Partial Specialization workaround

[Philip Lawatsch]

Hi

I'd like to implement some kind if type traits myself, but I have to
support broken compilers (like visual studio) that do not support
Partial Specialization.

My first shot was something like this:

----8<---
typedef char IsPODForListArr ayTrue;
typedef struct {char bla[2];} IsPODForListArr ayFalse;

IsPODForListArr ayTrue IsPODForListArr ayDummy(int);
IsPODForListArr ayTrue IsPODForListArr ayDummy(long);

IsPODForListArr ayFalse IsPODForListArr ayDummy(...);

#define IsPODForListArr ay(x) (sizeof(IsPODFo rListArrayDummy (x)) ==
sizeof(IsPODFor ListArrayTrue))

-----8<----

This seems to work, BUT, the problem here is that I cant just pass a
typename to IsPodForListArr ay but rather have to pass something real.

eg. IsPodForListArr ay (int) <-- wont work

int foo; IsPodForListArr ay(foo) <-- works

This is not acceptable, since I do want to use this in places where i
can only use types (eg template parameters).

I do want to implement it myself, and not add any non std. libs.


Any ideas of what I could do ?
I somehow have to work around the need for partial specialization ..


with kind regards Philip

[Philip Lawatsch]

Re: Partial Specialization workaround

[color=blue]
> Like this?
>[/color]

<snip code>


Yea, like this, only that this is partial specialization ....


with kind regards Philip

[Alf P. Steinbach]

Re: Partial Specialization workaround

On Wed, 16 Jul 2003 17:46:55 +0200, Philip Lawatsch <philip@waug.at > wrote:
[color=blue]
>Hi
>
>I'd like to implement some kind if type traits myself, but I have to
>support broken compilers (like visual studio) that do not support
>Partial Specialization.
>
>My first shot was something like this:
>
>----8<---
>typedef char IsPODForListArr ayTrue;
>typedef struct {char bla[2];} IsPODForListArr ayFalse;
>
>IsPODForListAr rayTrue IsPODForListArr ayDummy(int);
>IsPODForListAr rayTrue IsPODForListArr ayDummy(long);
>
>IsPODForListAr rayFalse IsPODForListArr ayDummy(...);
>
>#define IsPODForListArr ay(x) (sizeof(IsPODFo rListArrayDummy (x)) ==
>sizeof(IsPODFo rListArrayTrue) )
>
>-----8<----
>
>This seems to work, BUT, the problem here is that I cant just pass a
>typename to IsPodForListArr ay but rather have to pass something real.
>
>eg. IsPodForListArr ay (int) <-- wont work
>
>int foo; IsPodForListArr ay(foo) <-- works
>
>This is not acceptable, since I do want to use this in places where i
>can only use types (eg template parameters).
>
>I do want to implement it myself, and not add any non std. libs.
>
>
>Any ideas of what I could do ?
>I somehow have to work around the need for partial specialization ..[/color]

I fail to see where partial specialization enters the picture.

Off the cuff:


template< typename T >
struct IsBasicType{ enum{ value = 0 }; };

template<> struct IsBasicType<int > { enum{ value = 1 }; };
template<> struct IsBasicType<lon g> { enum{ value = 1 }; };


Checking for POD'ness is much more involved, but still I don't
see the partial specialization (which is a specialization where
just a subset of the template arguments are bound).

[John Harrison]

Re: Partial Specialization workaround


"Philip Lawatsch" <philip@waug.at > wrote in message
news:3f15780a@e-post.inode.at.. .[color=blue]
>[color=green]
> > Like this?
> >[/color]
>
> <snip code>
>
>
> Yea, like this, only that this is partial specialization ....
>
>
> with kind regards Philip
>[/color]

I don't understand. I understand what partial specialization, but my code
didn't use partial specialization so I don't see what the problem with it
is.

Perhaps if you posted the code that needs to distinguish between POD and
non-POD. Nested template types are often a solution to the lack of partial
specialization.

john

[Philip Lawatsch]

Re: Partial Specialization workaround



John Harrison wrote:

[color=blue]
>
> I don't understand. I understand what partial specialization, but my code
> didn't use partial specialization so I don't see what the problem with it
> is.[/color]

Yea, just realized this !
Sorry, i just saw "template" and didnt read much further, this was
really my fault!

Your code works fine for what i need, so thanks a lot !

with kind regards philip

[Philip Lawatsch]

Re: Partial Specialization workaround



John Harrison wrote:
[color=blue]
> I don't understand. I understand what partial specialization, but my code
> didn't use partial specialization so I don't see what the problem with it
> is.
>
> Perhaps if you posted the code that needs to distinguish between POD and
> non-POD. Nested template types are often a solution to the lack of partial
> specialization.[/color]

One little confusion on my side though
What i have now is:

#define IsPODForListArr ayDefine(x) template <>\
class IsPODForListArr ayDummy<x>\
{\
public:\
enum {Result = FC_True};\
};

template <class T>
class IsPODForListArr ayDummy
{
public:
enum { Result = false };
};

IsPODForListArr ayDefine(FC_Int )
IsPODForListArr ayDefine(FC_Uin t)


#undef IsPODForListArr ayDefine

#define IsPODForListArr ay(x) (IsPODForListAr rayDummy<x>::Re sult)


and this works fine.

But now it will get tricky:

template <class T> class foo;

template <class T>
class IsPODForListArr ayDummy < foo <T> >
{
public:
enum {result = FC_True};
};

This wont work, so what would I want to do in this case ?

with kind regards philip

[Andrey Tarasevich]

Re: Partial Specialization workaround

Philip Lawatsch wrote:[color=blue]
> ...
> Yea, like this, only that this is partial specialization ....
> ...[/color]

Where??? There's no partial specialization in this code. This is
_explicit_ specialization, which is supported by MSVC++ 6.0 compiler (if
that's the one you are talking about).

--
Best regards,
Andrey Tarasevich
Brainbench C and C++ Programming MVP

[Philip Lawatsch]

Re: Partial Specialization workaround



Andrey Tarasevich wrote:
[color=blue]
> Philip Lawatsch wrote:
>[color=green]
>>...
>>Yea, like this, only that this is partial specialization ....
>>...[/color]
>
>
> Where??? There's no partial specialization in this code. This is
> _explicit_ specialization, which is supported by MSVC++ 6.0 compiler (if
> that's the one you are talking about).[/color]

As I said 5 minutes later, i apologize for not thinking while reading
his code. My fault


with kind regards philip

[John Harrison]

Re: Partial Specialization workaround


"Philip Lawatsch" <philip@waug.at > wrote in message
news:3f157d49@e-post.inode.at.. .[color=blue]
>
>
> John Harrison wrote:
>[color=green]
> > I don't understand. I understand what partial specialization, but my[/color][/color]
code[color=blue][color=green]
> > didn't use partial specialization so I don't see what the problem with[/color][/color]
it[color=blue][color=green]
> > is.
> >
> > Perhaps if you posted the code that needs to distinguish between POD and
> > non-POD. Nested template types are often a solution to the lack of[/color][/color]
partial[color=blue][color=green]
> > specialization.[/color]
>
> One little confusion on my side though
> What i have now is:
>
> #define IsPODForListArr ayDefine(x) template <>\
> class IsPODForListArr ayDummy<x>\
> {\
> public:\
> enum {Result = FC_True};\
> };
>
> template <class T>
> class IsPODForListArr ayDummy
> {
> public:
> enum { Result = false };
> };
>
> IsPODForListArr ayDefine(FC_Int )
> IsPODForListArr ayDefine(FC_Uin t)
>
>
> #undef IsPODForListArr ayDefine
>
> #define IsPODForListArr ay(x) (IsPODForListAr rayDummy<x>::Re sult)
>
>
> and this works fine.
>
> But now it will get tricky:
>
> template <class T> class foo;
>
> template <class T>
> class IsPODForListArr ayDummy < foo <T> >
> {
> public:
> enum {result = FC_True};
> };
>
> This wont work, so what would I want to do in this case ?
>
> with kind regards philip
>[/color]

You want to say that any foo<T> is POD? That seems strange because if T is
not POD how can foo<T> be? Indeed can any template be POD? Not sure of the
answer to that one.

In any case looks like you need partial template specialisation again.

john

[Philip Lawatsch]

Re: Partial Specialization workaround



John Harrison wrote:
[color=blue]
>
> You want to say that any foo<T> is POD? That seems strange because if T is
> not POD how can foo<T> be? Indeed can any template be POD? Not sure of the
> answer to that one.[/color]

Well, foo is actually a coordinate class, which only has 3 T inside.
Anything besides PODs would not make sense in this class (and only these
are used)

I know that this is not nice, but i'm currently speed hacking our array
class and got huge performance increase (>300%) when using malloc et al
for these coordinates in comparison to c++ new [newsize]and then copy.

[color=blue]
> In any case looks like you need partial template specialisation again.[/color]
Yea, but I found a workaround.
I typedef'd some of the most likely cases (like double, float and int)
and used these.

Not nice but anyway.

Btw, i've added stuff like

if (IsPOD(T))
{
// speed hack code here
}
else
{
// old code here
}

into the array.

Now the array seems to be slower than befor for the old data types, any
ideas why ?

I thought the compiler would emit the same code as befor for the non pod
types since IsPOD is a compile time decision and every normal
optimizer should get rid of that .. ?


with kind regarsd philip