CODEWITHSUNDEEP
c++templatesstlstaticmember
user1186196
user1186196

Reputation:

Using static variables with templates

I am trying to create a template class which contains a static list of objects which there is only one of. What i have so far works but it gives me a copy of "mylist" for each different type of class B parameter. How can i change it so that i get one "mylist" for all instantiations of class B regardless of template parameters?

This is what i have:

template <class T> class A {
    ...
};
template <class T> class B {
    static list<A<T> > mylist;
    ...
};
template <class T> list< A<T> > B<T>::mylist;

Thanks in advance :)

Upvotes: 2

Views: 210

Answers (2)

Drew Dormann
Drew Dormann

Reputation: 63946

How can change it so that i have only ONE copy

You can inherit from a common (non-templated) base class to ensure that there isn't an instance for every template instantiation.

... that contains any type?

That depends on your expectations. I'm going to assume from your example that "any type" means "any instantiation of template class A".

Since those types could vary in size, you'll do best with holding pointers to the objects.

This is one example of a solution that would solve both those problems.

class ACommon {
    // Put a common virtual interface here.
};

template <class T> class A : public ACommon {
};

class BCommon {
    // You'll have a single, common static here.

    static list<shared_ptr<ACommon> > mylist;
};

template <class T> class B : public BCommon{

};

Upvotes: 4

enobayram
enobayram

Reputation: 4708

You can not "just" have ANY type in a list, living side by side. Depending on what you expect to do with that list, and the objects in it, there are several things to do.

An alternative to Drew's solution would be: You definitely need BCommon if you want a single list for all instantiations. If you don't want to hide your A types behind an interface, you could use Boost::any or Boost::variant to hold your A objects.

Upvotes: 1

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