Question

I have a base class that updates an extern reference, and I want to build an inherited class that would embed this reference as a member. A kind of default initialization of the reference.

I came up with the following solution:

#include<iostream>

class Statefull
{
public:
    Statefull( int& ref ) : _base_ref(ref) {}
    int& _base_ref;
    // update the extern variable
    void work() { std::cout << ++_base_ref << std::endl; }
};

class Stateless : public Statefull
{
public:
    // use a temporary allocation
    Stateless( int* p = new int() ) :
        // we cannot initialize local members before base class:
        // _dummy(), Statefull(_dummy)
        // thus, initialize the base class on a ref to the temporary variable
        Statefull(*p),
        _tmp(p),
        _dummy()
    {
        // redirect the ref toward the local member
        this->_base_ref = _dummy;
    }
    int* _tmp;
    int _dummy;
    // do not forget to delete the temporary
    ~Stateless() { delete _tmp; }
};

int main()
{
    int i = 0;
    Statefull full(i);
    full.work();

    Stateless less;
    less.work();
}

But the need of a temporary allocation in a default argument of the constructor seems quite ugly. Is there a more elegant way to achieve this kind of default initialization while keeping a reference in the base class constructor?

Answer 1

I think this might work:

StateLess(): Statefull(*new int) {}
~StateLess() { delete &_base_ref; }

You can’t do without temporaries, but they don’t have to be in the classes definitions.