Solved

function template conditional execution of code

Posted on 2002-05-01
6
288 Views
Last Modified: 2010-08-05
Hi,

I want to do the following

------------------------------------------------------------------------------------
     template <class T> void PutToSA(SAFEARRAY* &insa,long ElemNo,T& InVal)
     {
          VARIANT MyVar;
          VariantInit(&MyVar);
#if typeof(T) is CString
          MyVar.vt=VT_BSTR;
          MyVar.bstrVal=InVal.AllocSysString();
#else
          MyVar.vt=VT_I2;
          MyVar.intVal=InVal;
#end if
          SafeArrayPutElement(insa,&ElemNo,&MyVar);
     };

------------------------------------------------------------------------------------

However, I just made up the "#if typeof" statement

Is there another way of doing this ???

I know I can create another template just for the CString class, but this seems to defeat the purpose of templates...
0
Comment
Question by:smegghead
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
6 Comments
 
LVL 3

Expert Comment

by:Crius
ID: 6983911
Eee, well, bad to use precompiled directives like this in code. Probably wouldn't work well anyway.

Try using COleVariant. It has a constructor that takes in all sorts of different variable types.

    template <class T> void PutToSA(SAFEARRAY* &insa,long ElemNo,T& InVal)
    {
         COleVariant MyVar(T);

         SafeArrayPutElement(insa,&ElemNo,&MyVar);
    };

It also allocates and deallocates memory (And calls the VariantInit) functions automatically. If you allocate a BSTR, it must be deallocated somewhere afterall. :)
0
 
LVL 3

Expert Comment

by:Crius
ID: 6983913
Oops, sorry,

COleVariant MyVar(InVal);

Don't use the type of the variable, just the value. Hehe.
0
 
LVL 30

Accepted Solution

by:
Axter earned 50 total points
ID: 6984072
Here's one method to test for CString.

 template <class T>
bool IsCString(T)
 {
      return false;
 }

bool IsCString(CString)
 {
      return true;
 }

 template <class T>
void FuncXyz(T Src)
{
      if (IsCString(Src))
      {
           AfxMessageBox("Is CString");
      }
      else
      {
           AfxMessageBox("Not CString");
      }
}
0
Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
LVL 4

Expert Comment

by:ct.smith
ID: 6984086
Seems to me like this is a good case for template specialisation:

template <class T> void PutToSA(SAFEARRAY* &insa,long ElemNo,T& InVal)
    {
         VARIANT MyVar;
         VariantInit(&MyVar);
         MyVar.vt=VT_I2;
         MyVar.intVal=InVal;
         SafeArrayPutElement(insa,&ElemNo,&MyVar);
    };

template <CString> void PutToSA(SAFEARRAY* &insa,long ElemNo,CString& InVal)
    {
         VARIANT MyVar;
         VariantInit(&MyVar);
         MyVar.vt=VT_BSTR;
         MyVar.bstrVal=InVal.AllocSysString();
         SafeArrayPutElement(insa,&ElemNo,&MyVar);
    };

Small catch, I don't have VC6 handy to double check my example code, but the spirit of the solution is fairly straightforward.
0
 
LVL 1

Expert Comment

by:andymurd
ID: 6990398
ct.smith is correct, but I think an overload rather than template specialzation is more appropriate.

So you would have:

void PutToSA(SAFEARRAY* &insa,long ElemNo,CString& InVal)
   {
        VARIANT MyVar;
        VariantInit(&MyVar);
        MyVar.vt=VT_BSTR;
        MyVar.bstrVal=InVal.AllocSysString();
        SafeArrayPutElement(insa,&ElemNo,&MyVar);
   };

template <class T> void PutToSA(SAFEARRAY* &insa,long ElemNo,T& InVal)
   {
        VARIANT MyVar;
        VariantInit(&MyVar);
        MyVar.vt=VT_I2;
        MyVar.intVal=InVal;
        SafeArrayPutElement(insa,&ElemNo,&MyVar);
   };

NOTE: The CString version comes BEFORE the template so the compiler will pick that one first if appropriate.
0
 
LVL 10

Author Comment

by:smegghead
ID: 6991840
Hi,

Thanks for your advice everyone, I have, in the interim, gone for the function overload solution, as this works fine.

However, I really wanted a way of branching code depending on the type/class passed in... it seems strange that the compiler doesn't have some way of doing this. It would make code a lot simpler and avoid a lot of duplicate code.

crius... I wasn't suggesting using pre-processor directives, this was just an example of the sort of thing I wanted to do.

I'm inclined to give the points to axter, as he/she came up with a solution I'd not thought of/tried.

So I'm gonna pass the points on to axter...

Sorry everyone else... I do appreciate your suggestions...

Regards

Smg.
0

Featured Post

Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Often, when implementing a feature, you won't know how certain events should be handled at the point where they occur and you'd rather defer to the user of your function or class. For example, a XML parser will extract a tag from the source code, wh…
IntroductionThis article is the second in a three part article series on the Visual Studio 2008 Debugger.  It provides tips in setting and using breakpoints. If not familiar with this debugger, you can find a basic introduction in the EE article loc…
The viewer will learn how to pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

738 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question