Mission Impossible / How to create datatypes which cannot contain invalid state

adrianimboden

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01 Introduction

Introduction

02 Enums

Enums - Overview Enums - Hands On Enums - Example Solution

03 Optional

Optional - Overview Optional - Hands On Optional - Example Solution

04 Variant

Variant - Overview Variant - Hands On Variant - Example Solution

05 Final

Final - Hands On Final - Example Solution

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Possible solution

// {
class SerialPort {
    enum class State { Init, Connected, Sending, Receiving };
public:
    void        connect();
    std::string send_request(const std::string& request);
    void        print_state(std::ostream& os) const;
private:
    mutable std::mutex guard_;
    State              state_ = State::Init;
};
void SerialPort::connect() {
    long_running_task();
    std::scoped_lock lock{guard_};
    BOOST_ASSERT_MSG(state_ == State::Init, "port already connected");
    state_ = State::Connected;
}
std::string SerialPort::send_request(const std::string& request) {
    {
        std::scoped_lock lock{guard_};
        BOOST_ASSERT_MSG(state_ == State::Connected, "port not connected yet");
        state_ = State::Sending;
    }
    long_running_task();
    {
        std::scoped_lock lock{guard_};
        BOOST_ASSERT_MSG(state_ == State::Sending, "detected race");
        state_ = State::Receiving;
    }
    long_running_task();
    {
        std::scoped_lock lock{guard_};
        BOOST_ASSERT_MSG(state_ == State::Receiving, "detected race");
        state_ = State::Connected;
    }
    return "ack";