Re: Templates HowTo?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 22 Feb 2008 10:42:35 +0100

Message-ID:

<13rt68h4isbn31c@corp.supernews.com>

* Alf P. Steinbach:

* James Kanze:

On Feb 21, 10:20 am, "Alf P. Steinbach" <al...@start.no> wrote:

* ke...@bytebrothers.co.uk:

I must confess I'm completely new to templates, having
mainly used C++ for quite a while as 'C with Classes', but
now I have an application which seems to cry out for
templates, but I just can't get my head around how to use
them in this situation. Any assistance or pointers to other
resources would be welcomed.

Let's say I have a (third party, unmodifiable) library of C
functions which provide the following interfaces:

int AAA_setup(const uint8_t* p1, int p2, struct context* p3);
int AAA_process(const uint8_t* p1, uint8_t* p2, struct context* p3);
int AAA_done(struct context* p1);

int BBB_setup(const uint8_t* p1, int p2, struct context* p3);
int BBB_process(const uint8_t* p1, uint8_t* p2, struct context* p3);
int BBB_done(struct context* p1);

int CCC_setup(const uint8_t* p1, int p2, struct context* p3);
int CCC_process(const uint8_t* p1, uint8_t* p2, struct context* p3);
int CCC_done(struct context* p1);

What I want to do is provide a class wrapper for all this so that in
my C++ application I can do something like:

Work<BBB> thing;
thing.setup(&data1, val);
thing.process(&in, &out);
thing.done();

(The pointer to the context structure can obviously disappear as
private data within the class)

Now this seems from my reading to be the sort of thing that templates
are probably good at, but I can't for the life of me see how to do it,
without first creating a separate class for each of AAA, BBB, and CCC.

Can anyone enlighten me please?

Each of group AAA, BBB and CCC essentially constitute a functor object.

   class AbstractWork
   {
   public:
       virtual ~AbstractWork() {}
       virtual int operator( whatever ) = 0;
   };

You've lost me there. Each group proposes three functions, not
one.

Each group essentially consists of initialization, do-it, and
destructor, and that's a functor.

(And you never use AbstractWork later, either. Is it
maybe part of an idea you had to start with, that didn't pan
out, and you forgot to delete it?)

It's a means to be able to treat the functors polymorphically.

Oh, I see. Da keybrd divel, igain. The operotr should of course be
operator(), as noted elsewhere (hah, I saw that!), and the later
WorkWrapper class should derive from AbstractWork (obisouskly).

Cheers,

- Alf

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