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

adrianimboden

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This example implements a serial port protocol which is request/response based. It assumes a that:

the send and receive is long running
the current status can be obtained for debugging reasons
calls to send_request are synchronized by the caller

Example

// {
class SerialPort {
public:
    void        connect();
    std::string send_request(const std::string& request);
    void        print_state(std::ostream& os) const;
private:
    mutable std::mutex guard_;
    bool               is_connected_ = false;
    bool               is_sending_   = false;
    bool               is_receiving_ = false;
};
void SerialPort::connect() {
    long_running_task();
    std::scoped_lock lock{guard_};
    BOOST_ASSERT_MSG(not is_connected_, "port already connected yet");
    is_connected_ = true;
}
std::string SerialPort::send_request(const std::string& request) {
    {
        std::scoped_lock lock{guard_};
        BOOST_ASSERT_MSG(is_connected_, "port not connected yet");
        BOOST_ASSERT_MSG(not is_sending_, "detected race");
        BOOST_ASSERT_MSG(not is_receiving_, "detected race");
        is_sending_ = true;
    }
    long_running_task();
    {
        std::scoped_lock lock{guard_};
        BOOST_ASSERT_MSG(is_connected_, "detected race");
        BOOST_ASSERT_MSG(is_sending_, "detected race");
        BOOST_ASSERT_MSG(not is_receiving_, "detected race");
        is_sending_   = false;
        is_receiving_ = true;
    }
    long_running_task();
    {

The problem here is, that we have three bool variables which would result in $2 \times 2 \times 2 = 8$ possible state combinations, of which only $4$ are used:

is_connected	is_sending	is_receiving
false	ignored	ignored
true	false	false
true	false	true
true	true	false

This leads to the following results:

A person which reads the code has to look up all the various asserts in order to understand the existing invariants
A person which debuggs the code has to keep values of supposedly relevant variables in his head, which are actually not used
It is very easy for a different programmer to change the code in such a way that the previous invariant is no longer given

Now try to refactor the SerialPort example to use an enum instead.

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