Problem with static downcast of base type to derived type

Question posted by: Dom Jackson (Guest) on July 2nd, 2008 07:15 PM

I have a program which crashes when:

1 - I use static_cast to turn a base type pointer into a pointer to a
derived type

2 - I use this new pointer to call a function in an object of the
derived type

3 - this function then 'grows' the derived type object (by pushing
onto a vector).

Is this actually possible, or should I be doing something else? In
more detail, what I'm doing is (there's a complete test program below,
which crashes):

1 - I have a base class which doesn't do very much;

2 - I declare a derived class with public inheritance from the base
class;

3 - the derived class has a method which pushes onto a vector in the
derived class;

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.

Any help much appreciated.

Thanks -

Dom

===========================================
#include <vector>
#include <set>
#include <iostream>

using std::vector;
using std::set;
using std::cout;

class A {
public:
A(int v) :
val(v) {}
int val;
};

class D : public A {
public:
D(int v) :
A(v) {}

mutable vector<intvec;

void dfunc(void) const {
std::cout << "hello world " << val << "!\n";
// ********************************************
// Ok without this line, crashes with this line
vec.push_back(val);
// ********************************************
}
};

class ALess {
public :
bool operator() (const A& a1, const A& a2) const {
return a1.val < a2.val;
}
};

typedef set<A, ALess ASet;
typedef ASet::iterator ASetIter;
typedef std::pair<ASetIter, bool ASetInsRetVal;

int main() {
ASetInsRetVal retval;
ASet aset;

for(int i=0; i<4; i++) {
retval = aset.insert(D(i));
if(!retval.second)
cout << "insert failed\n";
else {
const D *dptr = static_cast<const D*>(&(*retval.first));
dptr->dfunc(); // crashes - see above
}
}
}
==============================================

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