c++ - Argument conversion in the presence of function templates -

i have set of function templates depend on 2 integers. these need instantiated, except 1 of function arguments needs converted. conversion operator provided purpose. compiler (vs 2013 express preview) refuses recognize possibility of conversion. see code below.

in conversion_op.h

template<int n> class proxy { };  template<int m, int n> void func(const proxy<m>& p1, const proxy<n>& p2) { }  typedef proxy<1> specialproxy;  class orig {     specialproxy p;  public:     operator specialproxy() const     {         return p;     } }; 

in conversion_op.cpp

#include "conversion_op.h"  //template void func(const specialproxy&, const specialproxy&); //template void func(const specialproxy&, const proxy<2>&); //template void func(const proxy<2>&, const specialproxy&); //template void func(const proxy<2>&, const proxy<2>&);  int test() {     orig a;      specialproxy p1;     proxy<2> p2;      func<1,1>(a, p1); //ok     func<1,2>(a, p2); //ok     func<1,1>(p1, a); //ok     func<2,1>(p2, a); //ok      func(a, p1); //error     func(a, p2); //error     func(p1, a); //error     func(p2, a); //error } 

the error is: "could not deduce template argument 'const proxy &' 'orig'. please see declaration of func."

i tried replacing

operator specialproxy() const; 

by

operator const specialproxy&() const; 

but error remains same.

also tried explicitly instantiating needed function (commented out code in conversion_op.cpp), no luck.

any insight appreciated. thank you.

the problem here c++ not allow implicit conversion when deducing template arguments.

when compiler tries instantiate func(a, p1) instance, can promptly deduce n -> 1, has no value m, nor allowed deduce out of implicit conversion of a proxy<1> defined in orig.