Accessing protected member in base class from a derived class

debugcn Published at Dev

Nilzone-

I have the following snippet of code:

const int DATE_LENGTH = 6;

class BaseClass {
    protected:
        int date[DATE_LENGTH];
        int year;
    public:
        BaseClass(){}
        BaseClass(int *d) {
            for (int i = 0; i < DATE_LENGTH; i++) { date[i] = d[i];}
            year = 1900 + date[4] * 10 + date[5];
        }
        void printYear() {
            cout << year << endl;
        }
};

class DerivedClass : public BaseClass {
    public:
        DerivedClass() {}
        void printYear() {
            cout << year << endl;
        }
};

int main(int argc, char *argv[]) {
    int dob[] = {1, 6, 1, 0, 9, 0};
    BaseClass base(dob);
    base.printYear(); // prints 1990

    DerivedClass derived;
    derived.printYear(); // prints 1439156608
}

I'm having trouble understanding why the output from printYear() in my derived class is outputting junk. Am I missing something very obvious?

Any help would be appreciated!

Vlad from Moscow

The default constructor of class BaseClass

BaseClass(){}

does not initislize data members date and year

This default constructor is called by the default constructor of class DerivedClass when you create object derived

DerivedClass derived;

So these data members have arbitrary values and your program has undefined behaviour.

Change the derived class the following way

class DerivedClass : public BaseClass {
    public:
        using BaseClass::BaseClass;
        DerivedClass() {}
        void printYear() {
            cout << year << endl;
        }
};

and create object derived as

DerivedClass derived( dob );

Or instead of the using declaration you can yourself explicitly define a constructor in class DerivedClass that has one parameter of type int * and calls the corresponding constructor of the base class. For example

class DerivedClass : public BaseClass {
    public:
        DerivedClass() {}
        DerivedClass( int *d ) : BaseClass( d ) {}
        void printYear() {
            cout << year << endl;
        }
};

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