STL typedefs and base class pointer problem

[emerth]

Suppose I have these 4 typedefs, where each of the 2nd parameter in
each is a simple class that just contains a different number of strings:

typedef map < char*, cctrlrec_data, ltstr > ctrlrec_table2;
typedef map < char*, ctyperec_data, ltstr > typerec_table2;
typedef map < char*, chotrec_data, ltstr > hotrec_table2;
typedef map < char*, cmtxtrec_data, ltstr > mtxtrec_table2;

I have a stream of XML data coming across a socket and I want to parse
it out into instances of cctrlrec_data, ctyperec_data, chotrec_data, or
cmtxtrec_data, which then will be stuffed into the appropriate typedef'd
map.

My parser will tell me which kind of object to instantiate, and
it produces the data for a number of each in series but does mix
them up in series.

eg: parser output is:
-- start parsing
cctrlrec_data strings
..
..
..
cctrlrec_data strings
ctyperec_data strings
..
..
..
ctyperec_data strings
chotrec_data strings
..
..
..
chotrec_data strings
cmtxtrec_data strings
..
..
..
cmtxtrec_data strings
-- done parsing

I'd like to have a pointer that I could just change to point at the
appropriate map when the parser code starts producing a different
type of output object.

A pointer to the base class seems ideal for this.

But how does one declare a pointer to the base class when that is a
template class? Can one do this? Am I completely out to lunch, & should
consider some other method of doing this? ;-)

TIA!
Eric

[root]

Re: STL typedefs and base class pointer problem

[color=blue]
>
> I'd like to have a pointer that I could just change to point at the
> appropriate map when the parser code starts producing a different
> type of output object.[/color]

Just curious, how do u plan to use this ptr in your code?
[color=blue]
>
> A pointer to the base class seems ideal for this.
>[/color]

[John Dibling]

Re: STL typedefs and base class pointer problem

On 7 Aug 2003 14:44:58 -0700, emerth@hotmail. com (emerth) wrote:
[color=blue]
>Suppose I have these 4 typedefs, where each of the 2nd parameter in
>each is a simple class that just contains a different number of strings:
>
>typedef map < char*, cctrlrec_data, ltstr > ctrlrec_table2;
>typedef map < char*, ctyperec_data, ltstr > typerec_table2;
>typedef map < char*, chotrec_data, ltstr > hotrec_table2;
>typedef map < char*, cmtxtrec_data, ltstr > mtxtrec_table2;[/color]

The keys should probably be something like std::strting, or you'll
BURG.
[color=blue]
>A pointer to the base class seems ideal for this.[/color]

You are wanting to use STL polymorphically . You can't; STL isn't
object-oriented.
[color=blue]
>But how does one declare a pointer to the base class when that is a
>template class? Can one do this? Am I completely out to lunch, & should
>consider some other method of doing this? ;-)[/color]

Consider a new method. Here's the thing: ctrlrec_table2 (etc) has no
base class; at least not one defined by the standard. (eg, if your
implementation of std::map is derived from something, that's
implementation-specific) Templated types aren't derived from base
classes. Each concrete instance of a template is it's own, seperate
thing.

Here's an example:

<code>
template<typena me T>
class templ
{
void f();
};

typedef templ<float> my_float_templ;
</code>

my_float_templ doesn't have a base class. It's not derived from
anything.

</dib>
John Dibling
Witty banter omitted for your protection

[emerth]

Re: STL typedefs and base class pointer problem

> template<typena me T>[color=blue]
> class templ
> {
> void f();
> };
>
> typedef templ<float> my_float_templ;
> </code>
>
> my_float_templ doesn't have a base class. It's not derived from
> anything.[/color]

Yes, I understand. Your example is most helpful. Thankyou very much!