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copied!<p>The following is off the cuff, and not fully tested. However I find myself wondering if the well known handle/body idiom might be a good match for your requirements here. It works by making <code>Matrix</code> nothing but a pointer to an implementation (what you currently call <code>Matrix</code>). And then derived clients derive from the implementation, not from <code>Matrix</code> itself.</p> <p>Here's a small sketch:</p> <pre><code>#include <memory> class Matrix { public: class Imp { public: virtual ~Imp() = default; virtual auto transposed() const -> std::unique_ptr<Imp> = 0; }; private: std::unique_ptr<Imp> data_; public: Matrix() = default; explicit Matrix(std::unique_ptr<Imp> data) : data_(std::move(data)) {} auto transposed() const -> Matrix { return Matrix(data_ ? data_->transposed() : nullptr); } }; class DenseMatrix : public Matrix::Imp { public: virtual auto transposed() const -> std::unique_ptr<Imp> { std::unique_ptr<DenseMatrix> res(new DenseMatrix); // Do some magic return std::move(res); } }; </code></pre> <p>It means all of your data must be on the heap. But that is probably going to happen anyway in a situation like this.</p> <p>External clients just deal with <code>Matrix</code>, which is not a base class, but a "handle" to a pointer to the base class. New implementations derive from the internal base class instead of from the <code>Matrix</code> class itself.</p> <p>It doesn't work all of the time. And I've glossed over details like how the client is going to specify he wants (e.g.) <code>DenseMatrix</code> as opposed to some other type of <code>Matrix</code>. But this general data structure can sometimes be quite useful.</p>

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