upcasting

using binaries = std::vector<char>;

class IClipboardReader
{
public:
    virtual binaries Read() const = 0;
};

class IClipboardWriter
{
public:
    virtual void Write(binaries bins) = 0;
};

class ClipboardSlot : public IClipboardReader, IClipboardWriter
{
public:
    binaries Read() const override
    {
        // ...
        return { 'a', 'b', 'c' };
    }

    void Write(binaries data) override
    {
        // ...
    }
};

template<typename CharType = char>
std::basic_ostream<CharType>& operator<<( // vypisanie vectoru
    std::basic_ostream<CharType>& out,
    const std::vector<CharType>& value
)
{
    for (auto& c : value) out << c; return out;
}

void main()
{
    auto slot = ClipboardSlot();
    IClipboardReader& upcasted1 = slot; // OK
    auto upcasted2 = (IClipboardReader&)slot; // Error: 'IClipboardReader': cannot instantiate abstract class
    auto upcasted3 = std::forward<const IClipboardReader&>(slot); // Error: 'IClipboardReader': cannot instantiate abstract class
    auto upcasted4 = static_cast<const IClipboardReader&>(slot); // Error: 'IClipboardReader': cannot instantiate abstract class

    auto slotPtr = std::make_shared<ClipboardSlot>();
    auto upcasted5 = static_cast<std::shared_ptr<IClipboardReader>>(slotPtr); // OK

    std::cout << "upcasted1:" << upcasted1.Read() << std::endl;
    std::cout << "upcasted2:" << upcasted2.Read() << std::endl;
    std::cout << "upcasted3:" << upcasted3.Read() << std::endl;
    std::cout << "upcasted4:" << upcasted4.Read() << std::endl;
    std::cout << "upcasted5:" << upcasted5->Read() << std::endl;
}