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 CreateABaseHold er()
{
return BaseHolder(new T());
}


int main()
{
BaseHolder h1 = CreateABaseHold er<Dev1>();
BaseHolder h2 = CreateABaseHold er<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]

Re: template confusion

Floogle wrote:[color=blue]
> 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 CreateABaseHold er()
> {
> return BaseHolder(new T());
> }
>
>
> int main()
> {
> BaseHolder h1 = CreateABaseHold er<Dev1>();
> BaseHolder h2 = CreateABaseHold er<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;
> }
> -------------------------------------------------------------------------[/color]

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_atla s_DOT_de

[Floogle]

Re: template confusion

On Mon, 09 Jan 2006 11:24:35 +0100, Stefan Näwe <please@nospam. net>
wrote:
[color=blue]
>Floogle wrote:[color=green]
>> 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 CreateABaseHold er()
>> {
>> return BaseHolder(new T());
>> }
>>
>>
>> int main()
>> {
>> BaseHolder h1 = CreateABaseHold er<Dev1>();
>> BaseHolder h2 = CreateABaseHold er<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;
>> }
>> -------------------------------------------------------------------------[/color]
>
>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[/color]


On planet MSVC6

[Ben Pope]

Re: template confusion

Floogle wrote:[color=blue]
> On planet MSVC6[/color]

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]

Re: template confusion

Floogle wrote:[color=blue]
> On Mon, 09 Jan 2006 11:24:35 +0100, Stefan Näwe <please@nospam. net>
> wrote:
>
>[color=green]
>>Floogle wrote:
>>[color=darkred]
>>>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 CreateABaseHold er()
>>>{
>>> return BaseHolder(new T());
>>>}
>>>
>>>
>>>int main()
>>>{
>>> BaseHolder h1 = CreateABaseHold er<Dev1>();
>>> BaseHolder h2 = CreateABaseHold er<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;
>>>}
>>>-------------------------------------------------------------------------[/color]
>>
>>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[/color]
>
>
>
> On planet MSVC6[/color]

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_atla s_DOT_de

[Floogle]

Re: template confusion

On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
<benpope81REMOV E@gmail.com> wrote:
[color=blue]
>Floogle wrote:[color=green]
>> On planet MSVC6[/color]
>
>http://msdn.microsoft.com/vstudio/ex...c/default.aspx
>
>It's free!
>
>Ben Pope[/color]


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

[Gernot Frisch]

Re: template confusion

[color=blue]
> I'm confused, too!
> It's probably one more of those annoying MSVC stupidities...[/color]

Lokks like a bug rather than a stupidity.

[Ben Pope]

Re: template confusion

Floogle wrote:[color=blue]
> On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
> <benpope81REMOV E@gmail.com> wrote:
>[color=green]
>> Floogle wrote:[color=darkred]
>>> On planet MSVC6[/color]
>> http://msdn.microsoft.com/vstudio/ex...c/default.aspx
>>
>> It's free!
>>
>> Ben Pope[/color]
>
>
> Alas, it's not that simple. It has to work using MSVC6... :o([/color]

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]

Re: template confusion

Gernot Frisch wrote:[color=blue][color=green]
>>I'm confused, too!
>>It's probably one more of those annoying MSVC stupidities...[/color]
>
>
> Lokks like a bug rather than a stupidity.
>
>[/color]

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<typena me T>
struct TypeHolder
{
typedef T OriginalType;
};

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

int main()
{
BaseHolder h1 = CreateABaseHold er(TypeHolder<D ev1>());
BaseHolder h2 = CreateABaseHold er(TypeHolder<D ev2>());

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_atla s_DOT_de

[Bo Persson]

Re: template confusion


"Floogle" <BananaSplits@N OSPAM.com> skrev i meddelandet
news:fgh4s1l5a5 t76cnmgqblfuq7p rmo7a95p6@4ax.c om...[color=blue]
> On Mon, 09 Jan 2006 11:05:13 +0000, Ben Pope
> <benpope81REMOV E@gmail.com> wrote:
>[color=green]
>>Floogle wrote:[color=darkred]
>>> On planet MSVC6[/color]
>>
>>http://msdn.microsoft.com/vstudio/ex...c/default.aspx
>>
>>It's free!
>>
>>Ben Pope[/color]
>
>
> Alas, it's not that simple. It has to work using MSVC6... :o([/color]


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 CreateABaseHold er()
{
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 CreateABaseHold er(const T* dummy = 0)
{
return BaseHolder(new T());
}

will work!


Bo Persson