Overriding static variables when subclassing

Question

I have a class, lets call it A, and within that class definition I have the following:

static QPainterPath *path;

Which is to say, I\'m dec

Answer 1

You probably don't want static variables to the overriden. Maybe you can store a pointer in your class instead?

class A
{
    public:
        A() :
            path(static_path)
        {
        }

    protected:
        A(QPainterPath *path)
            : path(path)
        {
        }

    private:
        QPainterPath *path;

        static QPainterPath *static_path;  /* Lazy initalization? */
};

class F : public A
{
    public:
        F() :
            A(F_static_path)
        {
        }

    private:
        static QPainterPath *F_static_path;  /* Lazy initalization? */
};

Answer 2

You can use virtual functions to achieve your result. This is probably your cleanest solution.

class A
{
    protected:
        virtual QPainterPath *path() = 0;

    private:
        static QPainterPath *static_path;  /* Lazy initalization? */
};

QPainterPath *A::path()
{
    return A::static_path;
}

class F : public A
{
    protected:
        virtual QPainterPath *path() = 0;

    private:
        static QPainterPath *F_static_path;  /* Lazy initalization? */
};

QPainterPath *A::path()
{
    return F::F_static_path;
}

Answer 3

Use a virtual method to get a reference to the static variable.

class Base {
private:
    static A *a;
public:
    A* GetA() {
        return a;
    }
};

class Derived: public Base {
private:
    static B *b;
public:
    A* GetA() {
        return b;
    }
};

Notice that B derives from A here. Then:

void Derived::paint() {
    this->GetA() ...
}

Answer 4

You might be able to do a variant on a mix in or Curiously recurring template pattern

#include <stdio.h>

typedef const char QPainterPath;

class Base
{
public:
    virtual void paint() { printf( "test: %s\n", getPath() ); }
    virtual QPainterPath* getPath() = 0;
};

template <class TYPE>
class Holder : public Base
{
protected:
    static QPainterPath* path;
    virtual QPainterPath* getPath() { return path; }
};

class Data1 : public Holder<Data1>
{
};

class Data2 : public Holder<Data2>
{
};

template <> QPainterPath* Holder<Data1>::path = "Data1";
template <> QPainterPath* Holder<Data2>::path = "Data2";

int main( int argc, char* argv[] )
{
Base* data = new Data1;
data->paint();
delete data;

data = new Data2;
data->paint();
delete data;
}

I have just run this code in CodeBlocks and got the following:

test: Data1
test: Data2

Process returned 0 (0x0)   execution time : 0.029 s
Press any key to continue.

Answer 5

If you don't care about the appearance just use A:: or F:: preceding the use of path to choose the correct one, or if you don't like :: name them differently.

Another option is to use a function to tidy this away, e.g. virtual QPainterPath* GetPath() { return A::path; } in A and QPainterPath* GetPath() { return F::path; } in F.

Really though this issue is just about how the code looks rather than what it does, and since it doesn't really alter readability I wouldn't fret about this...

Answer 6

You can't "override" static functions, let alone static member variables.

What you need is probably a virtual function. These can only be instance functions, so they will not be accessible without class instance.