mixing non-specialized and specialized templates

i'm writing an ADO-like wrapper class for SQLite. I'd like to emulate (and expand upon) their "AsType" methods, but i'm running into trouble mixing template specializations

Code:

---

template<> void castResult(String &arg,char *result)
{
        arg = (String)result;
}

template<> void castResult(double &arg,char *result)
{
        if(!TryStrToFloat(result,arg))
        {
                throw Exception((String)result+"cannot be cast to double");
        }
}

template<> void castResult(unsigned int &arg,char *result)
{
        if(!TryStrToInt(result,arg))
        {
                throw Exception((String)result+"cannot be cast to int");
        }       
}

template<> void castResult(__int64 &arg,char *result)
{
        if(!TryStrToInt64(result,arg))
        {
                throw Exception((String)result+"cannot be cast to double");
        }       
}

template<> void castResult(unsigned __int64 &arg,char *result)
{
        if(!TryStrToInt64(result,arg))
        {
                throw Exception((String)result+"cannot be cast to double");

        }       
}
template<> void castResult(bool &arg,char *result)
{
        if(!TryStrToBool(result,arg))
        {
                throw Exception((String)result+"cannot be cast to bool");

        }       
}

template<> void castResult(TDateTime &arg,char *result)
{
        if(!TryStrToTime(result,arg))
        {
                throw Exception((String)result+"cannot be cast to TDateTime");
        }
}

template<> void castResult(int &arg,char *result)
{
        if(!TryStrToInt(result,arg))
        {
                throw Exception((String)result+"cannot be cast to double");
        }
}


template<typename T> T __fastcall field(char *colName)
{
        T t;
        castResult<T>(t,results[colNumber(colName)+cols*recordOffset]);
        return t;
}

template<typename T> T __fastcall field(char *colName,unsigned int row)
{
        if(row>=rows)
        {
                invalidRow(row);
        }
        T t;
        return castResult(t,results[colNumber(colName)+cols*(row+1)]);
}

template<typename T> T __fastcall field(unsigned int col)
{
        if(col>=cols)
        {
                invalidCol(col);
        }
        T t;
        return castResult(t,results[col+cols*recordOffset]);
}

template<typename T> T __fastcall field(unsigned int col,unsigned int row)
{
        if(row>=rows)
        {
                invalidRow(row);
        }
        if(col>=cols)
        {
                invalidCol(col);
        }
        T t;
        return castResult(t,results[cols*(row+1)+col]);
}

---

Code:

---

        sqlitedb::sqlResult *result = db.exec("select reportData.RPTID,reportData.SVID from reportData,eventReports where reportData.RPTID = eventReports.RPTID and eventReports.CEID = 0");
        unsigned int row = 1;
        while(!result->eof)
        {
                int rptid = result->field<int>("RPTID");
                int svid = result->field<int>("SVID");
                row++;
                result->next();
        }
        delete result;

---

i've tried many different variations, including a T& as an argument to both field and castResult, but for some reason, my compiler won't deduce how to relate non-specialized and specialized templates.

suggestions on the best way to proceed? i'd like not to have to specialize each of the 4 incarnations of field for each type.

Without effort you can't specialize a template function. Very rarely is the situation such that it is necessary. In most situations, simply overloading the function is correct.

Just try 'castResult(t,results[colNumber(colName)+cols*recordOffset]);' in the relevant function.

Soma

AFAICT that's what i've done. I have gotten all manner of odd errors. Including but not limited to:

"call to undefined function castResult"

"undefined symbol arg" <- this one really bothers me, in that it resolves to the appropriately specialized function, but then says it can't recognize its argument O_o

I didn't even notice that you had called it differently in different functions.

From 'field(char *, unsigned int)': 'castResult(t,results[colNumber(colName)+cols*(row+1)]);'.

From ' field(char *)': 'castResult<T>(t,results[colNumber(colName)+cols*recordOffset]);'.

Soma

err - yeah i was experimenting with different incarnations. i backed up to the point where i was getting "undefined symbol arg"

the trick seems to be declaring a local variable and returning that (and changing the return types to return by value)

Code:

---

template<> int castResult(int d,char *result)
{
        int d;
        if(!TryStrToInt(result,d))
        {
                throw Exception((String)result+"cannot be cast to int");
        }
        return d;
}

//...

template<typename T> T __fastcall field(char *colName)
{
        T t;
        return castResult(t,results[colNumber(colName)+cols*recordOffset]);
}

//...

int RPTID = result->field<int>("RPTID");

---

i still don't understand why can't it recognize the function arguments.

What is the template<> doing in the castResult functions? Essentially it is not a template function / specialization.

It should be possible, though, to use or create a templated converter (e.g boost::lexical_cast can convert anything to and from string as long as they have overloaded operator<< and >>, and anything you create based on stringstreams would too).

ahh i get you; in this case, castresult has just been overloaded multiple times and they're not really specializations at all. i initially didn't want to supply a second argument to castResult, but specify which specialization to use as: castResult<T>(char *result), which does necessitate the use of templates AFAICT

i do want one true template version of castresult though; so that in the event that the supplied type is not one specified, it will attempt the cast from char * to T.