Check which subclass an object is in C++?

Question

Given that there are the following struct types:

struct A {};
struct B : A {};
struct C : A {};
struct D : B {};

And we have a piece of code:

A a = f();

How would you check what type the struct actually is? i.e. is it A, B, C or D?

I can give the context in which I'm trying to use the answer for this if needed, but I don't think that's needed.

It probably is needed (how do we know a virtual function won't solve your problem) and the type is clearly `A`, always, since any derived type returned would be sliced. — chris, Aug 11 '13 at 10:39

score 2 · Answer 1 · edited May 23 '17 at 12:05

2

Note that polimorphism in C++ only works with pointers and references, if you return A by value, it will always be an A. If the function f() returns a class of type B, slicing will occur.

If you change the function to return a pointer to A, then you can use a dynamic cast and check if returns null.

A *a=f();
B *b=dynamic_cast<B*>(a);
if(b!=NULL)
{
    //a was of type B
}

edited May 23 '17 at 12:05

Community

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answered Aug 11 '13 at 10:29

user2671945

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My question was less regarding how we're obtaining `a`, more how do we check if it's actually something else (e.g. `B`). The `dynamic_cast` approach works. Thanks :) – Bertie Wheen Aug 11 '13 at 10:33
Keep in mind that if you happen to do the same with references instead of pointers, `dynamic_cast` throws an exception of type `bad_cast` if the type doesn't match (as null references don't exist in C++). – user2671945 Aug 11 '13 at 10:40
@user2671945 And sometimes, an exception is exactly what you need. :-) – C. K. Young Aug 11 '13 at 10:42
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I wasn't implying anything negative with the exception behaviour. You just have to be aware of that, and remeber a `try..catch` block around a `dynamic_cast` among references when you're unsure if it'll succeed, just as the OP asked. – user2671945 Aug 11 '13 at 10:47

score 0 · Answer 2 · answered Aug 11 '13 at 10:25

0

It's type A. Why? Because of slicing.

If, however, f() returned an A&, and you used A& a = f(); instead, then you have two options (both involving RTTI):

You could use typeid(a) and either look at its name() or compare against the type_info objects for the various (sub)classes.
You could use a dynamic_cast: for example, dynamic_cast<D*>(&a) would return null if a is not of type D.

answered Aug 11 '13 at 10:25

C. K. Young

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Given something along the lines of `A f() { B b; return b; }`, it's of type B. EDIT: Also, this doesn't answer the question – Bertie Wheen Aug 11 '13 at 10:27
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@BertieWheen no, it is `A` *and only* `A`. That is, assuming OP wants to know the type of variable `a`. – juanchopanza Aug 11 '13 at 10:29
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@BertieWheen `A f() {B b; return b;}` always returns an object of type `A` because of slicing. But if you had something like `A* f() {return new B;}`, then the returned pointer would indeed reference an object of type `B`. – C. K. Young Aug 11 '13 at 10:32
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return new Something is a no no – Aug 11 '13 at 10:37
@DieterLücking, how would you write the function? I know return new something isn't good, but I'm not sure about the best practice alternative – Bertie Wheen Aug 11 '13 at 10:38
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@DieterLücking It's not a no-no if the return type is `unique_ptr` ;-), but in any case, I was simply trying to talk about how the main way to avoid slicing is to return by pointer/reference (and I didn't want to create a dangling reference). – C. K. Young Aug 11 '13 at 10:39
@BertieWheen Return a `unique_ptr` or a `shared_ptr`, depending on what ownership model you need. – C. K. Young Aug 11 '13 at 10:40
What BertieWheen says – Aug 11 '13 at 10:41

Check which subclass an object is in C++?

2 Answers2