C++: using typeinfo to test class inheritance

Question

I have a pointer to a polymorphic type, p. I also have a type_info for a class somewhere in the same hierarchy, ti.

If I just

Answer 1

Yes, you can use dynamic_cast<> for that purpose. If you try to cast to Base*, it performs a runtime check to see if your class really is derived from Base (or directly is a Base). In case of failure, dynamic_cast<> returns nullptr. Example :

struct Base {
     virtual ~Base() {}
};

struct AnotherBase {
     virtual ~Base() {}
};

struct Derived : Base {};


Base * basePtr = new Base();
Base * derivedPtr = new Derived();
AnotherBase * anotherBasePtr = new Base();

// is derivedPtr a pointer to a class derived from AnotherBase ?
AnotherBase* test2 = dynamic_cast(derivedPtr);    // test2 == nullptr

// is basePtr a pointer to a class derived from Derived ?
Derived * test3 = dynamic_cast(basePtr);    // test3 == nullptr

Side nodes :

• if dynamic_cast<> is used to convert pointers, it returns either nullptr or the converted pointer.

• BUT when dynamic_cast<> is used to convert references it throws an exception in case of failure.

• The runtime check of dynamic_cast<> is only possible for polymorphic types. If your Base doesn't contain any virtual function (= non polymorphic), it won't be possible to safely convert a Base* to a Derived*.