Static value inheritance

Question

I want each class to have its own static code, which can be requested from each object. I am thinking of this, but it doesn't seem to work:

#include <iostream>

class Parent {
protected:
    static int code;
public:
    int getCode();
};

int Parent::code = 10;
int Parent::getCode() {
  return code;
}

class Child : public Parent {
protected:
    static int code;
};

int Child::code = 20;

int main() {
  Child c;
  Parent p;

  std::cout << c.getCode() << "\n";
  std::cout << p.getCode() << "\n";

  return 0;
}

It outputs:

10

10

yet I expect

20

10

what do you mean "doesn't work", may be you can show some real code and the output vs expected out? — Nim, Jan 20 '15 at 15:26
"I want each class to have its own static code, which can be requested from each object." Theres a thing for that it's called a virtual method. — Borgleader, Jan 20 '15 at 15:26
@Borgleader I don't see what virtual member functions have to do with that. There's no polymorphism here. By "code", he appears to mean an integer value. — Joseph Mansfield, Jan 20 '15 at 15:30
I updated my question with a piece of compilable, runnable code. — Bart Friederichs, Jan 20 '15 at 15:31
is it needed to be able to change the Codes (the IDs) during runtime? Do the IDs have to be unique? — wonko realtime, Jan 20 '15 at 15:37
Because they are hardcoded to never change, I will update my answer to a simpler version. — abelenky, Jan 20 '15 at 15:53

abelenky · Answer 1 · 2015-01-20T15:54:54.507

1

class Parent {
public:
    virtual int getCode();

    // Looks like a variable, but actually calls the virtual getCode method.
    // declspec(property) is available on several, but not all, compilers.
    __declspec(property(get = getCode)) int code;
};


class Child : public Parent {
public:
    virtual int getCode();
};

int Parent::getCode() { return 10; }
int Child::getCode()  { return 20; }

int main() {
  Child c;
  Parent p;

  std::cout << c.code << "\n"; // Result is 20
  std::cout << p.code << "\n"; // Result is 10

  return 0;
}

edited Jan 20 '15 at 15:54

answered Jan 20 '15 at 15:32

abelenky

63,815
23
109
159

So I will always have to use a member variable per class? Not to mention implement `getCode` every time. – Bart Friederichs Jan 20 '15 at 15:34
1

Simpler to put the static value in the function body I think. – tenfour Jan 20 '15 at 15:38

score 1 · Accepted Answer · answered Jan 20 '15 at 15:48

1

You have to make the 'getCode()' function as virtual and have to implement every time as following codes:

class Parent {
protected:
    static int code;
public:
    virtual int getCode() { return code; }
};

int Parent::code = 10;

class Child : public Parent {
protected:
    static int code;
public:
    virtual int getCode() { return code; }
};

int Child::code = 20;

int main() 
{
    Child c;
    Parent p;

    std::cout << c.getCode() << "\n";
    std::cout << p.getCode() << "\n";
    return 0;
}

answered Jan 20 '15 at 15:48

Farhad Reza

424
4
16

The static variables are not needed when the values are constant (as described in OPs comments). – abelenky Jan 20 '15 at 16:04
@abelenky, you are correct but I've tried to give a solution with less change of the original source code. – Farhad Reza Jan 20 '15 at 16:09
You don't need to have virtual methods if you don't access the child object with a pointer to a base class like in his example, overriding the method is sufficient. – Johnmph Jan 20 '15 at 16:26

score 0 · Answer 3 · answered Jan 20 '15 at 15:38

0

Why use the member variable at all?

class Parent {
public:
    static int getCode();
};

int Parent::getCode() {
  return 10;
}

class Child : public Parent {
public:
    static int getCode();
};

int Child::getCode() {
  return 20;
}

Instead of one static member per class, you have one member function per class. Plain and simple.

answered Jan 20 '15 at 15:38

Angew is no longer proud of SO

167,307
17
350
455

1

A static value is not const, and could still be updated. Your code, however, can never change the value of the codes returned. (that may be OK for the poster... but was not described in the question) – abelenky Jan 20 '15 at 15:44
@abelenky Fair enough. – Angew is no longer proud of SO Jan 20 '15 at 15:48

score 0 · Answer 4 · edited Jan 20 '15 at 15:42

Your problem:

In the parent class, you don't declare your getCode function as virtual. So, whenever you call it with a class that inherits from your parent class, it will just return the int code from the parent class.

To fix this:

First, declare the getCode function as virtual in your parent class.
Second, write another getCode function in your inherited class and return the int code from the inherited class

Static value inheritance

4 Answers4