeitem,      vartype _vartype = _atl_automationtype<tcomitem>::type> class inoutcomarray { public: inoutcomarray(     tnativeitem* items, size_t length,     std::function<tcomitem(const tnativeitem&)> converttocom = noconverter<tnativeitem>(),     std::function<tnativeitem(const tcomitem&)> convertfromcom = noconverter<tnativeitem>()); };