[Q] Template: Undefined Constructors

[Eric]

Ok, this is very strange.

in TRegionGrower.h, I've got:

--
template <class T>
class TRegionGrower
{
public:

TRegionGrower( void );
~TRegionGrower( void );
};
--

In TRegionGrower.cp, I've got:

--
#include "TRegionGrower.h"

template <class T>
TRegionGrower<T>::
TRegionGrower( void )
{

}



template <class T>
TRegionGrower<T>::
~TRegionGrower( void )
{

}
--

I use this class by doing:

TRegionGrower<UInt8> regionGrower;

I get two link errors telling me that the constructor and destructor are
undefined.

However, if I remove the constructor and destructors from the .cp file
and change the .h file to:

--
template <class T>
class TRegionGrower
{
public:

TRegionGrower( void )
{
}

~TRegionGrower( void )
{
}
};

 

[Russell Hanneken]

I get two link errors telling me that the constructor and destructor are
undefined.

However, if I remove the constructor and destructors from the .cp file
and change the .h file to:

template <class T>
class TRegionGrower
{
public:

TRegionGrower( void )
{
}

~TRegionGrower( void )
{
}
};

The link errors go away.

The FAQ addresses this issue:

http://www.parashift.com/c++-faq-lite/containers-and-templates.html#faq-34.12

 

[Claudio Jolowicz]

Ok, this is very strange.

in TRegionGrower.h, I've got:

--
template <class T>
class TRegionGrower
{
public:

TRegionGrower( void );
~TRegionGrower( void );
};
--

In TRegionGrower.cp, I've got:

--
#include "TRegionGrower.h"

template <class T>
TRegionGrower<T>::
TRegionGrower( void )
{

}



template <class T>
TRegionGrower<T>::
~TRegionGrower( void )
{

}
--

I use this class by doing:

TRegionGrower<UInt8> regionGrower;

I get two link errors telling me that the constructor and destructor are
undefined.

1. How about giving us the definition of UInt8? With a sensible typedef,
there shouldn't be any errors. If its a macro, anything is possible.

2. Is the last line of code in a separate source file or in
TRegionGrower.cp?

3. What does the command line to (compile and) link the files look like?

 

[Michiel Salters]

Ok, this is very strange.

in TRegionGrower.h, I've got:

--
template <class T>
class TRegionGrower
{
public:

TRegionGrower( void );
~TRegionGrower( void );
};

[ the obvious implementation ]

The FAQ reference in the previous post is correct, although there
currently is an alternative solution. These templates need extra
bookkeeping, as the FAQ explains. This is enabled using the export
keyword on the class, but it is today only supported on the Comeau
compiler.

Regards,
Michiel Salters