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.