Use abstract classes as interfaces when complete separation of interface and implementation is needed

Reason

Such as on an ABI (link) boundary.

Example

struct Device {
    virtual ~Device() = default;
    virtual void write(span<const char> outbuf) = 0;
    virtual void read(span<char> inbuf) = 0;
};

class D1 : public Device {
    // ... data ...

    void write(span<const char> outbuf) override;
    void read(span<char> inbuf) override;
};

class D2 : public Device {
    // ... different data ...

    void write(span<const char> outbuf) override;
    void read(span<char> inbuf) override;
};

A user can now use D1s and D2s interchangeably through the interface provided by Device. Furthermore, we can update D1 and D2 in ways that are not binary compatible with older versions as long as all access goes through Device.

Enforcement

???

C.hierclass: Designing classes in a hierarchy: