Base pointer Blues!!!

avenger_sb25

Suppose i have a set up like this:

class Base
{
public:
int x;
virtual void f()
};
class Derv:public Base
{
public:
int y;
void f();
};

void main()
{
Base *p;
p=new Derv[4];

   //I WANT TO ACCESS EACH ENTRY OF THE ALLOCATED ARRAY USING POINTER OF BASE
   for(int i=0;i<4;i++)
      (p+i)->y=10;  //THIS CRASHES WHEN i>0 AS I HAVE EXPLAINED BELOW

    delete\[\]p;

}

NOW HERE IS THE PROBLEM- I can access the first element using pointer 'p' BUT if i try to acces the second element of the array by doing p+1, MY PROGRAM CRASHES... This is the problem: Suppose the derived class array was allocated at 0x00000000, then p contains 0x11111111. I thought that p+1 will be 0x11111111+sizeof(Derv), BUT it is 0x11111111+sizeof(Base) SO WHAT I WANT TO KNOW FORM YOU FRIENDS IS - HOW TO ACCESS THE ELEMENTS OF A DERIVED CLASS ARRAY USING BASE CLASS POINTER ??? thanx for ur time ...Avenger

Remember... testing & debugging are always part of programming ...so exterminate those stinking bugs

User 580446

Hey Avenger, Here's a funny workaround... DWORD dw = (DWORD) p; // move in the array using this Base* p2 = NULL; for(int i=0;i<4;i++) { p2 = (Base*) dw; p2->y=10; dw += sizeof(Derv); // move to the next elem } What do you think?? :-D Clinton

toxcct

hey, looking your code, i cannot see any inheritence between your two classes. you must tell that Derv is derived from Base (public, protected or private as you like...) :

class Base {
public:
int x;
virtual void f();
};

class Derv : public Base {
public:
int y;
void f();
};

avenger_sb25 wrote: Suppose the derived class array was allocated at 0x00000000 impossible, nothing can be allocated at the address 0x00000000 (and even the operating system don't !). you must test if the address returned by new is different from NULL ((void*)0x00000000) :

Base* p;
p = new Derv[4];
if (p != NULL) {
for (int i = 0; i < 4; i++) {
p[i]->y = 10;
}
delete[] p;
}

TOXCCT >>> GEII power
[toxcct][VisualCalc]

avenger_sb25

Nice trying Clinton!:-D If you come up with a better solution, do let me know chao!:rose: ...Avenger

Remember... testing & debugging are always part of programming ...so exterminate those stinking bugs