Inheritance question

[MUTOOLS]

class A {

public:
char String[20];
};

class B : public A {

public:
char String[20];
};

class C : public B {

public:
char String[20];
};

void Test() {

A a,*ap;
B b;
C c;

strcpy(a.String,"Adios");
strcpy(b.String,"ByeBye");
strcpy(c.String,"Ciao");

printf("%s\n",a.String); // prints 'Adios' -> OK
printf("%s\n",b.String); // prints 'ByeBye' -> OK
printf("%s\n",c.String); // prints 'Ciao' -> OK

ap=&a;
printf("%s\n",ap->String); // prints 'Adios' -> OK
ap=&b;
printf("%s\n",ap->String); // prints 'IIIIIIIIIIIIIIIIIIIIByeBye' -> Strange! Why?
ap=&c;
printf("%s\n",ap->String); // prints 'IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIICiao' -> Strange! Why?
}

When doing

ap=&b;

Why doesn't ap->String show "Adios"? Or "ByeBye"? (that was what i wanted to test)

I searched the net but couldn't find the answer. Lot of info about deriving member functions, but not about deriving member variables.

Any answer or links are much appreciated.

Thanks.

 

[Thomas J. Gritzan]

class A {

public:
char String[20];
};

class A contains a char array.

class B : public A {

public:
char String[20];
};

[...all of class C snipped...]

class B contains two char arrays

void Test() {

A a,*ap;
B b;

strcpy(a.String,"Adios");

Here you initialize the char array of a.

strcpy(b.String,"ByeBye");

Here you initialize one char array of b. The char array of b's base
class is left uninitialized. Accessing it yields undefined behaviour.

printf("%s\n",a.String); // prints 'Adios' -> OK
printf("%s\n",b.String); // prints 'ByeBye' -> OK
ap=&a;
printf("%s\n",ap->String); // prints 'Adios' -> OK
ap=&b;
printf("%s\n",ap->String); // prints 'IIIIIIIIIIIIIIIIIIIIByeBye' ->
Strange! Why?

Here you access the uninitialized char array of b's base class. That's
undefined behaviour.

It's like this:

int main()
{
int i;
printf("%d", i); // this might output strange numbers or
// crash your system
}

Always initialize your variables.
Also, don't use char arrays and strcpy for strings, use std::string.

 

[jo]

Here you initialize the char array of a.


Here you initialize one char array of b. The char array of b's base
class is left uninitialized. Accessing it yields undefined behaviour.




Here you access the uninitialized char array of b's base class. That's undefined behaviour.

Oh, of course, i'm sorry.

Thanks!