simple C++ inheritance

Question

With the following code I want accomplish two different values for intA, but I get:

clsA a=1? 1 clsB a=2? 1

//Code
#include <iostream>
using namespace std;
class clsA {
    static const int intA = 1; 
public:
    virtual int get_a() { return intA; }
};
class clsB: public clsA { 
    static const int intA = 2;   
}; 
int main() { 
    clsA a; clsB b;
    cout << "clsA intA =  " << a.get_a() << endl;
    cout << "clsB intA =  " << b.get_a() << endl;
}

How can I get intA=1 for object a and intA=2 for object b? Thanks for the reaction, André

What you are looking for is late static binding. It has been a while since I last did c++. I do not remember if it supports late static binding. — d_inevitable
– d_inevitable, Commented Mar 17, 2012 at 20:06
Actually I want clsA::intA=5 and clsB::intA=8. I have two intA s for the rest of the code, clsA delivers 5 random numbers and clsB delivers 8 random numbers — andre de boer
– andre de boer, Commented Mar 17, 2012 at 20:17

thb · Accepted Answer · 2012-03-17 20:04:57Z

1

Try this:

class clsB: public clsA {
    static const int intA = 2;
public:
    virtual int get_a() { return intA; }
};

The reason the other didn't work is that there literally existed no method clsB::get_a(). There only existed a vtable entry to which such a method might have been attached. The above attaches the method.

answered Mar 17, 2012 at 20:04

thb

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Comments

Jon · Accepted Answer · 2012-03-17 20:02:16Z

You need to tell the compiler that when get_a is called on clsB, it needs to return the intA defined in that same class:

class clsB: public clsA { 
    static const int intA = 2;   
    virtual int get_a() { return intA; }
};

However, this way of orchestrating things is not the best perhaps. Given that the allure of "automatically getting the correct value" is diminished by the fact that it's impossible to do that, giving the constants different names might be a good idea.

111111 · Accepted Answer · 2012-03-17 20:02:28Z

1

You are still calling the base function which can only see the bases version of a.

class clsA {
    static const int intA = 1; 
public:
    virtual int get_a() { return intA; }
};
class clsB: public clsA { 
    static const int intA = 2;   
    int get_a() { return intA; }
}; 

int main() { 
    clsA a; clsB b;
    cout << "clsA intA =  " << a.get_a() << endl;
    cout << "clsB intA =  " << b.get_a() << endl;
}

answered Mar 17, 2012 at 20:02

111111

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Comments

Greg Hewgill · Accepted Answer · 2012-03-17 20:01:21Z

0

Add an implementation of get_a() to clsB:

class clsB: public clsA { 
    static const int intA = 2; 
public:
    virtual int get_a() { return intA; }
};

However, it's confusing to have two different values for intA. I suggest a different naming scheme, such as intB in clsB.

answered Mar 17, 2012 at 20:01

Greg Hewgill

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2 Comments

d_inevitable Over a year ago

What he is trying to do is to overload the static variable. It is not really that confusing.

Greg Hewgill Over a year ago

@d_inevitable: I suggest that is confusing, because C++ doesn't have the concept of "overloading a static variable". That's pretty much the opposite of what static means.

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