Modifying a pointer pointer field from a method

Question

I'm trying to modify a bool field in a method. The method accepts a pointer pointer bool, but can't seem to figure it out how to do this correctly.

This is a basic example of something similar I want to do:

class WarningManager {
    bool seenWarningA; 

    void updateWarnings() {
        pushWarning(&seenWarningA)
    }

    void pushWarning(bool ** warning) {
        **warning = true;
    }
}

This code example errors (sending bool* but needs to be bool**) and I've tried other ways with no luck. Can't find any online examples but maybe I'm searching for the wrong terms. What is the right way to do this?

What do you think warning points to? The address of seenWarningA? Where do you think that address is stored? — Beta
– Beta, Commented Sep 2, 2018 at 23:39
@NeilButterworth If you can think of a better way of doing this without pointers, I would also be interested. Could do ref pointers? — Daniel Ryan
– Daniel Ryan, Commented Sep 2, 2018 at 23:39
I see no need for double-indirection in this code at all. At-most single indirection is suitable (or just a regular reference and no pointers). I can only assume pushWarning is doing something monumental an unseen; otherwise this is pointless and updateWarnings would simply seenWarningsA = true; would be done. — WhozCraig
– WhozCraig, Commented Sep 2, 2018 at 23:41
I can't provide a better way because you have not really explained fully what you are trying to do. Whatever it is, the chances of it requiring a pointer-to-pointer are pretty minimal. — user2100815
– user2100815, Commented Sep 2, 2018 at 23:43

Pete Becker · Accepted Answer · 2018-09-02 23:59:16Z

1

Since you have a class, no parameters are required.

class WarningManager {
bool seenWarningA; 

void updateWarnings() {
    pushWarning()
}

void pushWarning() {
    seenWarningA = true;
}
}

Using references rather than pointers is more elegant.

class WarningManager {
bool seenWarningA; 

void updateWarnings() {
    pushWarning(seenWarningA)
}

void pushWarning(bool & warning) {
    warning = true;
}
}

If you want to use pointers, the & operator just gives single pointer rather than a double point:

class WarningManager {
bool seenWarningA; 

void updateWarnings() {
    pushWarning(&seenWarningA)
}

void pushWarning(bool * warning) {
    *warning = true;
}
}

edited Sep 2, 2018 at 23:59

Pete Becker

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answered Sep 2, 2018 at 23:45

Matthew Fisher

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3 Comments

Daniel Ryan Over a year ago

Ah, I didn't realise the bottom example is all I need. I got confused in something I read. Thanks.

Tas Over a year ago

The bottom example isn't what you need, the second example is what you need. Notice when you're using pointers how confusing code becomes, and it's easy to mis-write it. That never happens with references, so it's far better to just use those (plus you wouldn't need to check for nullptr before use)

Daniel Ryan Over a year ago

Yes, I totally agree Tas. I don't use pointers until I need to.

Charlim · Accepted Answer · 2018-09-02 23:42:12Z

1

You appear to be trying to pass an argument of bool* into a function that takes bool**. Remove one of the layers of indirection from the parameter list, or add one to the value you're passing in. Either should work.

answered Sep 2, 2018 at 23:42

Charlim

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Comments

Coral Kashri · Accepted Answer · 2018-09-02 23:49:18Z

1

Two mistakes:

First- your declaration of pushWarning is with parameter of type bool**, and you are trying to send bool*.

Second- you can simply use reference:

using namespace std;

class WarningManager {
public:
    bool seenWarningA;

    void updateWarnings() {
        pushWarning(seenWarningA);
    }

    void pushWarning(bool &warning) { // You can simply use refference instead of pointer to pointer, or pointer at all..
        warning = true;
    }
};

int main()
{
    WarningManager w;
    w.seenWarningA = false;
    w.updateWarnings();
    cout << w.seenWarningA; // Prints 1

    return 0;
}

edited Sep 2, 2018 at 23:49

answered Sep 2, 2018 at 23:42

Coral Kashri

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1 Comment

Coral Kashri Over a year ago

@Tas I took the name before it been edited, so I didn't had much attention to class's name, but you right. And I prefer to use using namespace std, I just forgot to put it here :)

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