Question

I will regularly read some discrete states, and applying some rules to differences in states I will report some errors. Rules can be changed in time.

What are best-practices to solve such a problem?

Thanks.

Answer 1

I’d also use the singleton pattern, apart from the Strategy one.
One possible implementation (though this is quite open, if you want a flexible set of rules you shoud use another class for the “Rule” entity. However, this way it is simpler to understand):

class Rules {
public:
    virtual bool rule_1(Data *) = 0;
    // ...
    virtual bool rule_n(Data *) = 0;

    static Rules * getRules()
        {
            // The only place in which to change the rule set
            if ( ruleSet == NULL ) ruleSet = new Rules_September2010();
            return ruleSet;
        }
protected:
    Rules();
    static Rules * ruleSet;
};

class Rules_August2010 : public Rules {
public:
    bool rule_1(Data *);
    bool rule_n(Data *);
};

class Rules_September2010 : public Rules {
public:
    bool rule_1(Data *);
    bool rule_n(Data *);
};

Of course this is an indication of the header(s). The implementation files are missing. Hope this helps.