CODEWITHSUNDEEP
c++
Alex
Alex

Reputation: 876

reinterpret_cast does not have defined behavior

SO i have two classes Tuna and Carp which have the member methods becomeDinner and Talk respectively: Now I have this code:

Tuna* myDum = new Tuna();
auto myCastedDum = reinterpret_cast<Carp*>(myDummy);
myCastedDum->Talk();

In my book it says:

Again, definitely not. reinterpret_cast changed only the interpretation of the pointer, and did not change the object being pointed to (that is still a Tuna). Calling a Talk() function on a Tuna object will not give the results you are looking for, and could possibly cause an application error.

Now this seams to not be the case, when I try to call the talk method on myCastedDum it works as if it was a Carp object and when I try to call becomeDinner() it tells me that it isn't a member, I don't understand, we cast it to Carp explicitely with reinterpret_cast so why would that be unexpected if we explicitely tell the compiler to do it?

What does changing the interpretation of the pointer mean?

Thanks in advance.

Upvotes: 0

Views: 81

Answers (1)

john
john

Reputation: 87944

The cause of your misunderstanding seems to be that in this code 

Tuna* myDummy = new Tuna();
auto myCastedDum = reinterpret_cast<Carp*>(myDummy);
myCastedDum->Talk();

you think that reinterpret_cast is changing your Tuna object into a Carp object. That's not true. All it is doing is converting a Tuna pointer into a Carp pointer. But that Carp pointer is still pointing at a Tuna object, in other words the underlying object has not changed.

So when you use the Carp pointer to call a Carp method you get undefined behaviour, because there is no Carp object for the Carp method to work on.

The reason you are able to call Carp::Talk() is that all the compiler is checking is the type of the pointer. You do have a Carp pointer, even if it not pointing at a Carp object, so the compiler will let you call Carp::Talk. Using reinterpret_cast is a way of overriding the normal safety checks that the language has. That's why it should be used with care.

Upvotes: 1

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