Re: breaking template parameter dependence

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 3 Dec 2007 09:47:55 -0500
Message-ID:
<fj14us$j3l$1@news.datemas.de>
er wrote:

On Nov 30, 10:43 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

er wrote:

I have a class D<INT> which serves a small implementation purpose
(think for example D<INT> is Factorial<INT>). I don't want every
class that uses D<INT> to depend on template parameter INT. However
I know that I won't need more than say INT=5. So I do 2 things:
a) have D derive from a base class B (see below)
b) have a class Get with a member B& instance_of_B(unsigned int INT)
(see below). However I can't put references to B into an vector


You can't put references to anything in a vector. You can, however,
put _pointers_ to your objects in a vector, because while your D<>
are singletons, pointers to it can be multiplied at will. So can
pointers to B, which still provide polymorphism, don't they?


So far, I have created

class Wrap: public B{ private: B& ref;} which is copyable/assignable


Assignable? Really?

    class A {};
    class B { A& a; public: B(A& a) : a(a) {} };

    #include <vector>

    int main() {
        std::vector<B> vb;
        std::vector<B> vbb;
        A a;
        B b(a);
        vb.push_back(b);
        vbb = vb;
    }

so I can put it in a vector. Usually I'd use a pointer rather than a
reference, but in this case, the reference is guaranteed to refer to
an object at all time (i think?)


I don't think so. Please post your real code so I can see that it
is really assignable.

In the future, will you please specify how your class is going to be
used? You give some abstract representation of some idea you have,


Thanks, I will try to clarify, now and in the future.

D<INT> can be thought of as Factorial<INT> (INT!). Although it's more
complicated, I would have the same problem if it were in fact
Factorial<INT>. The client of this class, say A, represents a sequence
of monomials (computed recursively via horner's rule) whose
coefficients are D<INT>.


I guess I haven't explained my problem clearly. "How your class is
going to be used" is illustrated by the code that would construct
instances of your class, use the class interface, store the class
instances somewhere, etc. *Not* what you think they are going to
mean, but how they are going to be interacting with other parts of
your solution.

Anyhow, my point is that I find it cumbersome to use template
parameters for every part of the code that needs a factorial
computation.


I don't see you use any template parameters.

I don't want to define A<INT>, I want to define A::A(unsigned int
INT). The solution I propose is to save the results obtained at
compile time for D<0>....D<N> and return any of these values on demand
(hence Get). But I find the whole thing cumbersome, and I'm wondering
if there's a simpler alternative.


Again, I don't see how 'A' is used in the rest of the code. I can
try to imagine, but that requires me to invent a problem that is
going to be solved with 'A' instead of learning of your proposed
solution for *your* problem.

[..]

What's wrong with

    vector<B*>


The way that D is set up, I can only get a reference (B&) to a
singleton by calling D<INT>::instance(). You mean I should do B* p =
&D<INT>::instance()?


Probably.

[..]


V
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