在给定指向其基类的指针的情况下识别子类？

Question

Suppose that I have an abstract base class Parent and subclasses Child1 and Child2. If I have a function that takes a Parent*, is there a way (perhaps with RTTI?) to determine at runtime whether it's a Child1* or a Child2* that the function actually received?

My experience with RTTI here, so far, has been that when foo is a Parent*, typeid(foo) returns typeid(Parent*) regardless of the child class that foo's a member of.

Answer 1

You need to look at the typeid of the dereferenced pointer, not the pointer itself; Ie, typeid(*foo), not typeid(foo). Asking about the dereferenced pointer will get you the dynamic type; asking about the pointer itself will just get you the static type, as you observe.

Answer 2

You can use std::dynamic_cast for this.

Parent* ptr = new Child1();
if(dynamic_cast<Child1*>(ptr) != nullptr) {
    // ptr is object of Child1 class
} else if(dynamic_cast<Child2*>(ptr) != nullptr) {
    // ptr is object of Child2 class
}

Also if you are using smart pointers, like std::shared_ptr , you can check it like this:

std::shared_ptr<Parent> ptr(new Child1());
if(std::dynamic_pointer_cast<Child1>(ptr) != nullptr) {
    // ptr is object of Child1 class
} else if(std::dynamic_pointer_cast<Child2>(ptr) != nullptr) {
    // ptr is object of Child2 class
}

Answer 3

Sure:

BaseClass *bptr = // whatever, pointer to base class
SubclassOne *safe_ptr_one = dynamic_cast<SubclassOne *>(bptr);
if (safe_ptr_one != nullptr) {
    // Instance of SubclassOne
} else {
    // not an instance of SubclassOne, try the other one
    SubclassTwo *safe_ptr_two = dynamic_cast<SubclassTwo *>(bptr);
    if (safe_ptr_two != nullptr) {
        // Instance of SubclassTwo
    } else {
        // it wasn't either one :'(
    }
}