CODEWITHSUNDEEP
c++pointers
Raphael
Raphael

Reputation: 127

Casting pointer from one base type to another

-EDIT-

Thanks for the quick response, I'd been having really weird problems with my code and I changed my casts to dynamic_cast and its working perfectly now

-ORIGINAL POST-

Is it safe to cast a pointer of one base class to another base class? To expand on this a bit, will the pointer I've marked in the following code not result in any undefined behavior?

class Base1
{
public:
   // Functions Here
};


class Base2
{
public:
   // Some other Functions here
};

class Derived: public Base1, public Base2
{
public:
  // Functions
};

int main()
{
  Base1* pointer1 = new Derived();
  Base2* pointer2 = (Base2*)pointer1; // Will using this pointer result in any undefined behavior?
  return 1;
}

Upvotes: 9

Views: 2766

Answers (2)

flamingo
flamingo

Reputation: 506

You should use dynamic_cast operator. This function returns null if types are incompatible.

Upvotes: 1

R. Martinho Fernandes
R. Martinho Fernandes

Reputation: 234434

Will using this pointer result in any undefined behavior?

Yes. The C-style cast will only try the following casts:

  • const_cast
  • static_cast
  • static_cast, then const_cast
  • reinterpret_cast
  • reinterpret_cast, then const_cast

It will use reinterpret_cast and do the wrong thing.

If Base2 is polymorphic, i.e, has virtual functions, the correct cast here is dynamic_cast.

Base2* pointer2 = dynamic_cast<Base2*>(pointer1);

If it doesn't have virtual functions, you can't do this cast directly, and need to cast down to Derived first.

Base2* pointer2 = static_cast<Derived*>(pointer1);

Upvotes: 13

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