C++ mixins via templates: why doesn't this work?

Question

I've got a interface that's implemented as an abstract base class with a number of pure virtual public methods. These pure virtual functions can be implemented using a template since the differences between subclasses aren't large - so my idea was to use multiple inheritance to mix-in the appropriately templated helper-class that provides the implementation. However, the compiler complains that the base class is abstract; it isn't considering the helper mix-in's implementation so thinks there's no implementation of a required method.

For example:

class TrivialList {
    int count;
public:
    TrivialList(int count) : count(count){}
    virtual double Average() const=0;
    int Count() const {return count;}
    virtual ~TrivialList(){}
};
template<typename TIndexable> class AverageHelper {
public:
    double Average() const {
        TIndexable const & self = static_cast<TIndexable const &>(*this);
        double sum=0.0;
        for(int i=0;i<self.Count();++) sum += self.Get(i);
        return sum / self.Count();
    }
};
class IndexableList : public TrivialList, public AverageHelper<IndexableList> {
    std::vector<double> backend;
public:
    IndexableList(int count) : TrivialList(count), backend(count) { }
    double & Get(int i) { return backend[i];}
    double const & Get(int i) const { return backend[i];}
};
IndexableList * MakeList() {return new IndexableList(5);} //error!
//    cannot instantiate abstract class

I'm using MSC 10.0 (Visual Studio 2010); the code fails with a similar error using g++ 4.5.

Get or the real-world equivalents in my project cannot be virtual because they're extremely minor operations that need to be inlined for adequate performance (think put-pixel/get-pixel) - so I need the generic algorithms to be templated rather than generic via virtual function calls.

Answer 1

It doesn't work because AverageHelper<>::Average() doesn't override TrivialList::Average(). In order to override a virtual function, the overriding class must inherit from the class containing the function to be overridden.

You could change your template thus:

template<typename TIndexable, typename Base > 
class AverageHelper : public Base {
public:
  template< typename T >
  AverageHelper(T arg) : Base(arg) {}
  // ... 
};

class IndexableList : public AverageHelper<IndexableList,TrivialList> {
public:
  IndexableList(int count) : AverageHelper<IndexableList,TrivialList>(count) {}
  // ...
};

You might want to virtually derive from TrivialList:

template<typename TIndexable, typename Base > 
class AverageHelper : virtual public Base {
  // ... 
};

Answer 2

For implementing mix-ins via templates, you need the template implementing the abstract function to derive from the abstract base class.

So you may fix your code by changing it the following way:

// ...
template<typename TIndexable> class AverageHelper : public TriviaList{

// ...
class IndexableList : public AverageHelper<IndexableList> {

In general, if you want to provide more than one mix-in, you may either use a virtual inheritance in order not multiplying the instances of the base classes, or to use chain inheritance as in the following sample:

class Abstract {
public:
    virtual void foo() = 0;
    virtual void bar() = 0;
};

template<class Base>
class FooImpl : Base {
public:
    void foo() { /* default foo implementation */ }
};

template<class Base>
class BarImpl : Base {
public:
    void bar() { /* default bar implementation */ }
};

class Derived : public BarImpl<FooImpl<Abstract> > {
    // You have both foo() and bar() implementations available
};