How to change a behavior for all instances of a class in a header only class

Question

For a class, which is only defined in a header, I need a special behavior of one method for all instance of the class. It should be depending on a default value, which can be changed any time during runtime. As I do not want a factory class nor a central management class I came up with that idea:

class MyClass
{
public:
    void DoAnything() // Methode which should be act depending on default set.
    {
        // Do some stuff
        if(getDefaultBehaviour())
        {
            // Do it this way...
        }
        else
        {
            // Do it that way...        
        }
    }

    static bool getDefaultBehaviour(bool bSetIt=false,bool bDefaultValue=false)
    {
        static bool bDefault=false;
        if(bSetIt)
            bDefault=bDefaultValue;
        return bDefault;
    }
};

It works, but it looks a little awkward. I wonder if there is a better way following the same intention. In the case where I want to use it the software already created instances of that class during startup and delivered them to different parts of the code. Eventually the program gets the information how to treat the instances (for e.g. how or where to make themselves persistent). This decision should not only affect new created instances, it should affect the instances already created.

Which bit looks awkward to you? as that will affect the answer. In general use you might want to look at the Strategy pattern — mmmmmm, Jan 07 '14 at 16:18
Is this a compile time decision? In that case it might be a job for the preprocessor. — cmaster - reinstate monica, Jan 07 '14 at 16:19
In that case, what you are doing looks pretty fine to me. Any other methods to solve this would introduce more, unnecessary abstraction. — cmaster - reinstate monica, Jan 07 '14 at 16:21
you could simply make it a static data member of the class, and get rid of this awkward static getter method which also sets it :p (if that's what you feel is awkward...) — melak47, Jan 07 '14 at 16:27
@melak47 No, a static data member need to be implemented in a cpp file. This class (as I already mentioned) should be header only. — Martin Schlott, Jan 07 '14 at 16:29
@MartinSchlott: You may place the static var inside a static method... `static bool& getMyStaticBool() { static bool b = false; return b; }` — Jarod42, Jan 07 '14 at 17:05
@Jarod42 Good timing, looks like we both had the same idea. But make those things an answer next time, that's a lesson I had to learn on SO... ;) — Daniel Frey, Jan 07 '14 at 17:07
@Jarod42 Daniel Frey is right. You got the right answer but not as an answer :-) — Martin Schlott, Jan 07 '14 at 17:52

Daniel Frey · Accepted Answer · 2014-01-07T17:25:55.200

2

I'd advise to use a simple method to simulate a static data member, so the usage becomes more natural:

class MyClass
{
public:
    // get a reference (!) to a static variable
    static bool& DefaultBehaviour()
    {
        static bool b = false;
        return b;
    }

    void DoAnything() // Methode which should be act depending on default set.
    {
        // Do some stuff
        if(DefaultBehaviour())
        {
            // Do it this way...
        }
        else
        {
            // Do it that way...        
        }
    }
};

where the user can change the default at any time with

MyClass::DefaultBehaviour() = true;

edited Jan 07 '14 at 17:25

answered Jan 07 '14 at 17:06

Daniel Frey

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Yeah, that looks much better. The only trade off is that it is not so good to understand for c++ beginners as it manipulates a return value. Anyway, it is better than my solution. – Martin Schlott Jan 07 '14 at 17:57

score 0 · Answer 2 · answered Jan 08 '14 at 13:07

My thanks to Daniel Frey with his answer which I already marked as the best. I wanted to add my final solution which is based on the answer from Frey. The class is used by some c++ beginners. As I told them to use always getter and setter methods, the way described by Frey looks very complex to beginners ("uuuh, I can give a function a value?!?!"). So I wrote the class like followed:

class MyClass
{
public:
    // get a reference (!) to a static variable
    static bool& getDefaultBehaviour()
    {
        static bool b = false;
        return b;
    }
    static void setDefaultBehaviour(bool value)
    {
        getDefaultBehaviour()=value;
    }

    void DoAnything() // Methode which should be act depending on default set.
    {
        // Do some stuff
        if(getDefaultBehaviour())
        {
            // Do it this way...
        }
        else
        {
            // Do it that way...        
        }
    }
};

for the user, I looks now like a usual getter and setter.

How to change a behavior for all instances of a class in a header only class

2 Answers2

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