Re: Separate Template Definition I wrote class Data in header. The C++ Compiler compiled without errors. I decided to move all member functions into source code because they are for implementation. I do not like that they are placed in class body.

Immortal Nephi <>
Thu, 29 Jul 2010 12:50:07 -0700 (PDT)
On Jul 29, 2:13 pm, Paul Bibbings <> wrote:

On Jul 29, 7:26 pm, Immortal Nephi <> wrote:

On Jul 29, 1:06 pm, Paul Bibbings <> wrote:

Jonathan Lee <> writes:

On Jul 29, 12:39 pm, Immortal Nephi <>

        I wrote class Data in header. The C++ Compiler =

compiled without

errors. I decided to move all member functions into source code
because they are for implementation. I do not like that they ar=


placed in class body.

        I got error message:

Main.obj : error LNK2001: unresolved external symbol "public:
__thiscall Data<unsigned char>::operator unsigned char(void)" (??B=


C:\Main.exe : fatal error LNK1120: 1 unresolved externals

        Could be problem with member function cast operato=

r. All other

member functions complied without any problems.

GCC compiled this without a complaint, so it's legal-ish (?).
BUT you have 3 specializations declared in data.cpp without
definitions. Maybe MS is expecting the definitions.

(I actually have no idea if this is legal or not since I've
never declared a specialization and not immediately implemented

Is this right, what you are proposing here? If you look back at th=

e OP's

code, what he provided were not specializations at all, but rather
*explicit instantiations*, including:

   template< typename T > // primary =


   Data< T >::operator unsigned char() {
      return x;

   template =

  // explicit inst. for uchar

   Data< unsigned char >::operator unsigned char();

I'm not that up to speed on this because I have never found myself us=


explicit instantiations. Still, it is not my /expectation/ that any
further `implementation' is required here. The OP's code is, surel=


merely saying "I have this template. I shall be needing to use it on
Data< unsigned char>. Instantiate the relevant member functions (ctor=


dtor, op uchar) for me." Of course, this is necessary here, owing =

to the

fact that the primary definitions are themselves present in a source,
rather than a header, file. Beyond that, I'm not immediately seein=


that there is any problem with this.

Do Google and look for Q239436. It answers your question why Visual =


+ 2008 fails to link template member function.

I think that there is a problem with VC over this. If you compile
only your
single file Data.cpp and then run objdump -t Data.obj, you will see
there is no symbol for the explicitly instantiated conversion operator
the table. On the other hand, even though we already have the
that it has no problem with explicit instantiations for the ctor and
add a member function:

   // Data.h
   template< typename T >
   class Data {
      // ...
      unsigned char get() const;

You can add many member functions in source code you like.

   // Data.cpp
   template< typename T >
   unsigned char Data<T>::get() const {
      return x;

Don't add that line below.

   unsigned char Data< unsigned char >::get() const;

I successfully resolved linkage failure. I fixed template class
specification in source code. Cast operator function is working

    Add line below.

class Data< unsigned char >;

    ...Before...add many member functions.

Review my code again.

// Data.h
#if !defined(DATA_H )
#define DATA_H

template< typename T >
class Data {

    operator unsigned char ();

    unsigned char x;

#endif // end macro !defined( DATA_H )

// Data.cpp

class Data< unsigned char >;

template< typename T >
Data< T >::Data() : x( 5 ) {}

template< typename T >
Data< T >::~Data() {}

template< typename T >
Data< T >::operator unsigned char () {
    return x;

// main.cpp

int main() {
    Data< unsigned char> data;
    unsigned char x = data;

    return 0;

    MS Visual C++ Compiler compiled successfully without errors.

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