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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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

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// { autofold
#include <boost/assert.hpp>
#include <iostream>
#include <mutex>
#include <thread>
void long_running_task() {
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
// }
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

The problem here is, that we have three bool variables which would result in possible state combinations, of which only 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