Re: Problem with static downcast of base type to derived type

Greg Herlihy <>
Thu, 3 Jul 2008 13:27:37 -0700 (PDT)
On Jul 2, 1:31 pm, Erik Wikstr=F6m <> wrote:

On 2008-07-02 21:13, Dom Jackson wrote:

4 - I create an STL set of base class objects;

5 - I then insert an object of the derived class into this set. I use
the set 'insert' method, which returns an iterator to the new object
in the set, but this is an iterator to a *base* object (I hope)

6 - I convert the iterator to a pointer, and static_cast the pointer
to a pointer to an object of the derived class

7 - I call my new method via this pointer, pushing onto the vector in
the derived-class-object. Bang.

This will not work, std::set holds objects of the type given as a
parameter, in this case base-objects and not derived objects, the
derived objects will be sliced into base objects. So what you are doing
is trying to call a function which does not exist to manipulate a vector
that does not exist.

To solve your problem you could use a set of pointers to base instead of
a set of base (e.g. std::set<Base*> instead of std::set<Base>) and
create the objects using new (e.g. set.insert(new Derived());).

You can always cast from pointer to derived to pointer to base using
static_cast, but when going the other way you shall use dynamic_cast.

No, a dynamic_cast<> cannot be used in the original program to cast
from an A to a B pointer - because the A class is not a polymorphic


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