Unhandled exception - c++ program stops on cast

Question

I have several structs:

struct Token
{
 //some content
}

Then follows a bunch of structs that inherit from Token:
struct A : public Token{
 //Stuff
}

.
.
.
struct Z : public Token{
 //Other stuff
}

I have a vector std::vector filled with subclasses A through Z and my program crashes when I try to cast any element in the the vector to the subclass. I'm casting by doing the following:

A subclass = *((A * ) &vector[0]);

What am i doing wrong?

use [`dynamic_cast`](http://en.cppreference.com/w/cpp/language/dynamic_cast) when downcasting, a [C style cast](http://en.cppreference.com/w/cpp/language/explicit_cast) is not guaranteed to be safe.. — Mgetz, May 17 '14 at 12:59
You cannot possibly have "a vector filled with subclasses". Vectors store elements of one single, fixed type only. — Kerrek SB, May 17 '14 at 13:00
Probably the vector stores pointers to parent class and the OP is casting the child class pointers before storing them in the vector. — Cool_Coder, May 17 '14 at 13:03

score 1 · Answer 1 · answered May 17 '14 at 12:46

1

A meaningful usage would be

A &subclassref = vector[0]);

In above line, no new object is created.

BTW what is the type of your vector and what exactly do you want to achieve? If you store objects of type A to Z in a single vector, it may at some point of time suffer object slicing.

answered May 17 '14 at 12:46

Mohit Jain

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Thank you. I have a string that i convert into different tokens that all inherit from the superclass Token. I want to parse this and create objects based on the contents of the vector of tokens. – Grisungen May 17 '14 at 12:52
1

A token_container class can be prepared using [smart pointer](http://en.cppreference.com/w/cpp/memory/unique_ptr) sounds like a better solution. Smart pointer type would be token and it would contains objects of different token sub-class types. Now you can have a `std::vector` of token_container. – Mohit Jain May 17 '14 at 12:56
This would be more "meaningful" if the syntax were valid. – Lightness Races in Orbit May 17 '14 at 13:32
A solution using pointers ended up being a much better idea. Thanks for the tip! – Grisungen May 18 '14 at 09:31

score 1 · Accepted Answer · edited May 23 '17 at 11:50

1

You should use dynamic_cast when casting pointers from one type to another in your use case. The one you are using is a C style cast and I strongly suggest you to go with a dynamic_cast. So your code should look something like:

if(dynamic_cast<A *>(vector[0]))
    A subclass = *(dynamic_cast<A *>(vector[0]));

When a dynamic_cast fails it will return a NULL pointer and you should take care of it appropriately. Refer dynamic_cast and static_cast in C++ for more information. Additionally When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? will help you understand a lot more types of casts.

edited May 23 '17 at 11:50

Community

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answered May 17 '14 at 13:01

Cool_Coder

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`You should always use dynamic_cast when casting pointers from one type to another.` Not true. _You_ read those posts! – Lightness Races in Orbit May 17 '14 at 13:32
I meant it in the context of the question only, where dynamic_cast is the correct cast. Do you disagree on that? And BTW what is your problem with me? Do you want to prove me down on the internet? For what? What wrong have I done? – Cool_Coder May 17 '14 at 16:07
1

I have no "problem with you", no. I respond to each post on its own merits, as is right, and this post asserts an "always" rule that is bad advice. I am simply pointing that out. If you only meant _in this case_, then say "in this case", not "always". – Lightness Races in Orbit May 17 '14 at 18:43
If dynamic_cast fails, you would end up de-referencing a null pointer. – Mohit Jain May 18 '14 at 08:03
Whoever downvoted would please care to give some reason? – Cool_Coder May 19 '14 at 13:31
With C++11, you can write it like this: `if(A* pointer = dynamic_cast(vector[0])) { A subclass = *pointer; }` Slightly shorter and less repetitive. – cmaster - reinstate monica May 19 '14 at 17:50
@Cool_Coder I downvoted it because null pointer check was missing and left an appropriate comment. Now because you edited your answer, I reverted my vote. – Mohit Jain May 20 '14 at 05:46
Moreover you don't need a cast (implicit static casting) for casting from Derived class pointer to Base class pointer. – Mohit Jain May 20 '14 at 07:58

score 1 · Answer 3 · answered May 17 '14 at 18:41

This answer may be wrong because I'm making a guess as to how you have filled the std::vector<>.

You simply cannot put objects of subclasses into an std::vector<Base>. All objects in an std::vector<Base> are precisely of type Base. If you try something like this:

std::vector<Base> myVec;
myVec.push_back(Derived1(...));

you first construct an object of class Derived1 which is subsequently spliced into an object of class Base, i. e. a new object of class Base is copy-constructed from the derived object. Only this copy constructed base class object ends up in the std::vector<>.

If you want to have an std::vector<> of polymorphic objects, you must use a pointer type as the template argument (and consequently allocate the contained objects with new).

Maybe he is storing pointers to base class in the vector (although the code in the question suggests otherwise). — Cool_Coder, May 19 '14 at 13:35
@Cool_Coder That's precisely why I put the disclaimer up front. And judging by the fact that your answer was accepted, it seems like I *was* wrong. — cmaster - reinstate monica, May 19 '14 at 17:47

Unhandled exception - c++ program stops on cast

3 Answers3