C++ Inheritance member functions using static variables

Question

I am trying to convert some Python classes into c++ but am having some trouble. I have a Base class which has a class (static) variable and a method which returns it. I also have a derived class which overrides the class (static) variable like so,

In Python:

class Base:
   class_var = "Base"
   @classmethod
   def printClassVar(cls):
      print cls.class_var

class Derived(Base):
   class_var = "Derived"

d = Derived()
d.printClassVar()

which prints out the desired derived class variable, "Derived". Any idea how I can get the same functionality in c++? I have tried but end up getting the class variable of the Base class.

In c++

class Base
{
public:
    static void printStaticVar(){cout << s_var << endl;}
    static string s_var;
};
string Base::s_var = "Base";

class Derived : public Base
{
public:
    static string s_var;
};
string Derived::s_var = "Derived";

void main()
{
    Derived d;
    d.printStaticVar();
}

Pubby · Accepted Answer · 2012-12-23 12:38:40Z

3

Write a virtual function which returns a reference to the static member:

class Base
{
public:
    void printStaticVar() {cout << get_string() << endl;}
    static string s_var;
    virtual string const& get_string() { return Base::s_var; }
};
string Base::s_var = "Base";

class Derived : public Base
{
public:
    static string s_var;
    virtual string const& get_string() { return Derived::s_var; }
};
string Derived::s_var = "Derived";

void main()
{
    Derived d;
    d.printStaticVar();
}

Note that printStaticVar shouldn't be static.

You could also make the string static local inside the getter:

class Base
{
public:
    void printStaticVar() {cout << get_string() << endl;}
    virtual string const& get_string() { 
        static string str = "Base";
        return str;
    }
};

class Derived : public Base
{
public:
    virtual string const& get_string() { 
        static string str = "Derived";
        return str;
    }
};

void main()
{
    Derived d;
    d.printStaticVar();
}

answered Dec 23, 2012 at 12:38

Pubby

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1 Comment

Mats Petersson Over a year ago

Or, instead of having a static variable at all, simply return a constant, in this example. However that may not be suitable for all solutons.

mistapink · Accepted Answer · 2012-12-23 15:43:47Z

0

Another possibility might be:

class Base
{
  const std::string var;
public:
  Base(std::string s="Base") : var(s) {}
  void printVar() { std::cout << var << std::endl }
};
class Derived : public Base
{
public:
  Derived(std::string s="Derived") : Base(s) {}
};

answered Dec 23, 2012 at 15:43

mistapink

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