Polymorphism with static calls

[niko]

Hello,

Is it possible to get when calling B::test() 2 printed out?

class A {
private:
    static int i;
public:
    static void test() {
        cout << i << endl;
    }
};
 
class B : public A {
    static int i;
};
 
int A::i = 1;
int B::i = 2;
 
int main(char** argv, int argc) {
    A::test(); //prints 1
    B::test(); //prints 1
    return 0;
}

class A {
private:
    static int i;
public:
    static void test() {
        cout << i << endl;
    }
};

class B : public A {
    static int i;
};

int A::i = 1;
int B::i = 2;

int main(char** argv, int argc) {
    A::test(); //prints 1
    B::test(); //prints 1
    return 0;
}

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thanks,
niko

[high_flyer]

you are shadowing (redeclaring) 'i' in your B class.
Remove that and it should do what you want.

[marcel]

And make i protected in A and provide a public getter and setter, in A or B. This way you will be able to set it from B too.

[niko]

sorry, but I can't get this working.

i made i now protected - doesn't help:

class A {
protected:
    static int i;
public:
    static void test() {
        cout << i << endl;
    }
};
 
class B : public A {
protected:
    static int i;
};
 
int A::i = 1;
int B::i = 2;
 
int main(char** argv, int argc) {
    A::test(); //prints 1
    B::test(); //prints 1
    return 0;
}

class A {
protected:
    static int i;
public:
    static void test() {
        cout << i << endl;
    }
};
 
class B : public A {
protected:
    static int i;
};
 
int A::i = 1;
int B::i = 2;
 
int main(char** argv, int argc) {
    A::test(); //prints 1
    B::test(); //prints 1
    return 0;
}

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And make i protected in A and provide a public getter and setter, in A or B. This way you will be able to set it from B too.

I don't understand what you mean - I do have static calls only!

[high_flyer]

class B : public A {
//protected:
 
//    static int i;  //you don't want that
};

class B : public A {
//protected:

//    static int i;  //you don't want that
};

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[niko]

I did try that too, it results in this compiler error:

main.cpp:19: error: ISO C++ does not permit 'A::i' to be defined as 'B::i'
main.cpp:19: error: redefinition of 'int A::i'
main.cpp:18: error: 'int A::i' previously defined here

main.cpp:19: error: ISO C++ does not permit 'A::i' to be defined as 'B::i'
main.cpp:19: error: redefinition of 'int A::i'
main.cpp:18: error: 'int A::i' previously defined here

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[high_flyer]

because its protected.
If your want to initialize it in global scope you will have to make it public.

[niko]

i get exactly the same compiler error

class A {
public:
    static int i;
    static void test() {
        cout << i << endl;
    }
};

class A {
public:
    static int i;
    static void test() {
        cout << i << endl;
    }
};

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[high_flyer]

Well I don't think this is possible (but others may correct me on this), since AFAIK a static member of a class is nothing more then a global variable that exists in the class's namespace.

So when you initialize your i member for B, you might try B::A::i = 2; (I didn't try this though)
But that would make no sense, since the 'i' member of A IS the same member of B, there is only ONE instance of it, not one per object!

What it is you need? I am sure if you explain what it is your are trying to achieve we can help you find a better way than using static members.

[niko]

I want classes that have some kind of static settings, take the code example below:

#include <QVariant>
#include <QMap>
#include <QDebug>
 
class A {
public:
    static QVariant getSetting(QString s) {
        return getSettings()[s];
    }
protected:
    static QMap<QString, QVariant> getSettings() {
        QMap<QString, QVariant> settings;
        settings["foo"] = "bar";
        settings["fooo"] = "baar";
        return settings;
    }
};
class B : public A {
    static QMap<QString, QVariant> getSettings() {
        QMap<QString, QVariant> settings = A::getSettings();
        settings["foo"] = "bar1";
        return settings;
    }
};
 
int main(char** /*argv*/, int /*argc*/) {
    qDebug() << A::getSetting("foo");
    qDebug() << A::getSetting("fooo");
    qDebug() << B::getSetting("foo"); //would like "bar1" here
    qDebug() << B::getSetting("fooo");
    return 0;
}

#include <QVariant>
#include <QMap>
#include <QDebug>

class A {
public:
    static QVariant getSetting(QString s) {
        return getSettings()[s];
    }
protected:
    static QMap<QString, QVariant> getSettings() {
        QMap<QString, QVariant> settings;
        settings["foo"] = "bar";
        settings["fooo"] = "baar";
        return settings;
    }
};
class B : public A {
    static QMap<QString, QVariant> getSettings() {
        QMap<QString, QVariant> settings = A::getSettings();
        settings["foo"] = "bar1";
        return settings;
    }
};

int main(char** /*argv*/, int /*argc*/) {
    qDebug() << A::getSetting("foo");
    qDebug() << A::getSetting("fooo");
    qDebug() << B::getSetting("foo"); //would like "bar1" here
    qDebug() << B::getSetting("fooo");
    return 0;
}

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[high_flyer]

Why not use QSettings then?

[niko]

these are not user settings - these are static settings that are compiled into the application.

but thanks for your replys...

[high_flyer]

I still fail to understand what it is you are trying to do, and why does it have to be static.
You can use QSettings to store settings, be it user, or non user settings.
It does nothing more then store values for keys, just like you are trying to do manually in your code.
Then you can have either a settings file for each class, or all settings in one file separated by sections or what have you.

From your code, I don't see the need to use static variales at all, and the fact you want polymorphism speak against it.