无法将列表迭代器强制转换为对象

Question

I get the error:

error C2682: cannot use 'dynamic_cast' to convert from 'std::_List_iterator<_Mylist>' to 'UserBean *'

When executing:

list<UserBean> * userBeans = getUserBeans();

for(list<UserBean>::iterator i = userBeans->begin(); i != userBeans->end(); i++)
   UserBean * newUser = dynamic_cast<UserBean*>(i);

Am I doing something wrong, or can you not convert iterator items to objects?

Answer 1

Sometimes iterators are implemented as raw pointers to container items, but more times than not, they are not pointers at all, so don't treat them that way. The correct way to access the item that an iterator refers to is to dereference the iterator, eg:

UserBean &newUser = *i;

Or:

UserBean *newUser = &(*i);

Iterators usually override the -> operator so you can access members of the referenced item, in cases where the iterator refers to an actual object instance (which yours does) and not a pointer to an object instance, eg:

i->SomeMemberHere

Answer 2

Am I doing something wrong, or can you not convert iterator items to objects?

No, you can't. You can dereference iterators to access objects:

UserBean & newUser = *i;

Answer 3

You can't convert an iterator to a pointer like this - that's not what dynamic_cast is for. You should only be using dynamic_cast when you're dealing with polymorphic behaviour (if at all). You can, however, do it like so:

UserBean* newUser = &*i;

This dereferences the iterator to get the object and then takes the address of the object.

Answer 4

The type of your container is list<UserBean> not list<*UserBean>

That's why your iterator is wrong. Its type is UserBean . Not UserBean* .

UserBean userBean = *i;

or

UserBean& userBean = *i;