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

Andre Kostur <>
Wed, 2 Jul 2008 20:56:38 +0000 (UTC)
Comments inline:

Dom Jackson <> wrote in

I have a program which crashes when:

[snip.. the code is more interesting]

Any help much appreciated.

Thanks -


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

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

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

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

   mutable vector<int> vec;

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

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));

This will slice your temporary D object into an A object. ASet contains
objects of type A, and only type A.

         cout << "insert failed\n";
      else {
         const D *dptr = static_cast<const D*>(&(*retval.first));

Undefined behaviour. retval->first is an A object. Which you then
force the compiler to try to treat it like a D object...

         dptr->dfunc(); // crashes - see above

.... particularly when you attempt to access members which only exist in
D objects.


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