Using a template for an internal method only

KeetL
KeetL used Ask the Experts™
on
Is it possible to use a template for an internal method "only" without defining the class to be a template class itself....

My Compilers (M$VC6,gcc,cxx) don't seem to think so. They all compile but have trouble finding the method signature during link time.

class LibLoader
{
public:
  static  LibLoader* Instance ();
 
  bool    doesLibExist     (const char* libName) const;
  TLIB_ID loadLib          (const char* libName);
  bool    unLoadLib        (TLIB_ID libID);
 
  template <typename T>
  T   loadSymbol       (const char* symbolName, TLIB_ID libID,T a) const;
}

As you can probably tell the class is a dynamic library loader which is also a singleton.

The original problem stemed from loading the symbol to a correct type (void*) -> (definable function pointer). I achieved this easily with a simple template function however my efforts to clean it all up and put it in the class are proving problematic.


This is what i would ideally like to have....

//load library
TLIB_ID libID = LibLoader->loadLib("msvcrt.dll");

// define symbol type
typedef int (*myFunctionType)(const char*);

// create a variable for use    
myFunctionType a = 0;

// load the symbol with the help of a template method nested inside a non-templated class
a  = LibLoader->loadSymbol("atoi",libID,a);
   
I32 x = a("1234");

This is what works for now with the help of the templated function FuncEntry(.....) as defined below

template <class T>
T FuncEntry(T a,void *vEntry)
{
  long lEntry = ACE_reinterpret_cast (long, vEntry);
  return ACE_reinterpret_cast (T, lEntry);
}


void *entry;
   
typedef int (*myFunctionType)(const char*);
   
myFunctionType a = 0; // just to initialise b 4 use

TLIB_ID libID = LibLoader->loadLib("msvcrt.dll");
   
entry  = LibLoader->loadSymbol("atoi",libID);
   
a = FuncEntry(a,entry);

int x = a("1234");

/////////////////////////\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

Cheers


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Commented:
VS6 fixed this

class C{

public:

     template <typename T>
     T add(T value) const
     {
          return value + value;    
     }
};

void main()
{    
     C c;
        int a = 10;
     a = c.add(a);
}

Commented:
your template method:

 template <typename T>
 T   loadSymbol(const char* symbolName, TLIB_ID libID,T a)const;

The argument a is of type T and you are returning same type.

When I read further down, you have declared a function pointer with const char* argument and int as return value.

So to make your template to work, it should be:

template <typename T>
 int   loadSymbol(const char* symbolName, TLIB_ID libID,T a)const;

or why not declare template like this:
template<typename T, typename U>
U loadSymbol(const char*, symbolName, TLIB_ID libID, T a) const;


Commented:
discard the last template with two types, it won't work.
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Author

Commented:
The problem is that the symbol signature is going to change every time. I only used atoi as an example and seeing that atoi's sig is
 
int (__cdecl*)(const char*)

that is why you cannot embed the types into the method...

The real problem here is using templates inside a class without defining the class to be a template class. This also must be done in a platform independent manner.  

I.E. This means abiding by ANSI c++ guidelines.

The first comment Kimpan is cool if it works on VC++ however it doesn't seem to work on my vc++ (w/sp3) as i outlined in my previos post nor does it work with gcc or cxx. I assume i'm breaking ANSI rules however I'm just curious to see if there is a way to get around this.

I just realised somthing. My implementaion was outside of the header defenition. I've just tested my old code using the code inside the header and all works fine on vc++... but now I'm not happy about code being in a def file... :<

There's never a totally happy moment in coding.

Author

Commented:
Here's the error as it appears if i put the implementation in the cpp file... if it goes in the header i get no problems....

any ideas anyone....???


Linking...
TestBase.obj : error LNK2001: unresolved external symbol "public: int (__cdecl*__thiscall TeccoLibLoader::loadSymbol(char const *,int,int (__cdecl*)(char const *)))(char const *)" (?loadSymbol@TeccoLibLoader@@QAEP6AHPBD@Z0HP6AH0@Z@Z)
..\..\bin\test1.exe : fatal error LNK1120: 1 unresolved externals

Here's the header def
class LibLoader
{
template <typename T>
T  loadSymbol       (const char* symbolName, TLIB_ID libID,T a);
}

and the impl

template <typename T>
T LibLoader::loadSymbol(const char* symbolName, TLIB_ID libID,T a)
{
  void* vEntry;
  long  lEntry;
 
  vEntry = loadSymbol(symbolName,libID);
  lEntry = ACE_reinterpret_cast (long, vEntry);
 
  return ACE_reinterpret_cast (T, lEntry);
}

Why oh why???

Commented:
eeh, templates are declarations and they must be in in the headerfile. Am I right?

Author

Commented:
you may just be.... my C++ primer 3rd edition says nothing of it but it does say templates are defenitions... does this mean i can make a .tpl file and inlcude it in the header....

I've seen people creating a template file before for template                
implementations... is this the reason...???

Author

Commented:
Well here's my problem for anyone interested...

templates are not functions ... duh!.. they can't be compiled seperately. Templates have to be used in conjunction with requests for particular instatiation of templates...

They must be included with the header file.

Only newer compilers implementing the export keyword can do have template methods in the implemenation file. How they do this my book doesn't say so anyone else who wishes to elaborate further on this issue please stand up.

Commented:
My instructor in a course I had recently told me that some compilers support having templates in ~headerfiles. You use export keyword. As the link tells you Comeau Compiler allows that.

Author

Commented:
cheers Kimpan...

Thanks for the pointers...

Commented:
This question didn't show any activity for more than 21 days. I will ask Community Support to close it unless you finalize it yourself within 7 days.
You can always request to keep this question open. But remember, experts can only help if you provide feedback to their comments.
Unless there is objection or further activity,  I will suggest to accept

    "Kimpan"

comment(s) as an answer.

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