Add subclasses of templated base-class to container without super-base-class? - c++

Question

I'm trying to create a vector (or any STL container, really) that could hold a set of various objects that are subclasses of one specific type. The problem is that my base class is templated.
From what I can tell, I have to create an interface/abstract super base class (not sure what the preferred C++ terminology is). I'd prefer not to do this, and just use my (templated) abstract base class. Below is some example code.
Basically, is there a way not to require the WidgetInterface? Someway to tell the compiler to ignore template requirements? If I must have WidgetInterface, am I going the right way with the following?
#include <vector>
#include "stdio.h"
enum SomeEnum{
LOW = 0,
HIGH = 112358
};
// Would like to remove this WidgetInterface
class WidgetInterface{
public:
// have to define this so we can call it while iterating
// (would remove from Widget if ended up using this SuperWidget
// non-template baseclass method)
virtual void method() = 0;
};
template <class TDataType>
class AbstractWidget : public WidgetInterface{
public:
TDataType mData;
virtual void method() = 0;
// ... bunch of helper methods etc
};
class EnumWidget : public AbstractWidget<SomeEnum>{
public:
EnumWidget(){
mData = HIGH;
}
void method(){
printf("%d\n", mData); // sprintf for simplicity
}
};
class IntWidget : public AbstractWidget<int>{
public:
IntWidget(){
mData = -1;
}
void method(){
printf("%d\n", mData); // sprintf for simplicity
}
};
int main(){
// this compiles but isn't a workable solution, not generic enough
std::vector< AbstractWidget<int>* > widgets1;
// only way to do store abitary subclasses?
std::vector<WidgetInterface*> widgets2;
widgets2.push_back(new EnumWidget());
widgets2.push_back(new IntWidget());
for(std::vector<WidgetInterface*>::iterator iter = widgets2.begin();
iter != widgets2.end(); iter++){
(*iter)->method();
}
// This is what i'd _like_ to do, without needing WidgetInterface
// std::vector< AbstractWidget* > widgets3;
return 0;
}

Answer 1

No, you can't use directly AbstractWidget as a parameter of STL container or anything else.
The reason is that class AbstractWidget does not exist. It is only a template for compiler to construct classes from.
What exists is AbstractWidget<SomeEnum> and AbstractWidget<int> only because of EnumWidget and IntWidget inheriting from them.
Templates exist at compiler-level only. If AbstractWidget<T> weren't used anywhere in your code, there would be no traces of it during the runtime.
Therefore, the code you posted seems to be the best (if not only) solution for your problem.

Answer 2

What you've done is the solution: you need a common class/interface, and since AbstractWidget is class template, therefore it cannot be used as common class for all concrete classes for which the template argument is different. So I think, you should go with this class design. It seems to be quite reasonable solution.

Answer 3

In fact the classes AbstractWidget<int> and AbstractWidget<double> are different classes, so your class IntWidget is a subclass of the first but is in no relation with the second. You need to have a common parent class to put in the vector so unfortunately you can not avoid the common interface that is not templated.

Answer 4

This could be completely in the wrong direction, but could you do something like this:
template <class T>
class ConcreteWidget : public AbstractWidget<T>
{
};
and then use template specialization to define your specific widgets like this:
template <>
class ConcreteWidget : public AbstractWidget<int>
{
public:
ConcreteWidget() : mData(-1) {}
};
template <>
class ConcreteWidget : public AbstractWidget<SomeEnum>
{
public:
ConcreteWidget() : mData(HIGH) {}
};
So rather than having an IntWidget and an EnumWidget, you'd have a ConcreteWidget and ConcreteWidget and then could simply have a vector<WidgetInterface> that would be the super of all of these generic children?
I'm not sure if this solves your problem, or would even work. I'd love feedback on this answer.