template confusion

[Floogle]

I cannot figure out why the following program instantiates two type
"2" instances instead of one "1" and one "2".

Could someone plz enlighten me?

Many thanks

Here's the code

-------------------------------------------------------------------------------
#include <iostream>
using namespace std;

class Base
{
public:

virtual int GetType()const=0;
};

class Dev1 : public Base
{
public:

int GetType()const{return 1;}
};

class Dev2 : public Base
{
public:

int GetType()const{return 2;}
};

struct BaseHolder
{
Base* pBase;

BaseHolder(Base* pB):pBase(pB)
{}
};

template <class T>
BaseHolder CreateABaseHolder()
{
return BaseHolder(new T());
}
int main()
{
BaseHolder h1 = CreateABaseHolder<Dev1>();
BaseHolder h2 = CreateABaseHolder<Dev2>();

cout << "\nID of base in h1 (should be == 1) = " <<
h1.pBase->GetType();
cout << "\nID of base in h2 (should be == 2)= " <<
h2.pBase->GetType();

return 0;
}
-------------------------------------------------------------------------

[Stefan Näwe]

Floogle wrote:

I cannot figure out why the following program instantiates two type
"2" instances instead of one "1" and one "2".

Could someone plz enlighten me?

Many thanks

Here's the code

-------------------------------------------------------------------------------
#include <iostream>
using namespace std;

class Base
{
public:

virtual int GetType()const=0;
};

class Dev1 : public Base
{
public:

int GetType()const{return 1;}
};

class Dev2 : public Base
{
public:

int GetType()const{return 2;}
};

struct BaseHolder
{
Base* pBase;

BaseHolder(Base* pB):pBase(pB)
{}
};

template <class T>
BaseHolder CreateABaseHolder()
{
return BaseHolder(new T());
}
int main()
{
BaseHolder h1 = CreateABaseHolder<Dev1>();
BaseHolder h2 = CreateABaseHolder<Dev2>();

cout << "\nID of base in h1 (should be == 1) = " <<
h1.pBase->GetType();
cout << "\nID of base in h2 (should be == 2)= " <<
h2.pBase->GetType();

return 0;
}
-------------------------------------------------------------------------

I get this:
$ ./baseholder

ID of base in h1 (should be == 1) = 1
ID of base in h2 (should be == 2) = 2
On which planet did you compile this?

/S
--
Stefan Naewe
naewe.s_AT_atlas_DOT_de

[Floogle]

On Mon, 09 Jan 2006 11:24:35 +0100, Stefan Näwe <pl****@nospam.net>
wrote:

Floogle wrote:

I cannot figure out why the following program instantiates two type
"2" instances instead of one "1" and one "2".

Could someone plz enlighten me?

Many thanks

Here's the code

-------------------------------------------------------------------------------
#include <iostream>
using namespace std;

class Base
{
public:

virtual int GetType()const=0;
};

class Dev1 : public Base
{
public:

int GetType()const{return 1;}
};

class Dev2 : public Base
{
public:

int GetType()const{return 2;}
};

struct BaseHolder
{
Base* pBase;

BaseHolder(Base* pB):pBase(pB)
{}
};

template <class T>
BaseHolder CreateABaseHolder()
{
return BaseHolder(new T());
}
int main()
{
BaseHolder h1 = CreateABaseHolder<Dev1>();
BaseHolder h2 = CreateABaseHolder<Dev2>();

cout << "\nID of base in h1 (should be == 1) = " <<
h1.pBase->GetType();
cout << "\nID of base in h2 (should be == 2)= " <<
h2.pBase->GetType();

return 0;
}
-------------------------------------------------------------------------

I get this:
$ ./baseholder

ID of base in h1 (should be == 1) = 1
ID of base in h2 (should be == 2) = 2
On which planet did you compile this?

/S

On planet MSVC6

[Ben Pope]

Floogle wrote:

On planet MSVC6

http://msdn.microsoft.com/vstudio/ex...c/default.aspx

It's free!

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[Stefan Näwe]

Floogle wrote:

On Mon, 09 Jan 2006 11:24:35 +0100, Stefan Näwe <pl****@nospam.net>
wrote:

Floogle wrote:

I cannot figure out why the following program instantiates two type
"2" instances instead of one "1" and one "2".

Could someone plz enlighten me?

Many thanks

Here's the code

-------------------------------------------------------------------------------
#include <iostream>
using namespace std;

class Base
{
public:

virtual int GetType()const=0;
};

class Dev1 : public Base
{
public:

int GetType()const{return 1;}
};

class Dev2 : public Base
{
public:

int GetType()const{return 2;}
};

struct BaseHolder
{
Base* pBase;

BaseHolder(Base* pB):pBase(pB)
{}
};

template <class T>
BaseHolder CreateABaseHolder()
{
return BaseHolder(new T());
}
int main()
{
BaseHolder h1 = CreateABaseHolder<Dev1>();
BaseHolder h2 = CreateABaseHolder<Dev2>();

cout << "\nID of base in h1 (should be == 1) = " <<
h1.pBase->GetType();
cout << "\nID of base in h2 (should be == 2)= " <<
h2.pBase->GetType();

return 0;
}
-------------------------------------------------------------------------

I get this:
$ ./baseholder

ID of base in h1 (should be == 1) = 1
ID of base in h2 (should be == 2) = 2
On which planet did you compile this?

/S

On planet MSVC6

OK.
I initially tried that with GCC 3.4.4 from Cygwin.

The test with MSVC gave me:

$ ./baseholder.exe

ID of base in h1 (should be == 1) = 2
ID of base in h2 (should be == 2) = 2
I'm confused, too!
It's probably one more of those annoying MSVC stupidities...
/S
--
Stefan Naewe
naewe.s_AT_atlas_DOT_de

[Floogle]

On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
<be*************@gmail.com> wrote:

Floogle wrote:

On planet MSVC6

http://msdn.microsoft.com/vstudio/ex...c/default.aspx

It's free!

Ben Pope

Alas, it's not that simple. It has to work using MSVC6... :o(

[Gernot Frisch]

I'm confused, too!
It's probably one more of those annoying MSVC stupidities...

Lokks like a bug rather than a stupidity.

[Ben Pope]

Floogle wrote:

On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
<be*************@gmail.com> wrote:

Floogle wrote:

On planet MSVC6

http://msdn.microsoft.com/vstudio/ex...c/default.aspx

It's free!

Ben Pope

Alas, it's not that simple. It has to work using MSVC6... :o(

It never is. I have an MSVC6 program, having spent the time to port it
to MSVC2005 it now behaves "strangely".

With MSVC6 you are restricting your template abilities considerably...
Perhaps you can poke around within boost code for MSVC6 workarounds.

Good luck.

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[Stefan Näwe]

Gernot Frisch wrote:

I'm confused, too!
It's probably one more of those annoying MSVC stupidities...

Lokks like a bug rather than a stupidity.

It's a stupidity when you can get around it:

<----CODE---->

#include <iostream>
using namespace std;

class Base
{
public:
virtual ~Base() { }
virtual int GetType()const=0;
};

class Dev1 : public Base
{
public:

int GetType()const{return 1;}
};

class Dev2 : public Base
{
public:

int GetType()const{return 2;}
};

struct BaseHolder
{
Base* pBase;

BaseHolder(Base* pB):pBase(pB)
{}
};

template<typename T>
struct TypeHolder
{
typedef T OriginalType;
};

template <class T>
BaseHolder CreateABaseHolder(TypeHolder<T>)
{
return BaseHolder(new T());
}

int main()
{
BaseHolder h1 = CreateABaseHolder(TypeHolder<Dev1>());
BaseHolder h2 = CreateABaseHolder(TypeHolder<Dev2>());

cout << "\nID of base in h1 (should be == 1) = " << h1.pBase->GetType();
cout << "\nID of base in h2 (should be == 2) = " << h2.pBase->GetType();

return 0;
}

<----/CODE---->

/S
--
Stefan Naewe
naewe.s_AT_atlas_DOT_de

[Bo Persson]

"Floogle" <Ba**********@NOSPAM.com> skrev i meddelandet
news:fg********************************@4ax.com...

On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
<be*************@gmail.com> wrote:

Floogle wrote:

On planet MSVC6

http://msdn.microsoft.com/vstudio/ex...c/default.aspx

It's free!

Ben Pope

Alas, it's not that simple. It has to work using MSVC6... :o(

It is a known bug in VC6, that function templates that don't use the
template as a function parameter will confuse the linker. If you have
several overloads, one of them will be used (at random).

template <class T>
BaseHolder CreateABaseHolder()
{
return BaseHolder(new T());
}

doesn't work (as you have noticed).

The solution is to add a dummy parameter of type T, to generate a
different internal name.

template <class T>
BaseHolder CreateABaseHolder(const T* dummy = 0)
{
return BaseHolder(new T());
}

will work!
Bo Persson