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?