Code worked in VS6 but not in VS2017

[Alan9]

The following code worked before with VS6 VC++. But it does not complete the compiling with VS2017 VC++ (Link error is shown below). The complete template is in a .h file. What was changed in VS2017? What needs to be done to resolve the error? Please help. Thanks.

error LNK2019: unresolved external symbol "class CArrayEx<double,double> __cdecl operator-(class CArrayEx<double,double> const &,class CArrayEx<double,double> const &)" (??G@YA?AV?$CArrayEx@NN@@ABV0@0@Z) referenced in function "public: int __thiscall CMathEx::QNSS(class CArrayEx<double,double> const &,class CArrayEx<double,double> &,class CArrayEx<double,double> &,double &,double &,int &,int)" (?QNSS@CMathEx@@QAEHABV?$CArrayEx@NN@@AAV2@1AAN2AAHH@Z)

//
template<class TYPE, class ARG_TYPE> 
class CArrayEx : public CArray<TYPE,ARG_TYPE>
{
public:
	CArrayEx(){}
	......
	friend CArrayEx<TYPE,ARG_TYPE>	operator + (const CArrayEx<TYPE,ARG_TYPE>& za1,const CArrayEx<TYPE,ARG_TYPE>& za2);
	......
};

template<class TYPE, class ARG_TYPE> 
CArrayEx<TYPE,ARG_TYPE> operator + (const CArrayEx<TYPE,ARG_TYPE>& za1,const CArrayEx<TYPE,ARG_TYPE>& za2)
{
	CArrayEx<TYPE,ARG_TYPE> za;
	za.SetSize(min(za1.GetSize(),za2.GetSize()));

	for (int i=0;i<za.GetSize();i++)
	{
		za[i]=za1[i]+za2[i];
	}
	return za;
}