Using base class pointer against derived class object on stack vs. heap

Question

I have a question on assigning a derived class object with base class pointer...

class Base 
{ 
    void print() { cout<<"Class Base"; }

};

class Derived: public Base
{ 
    void print() {  cout<<"class Derived"; }

};

int main()
{
    Base b, *bp;
    Derived d, *dp;

    b.print();
    d.print();
    bp = d; // why is this a conversion error? getting an error "cannot convert ‘Derived’ to ‘Base*’ in assignment"

    bp = new B(); // this works fine...

}

Does it mean that we can only assign a dynamically allocated derived class object to a base class pointer?? why is that so???

Where is `ap` declared? This code won't even compile because `ap` is not declared.Show us the *actual* code.Also, You cannot assign objects to pointers,You can assign addresses to pointers.You should pick up a [good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Alok Save, Nov 23 '11 at 16:12

score 7 · Answer 1 · edited Nov 23 '11 at 16:13

7

bp is a pointer here, and you are trying to assign an object to it. Try assigning the address of that object instead: bp = &d;

edited Nov 23 '11 at 16:13

jweyrich

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answered Nov 23 '11 at 16:11

Brett Hale

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@w00te, I cheated ... I added the C++ tag. – Brett Hale Nov 23 '11 at 16:29

score 0 · Answer 2 · answered Nov 23 '11 at 16:12

0

Derived isn't a pointer, its an object. You need to get the address of that object to assign it to a pointer (which holds an address!):

bp = &d;

Works fine.

answered Nov 23 '11 at 16:12

John Humphreys

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Using base class pointer against derived class object on stack vs. heap

2 Answers2