Re: Template function parameters
From: nevlis (silven_at_removesilven.canada.com)
Date: 09/27/04
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Date: Mon, 27 Sep 2004 15:53:58 GMT
"Doug Harrison [MVP]" <dsh@mvps.org> wrote in
news:opagl0puoccecajsqq7sc39rpejoidlcr8@4ax.com:
> nevlis wrote:
>
>>I am trying to create a Template function that will take
>>differnet parameter types. However all my attempts have failed
>>can someone please tell me what I am doing wrong here is the code.
>>
>>It seems to work when I pass integers however will not compile when
>>I have a float or double
>>
>>I get this error when compiling
>>
>>initializing : cannot convert from 'unsigned short *' to 'double'
>>
>>I do not understand why it has unsigned short when I am passing
>>a double for the intData parameters
>>
>>
>>//function prototype
>>template <class Z>void StoreInArray(VARIANT* arr_rgElems,const
>>inDataType, Z inData, int intPosition);
>>
>>//Test Calling this works
>>strcpy (strAsterisk, "*");
>>strTemp = A2BSTR(strAsterisk);
>>StoreInArray(rgElems,VT_BSTR, strTemp, intCounter++);
>>
>>//this works
>>StoreInArray(rgElems,VT_I2, 99, intCounter++);
>>
>>//I tried different many differnet ways but always get
>>//the same type of error when compiling
>>
>>double tempX = 8.99
>>StoreInArray<double>(rgElems,VT_R8, tempX, intCounter++);
>>StoreInArray<double>(rgElems,VT_R8, (double)tempX, intCounter++);
>>StoreInArray(rgElems,VT_R8, (double)tempX, intCounter++);
>>
>>
>>//function
>>template <class Z> void StoreInArray(VARIANT* arr_rgElems,const
>>inDataType, Z inData, int intPosition){
>
> Note that implicit int is not legal, so that should be "const int",
//int should be integer here
> not just "const".
>
>> COleDateTime myDateTime;
>>
>> double douInData = inData;
>
> The statement above is flawed, and I don't see how the compiler can
> possibly accept your earlier call where Z is BSTR. It would be helpful
> to know what compiler version you're using, and it would be even more
> helpful if you were to present this as a small console program
> fragment that can be compiled and which demonstrates the problem. Get
> rid of all the irrelevant things like BSTR, COleDateTime, etc and
> write it in terms of basic types.
//compiler version VC++ 6
>
> You should not be converting inData to some other type, only to
> convert it back below. It won't work for non-arithmetic types like
> pointers, and it can lose precision for 64 bit integer types. You'd be
> better off forgetting about templates and making inData void*, passing
> objects by address and casting to the correct type with, for example:
>
> // inData now void*
> arr_rgElems[intPosition].intVal = *(int*) inData;
>
>> VariantInit(&arr_rgElems[intPosition]);
>> arr_rgElems[intPosition].vt = inDataType;
>> switch (inDataType) {
>> case VT_BSTR:
>> arr_rgElems[intPosition].bstrVal = (BSTR) inData;
>> break;
>>
>> case VT_I2:
>> arr_rgElems[intPosition].intVal = (int)inData;
>> break;
>>
>> case VT_I4:
>> arr_rgElems[intPosition].intVal = (long)inData;
>> break;
>>
>> case VT_R4:
>> arr_rgElems[intPosition].fltVal = (float)inData;
>> break;
>>
>> case VT_R8:
>> arr_rgElems[intPosition].dblVal = (double)inData;
>> break;
>>
>> case VT_DATE:
>> myDateTime = (time_t) *(long*)inData;
>> arr_rgElems[intPosition].date = (DATE)myDateTime;
>> break;
>>
>> default:
>> break;
>> }
>>}
>
> When all is said and done, I just don't see the advantage in making
> this function a template. I could see writing a bunch of overloaded
> functions to relieve you from having to specify the VT_xxx type,
> though, and they could be one-liners that defer to the generic form
> that takes the VT_xxx type and void*.
>
Hi Doug,
Thanks for the quick response... To begin let me just say that I do not
use C++ or C very often so forgive me if some things are not correct.
My origninal function was
void StoreInArray(VARIANT* arr_rgElems,const inDataType, void* inData,
int intPosition){
//I will change int to integer
COleDateTime myDateTime;
VariantInit(&arr_rgElems[intPosition]);
arr_rgElems[intPosition].vt = inDataType;
switch (inDataType) {
case VT_BSTR:
arr_rgElems[intPosition].bstrVal = (BSTR) inData;
break;
case VT_I2:
arr_rgElems[intPosition].intVal = *(int*)inData;
break;
case VT_I4:
arr_rgElems[intPosition].intVal = *(long*)inData;
break;
case VT_R4:
arr_rgElems[intPosition].fltVal = *(float*)inData;
break;
case VT_R8:
arr_rgElems[intPosition].dblVal = *(double*)inData;
break;
case VT_DATE:
myDateTime = (time_t) *(long*)inData;
arr_rgElems[intPosition].date = (DATE)myDateTime;
break;
default:
break;
}
}
Here was my orginal funtion so I guess I will stick to it... I will look
into overloading the function as you mentionned.
I guess my attempt to use my first template should not have been here.
Thanks nevlis
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