When is C++ covariance the best solution?

Question

This question was asked here a few hours ago and made me realise that I have never actually used covariant return types in my own code. For those not sure what covariance is

Answer 1

I think covariance can be useful when declaring factory methods that return a specific class and not its base class. This article explains this scenario quite well, and includes the following code example:

class product
{
    ...
};

class factory
{
public:
    virtual product *create() const = 0;
    ...
};

---

class concrete_product : public product
{
    ...
};

class concrete_factory : public factory
{
public:
    virtual concrete_product *create() const
    {
        return new concrete_product;
    }
    ...
};