CODEWITHSUNDEEP
c++
jensa
jensa

Reputation: 2890

Is polymorphic class required for dynamic_cast?

I've done some searching for this, but just to make sure:

Using dynamic_cast to cast a base-class pointer to a derived-class pointer requires the base class to be polymorphic? Otherwise we won't even be able to compile?

Thanks.

Upvotes: 1

Views: 627

Answers (2)

Columbo
Columbo

Reputation: 60979

For Upcasts, the base class type - which is the type of the casted expression - has to be polymorphic, otherwise you get a compiler error. That is because dynamic_cast uses RTTI to perform the cast and check whether it is successful or not - and RTTI is only generated for polymorphic types.

static_cast on the other hand doesn't check whether the cast is valid; if it isn't, the cast invokes undefined behavior. static_cast just adds or subtracts an offset from the pointer (or internal pointer of the reference). dynamic_cast does a runtime type check.

Upvotes: 1

Christophe
Christophe

Reputation: 73376

You could use dynamic_cast to cast a derived class to a non polymorphic base class. But you cannot dynamic_cast a non polymorphic base to a derived class.

The standard explains (§5.2.7):

(1) The result of the expression dynamic_cast(v) (...)

(5) If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v.

(6) Otherwise, v shall be a pointer to or a glvalue of a polymorphic type

The polymorphic is meant here in a very humble sense: "A class that declares or inherits a virtual function is called a polymorphic class." (§10.3/1).

Upvotes: 2

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