Can I use inherited class pointers just as parent's?

Question

Both of my classes: Parent and Child are the same (for now) and have the same constructor.

class Parent{
protected:
  string name;
public:
  Parent(string &n, vector <int> &v) {
  /* read n and v into vars */
};

class Child : public Parent {
public:
  Child(string &n, vector <int> &v) : Parent(n, v) {}
};
vector <int> val;
string nam, numb;
if(val[0] == 0) 
  Child* ptr = new Child(nam, val);
else
  Parent* ptr = new Parent(nam, val);

myMap.insert(Maptype::value_type(numb, ptr) );

Is it legal to pass Child* ptr object as Parent* ptr object? I've heard that they have the same pointer type, so it should be alright. Then why am I getting warning: unused variable 'ptr' warning: unused variable 'ptr' error: 'ptr' was not declared in this scope ? My program works fine with Parent class only. I feel like I haven't inherited Parent right.

"parent/child" is not a good metaphor of the C++ object model and in particular of public inheritance. "Base" and "Derived" are much better. A child *is* not a parent, but derived classes *are* in a sense a version of their public bases. — Kerrek SB, Jun 10 '12 at 13:51

score 6 · Accepted Answer · edited May 23 '17 at 10:34

6

The code creates two separate variables called ptr, both with very limited scope.

Consider the following:

if(val[0] == 0) 
  Child* ptr = new Child(nam, val);
else
  Parent* ptr = new Parent(nam, val);

It is equivalent to:

if(val[0] == 0) {
  Child* ptr = new Child(nam, val);
} else {
  Parent* ptr = new Parent(nam, val);
}
// neither of the `ptr' variables is in scope here

Here is one way to fix your code:

Parent* ptr;
if(val[0] == 0) 
  ptr = new Child(nam, val);
else
  ptr = new Parent(nam, val);

Once you do this, you also need to make sure that Parent has a virtual destructor. See When to use virtual destructors?

edited May 23 '17 at 10:34

Community

1
1

answered Jun 10 '12 at 08:25

NPE

438,426
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887
970

Thank you. It's still kinda mysterious for me why those pointer have the same type. – user1078719 Jun 10 '12 at 08:30
1

@user1078719: Actually, they don't. It is, however, the case that `Child*` can be converted to `Parent*`. http://www.parashift.com/c++-faq-lite/basics-of-inheritance.html#faq-19.4 – NPE Jun 10 '12 at 08:32

score -1 · Answer 2 · answered Jun 10 '12 at 11:21

-1

because you declare the ptr in the if statement only, try declaring it just above the if statement so it could be like aix answer

answered Jun 10 '12 at 11:21

justcoder

56
4

Can I use inherited class pointers just as parent's?

2 Answers2