Modifying a variable in class that calls function of another class

Question

I have a problem, and I am almost certain I can find a solution by restructuring the code I have to eliminate the problem, but I was wondering if there was a way to achieve the same result with what I have right now.

Suppose we have class A:

class A
{
public:
    int thing;
    void dostuff();
    std::list<B> mList;
}

void A::doStuff()
{
    for(auto x : mList)
        x.doMoreStuff();
}

And then there is class B.

class B
{
    void doMoreStuff();
}

Is it possible in any way to make B's doMoreStuff change the 'int thing' variable of class A without passing the instance of class A to B or similar convoluted methods?

Yeah, I was hoping to do it without that, or a superclass or something of that sort. — NeedForSleep
– NeedForSleep, Commented Mar 16, 2017 at 8:10
How should B know about A if it doesn't get a reference or pointer to A? — zett42
– zett42, Commented Mar 16, 2017 at 8:11
You could go down the 'static' route but then you'll be limited to a single variable 'thing' amongst all other instances of A — Alfie J. Palmer
– Alfie J. Palmer, Commented Mar 16, 2017 at 8:13
@juanchopanza Sorry about the confusion, I was in a hurry. Presumably it would be the instance of class B, also referred to as 'B'. I probably should edit that, and make it clearer. — NeedForSleep
– NeedForSleep, Commented Mar 16, 2017 at 8:20

Shogunivar · Accepted Answer · 2017-03-16 08:34:32Z

2

If you only want to change the value of a certain instance you'll have to reference.

Tho something you could do is use the return value of the method in B

class A
{
    int thing;
    B b;
    void dostuff() {
        thing += b.doMoreStuff();
    };
}

class B
{
    int doMoreStuff() {
       return 1;
    };
}

Or you can make either int thing or void doMoreStuff() a static so it can be accessed from class memory instead of instance memory like so:

class A
{
    static int thing;

    void dostuff() {
        B.DoMoreStuff();
    };
}


class B
{
    static void doMoreStuff() {
       A.thing += 2;
    };
}

edited Mar 16, 2017 at 8:34

answered Mar 16, 2017 at 8:28

Shogunivar

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1 Comment

Loay Ashmawy Over a year ago

There is an error in your code. you can't do B b = new B(); it's either B * b = new B(); or B b; and if you choose to do the first option with pointers you should also change b.doMoreStuff(); to b->doMoreStuff();

Loay Ashmawy · Accepted Answer · 2017-03-16 08:27:56Z

1

Yes you can by simply passing the address of the int thing of A for example:

B.doMoreThing(&myInt);

and the definition of doMoreThing would be: doMoreThing(int * myInt) and when modifying the value of the int inside the function you should do for example *myInt+=5 the * is important to access the variable.

answered Mar 16, 2017 at 8:27

Loay Ashmawy

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6 Comments

NeedForSleep Over a year ago

I guess that's the best I am going to get. I really hoped there was a way to do it without references -- for no particular reason.

Loay Ashmawy Over a year ago

I don't think so. I guess that this is the best way to do it. I don't really like the solutions making the variable static because it becomes accessible by everyone and everywhere

NeedForSleep Over a year ago

I switched the answer to the static one as that one achieves the same result without the reference. I don't like the static either, but in my case, doMoreStuff() has about 5 lines of inputs, having to put a reference in just makes things a hell of a lot more complicated.

Loay Ashmawy Over a year ago

No problem but take care because declaring a variable as static makes it accessible from everywhere but also makes instantiated once which means that if you do A a; a.thing=9 and then do A a2; a2.thing=46 that will also change the a.thing and makes it equal 46

NeedForSleep Over a year ago

I know, that's fine. I only need one a.thing-- it's practically a global variable anyway. This saves me the trouble of having to pass it by reference to other things.

|

inox · Accepted Answer · 2017-03-16 08:17:42Z

0

You're not going to be able to write to thing without a reference or a pointer to it. That said, thing is private anyway, so you'd need a setter. Alternatively you could make B a friend of A, or give B a pointer to member to thing.

answered Mar 16, 2017 at 8:17

inox

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