Question

I need to call a virtual method for all classes derived from a given base base class right after the construction of the derived object. But doing so in the base class constructor will result in a pure virtual method call

Here is a simplified example:

struct Loader {
    int get(int index) { return 0; }
};

struct Base{
    Base() {
        Loader l; 
        load( l ); // <-- pure virtual call!
    }
    virtual void load( Loader & ) = 0;
};

struct Derived: public Base {
    int value;
    void load( Loader &l ) {
        value = Loader.get(0);
    }
};

I can call load at the Derived constructor, but Derived could not know how to create a Loader. Any ideas/workarounds?

Answer 1

Use the PIMPL pattern:

template<typename T>
class Pimpl
{
    public:
        Pimpl()
        {
            // At this point the object you have created is fully constructed.
            // So now you can call the virtual method on it.
            object.load();
        }
        T* operator->()
        {
            // Use the pointer notation to get access to your object
            // and its members.
            return &object;
        }
    private:
        T    object;   // Not technically a pointer
                       // But otherwise the pattern is the same.
                       // Modify to your needs.
};

int main()
{
    Pimpl<Derived>   x;
    x->doStuff();
}