Re: Problem with static downcast of base type to derived type
Comments inline:
Dom Jackson <nospam@mci2000.com> wrote in
news:6qjn645kup1jke5279liv592rced2lkn4j@4ax.com:
I have a program which crashes when:
[snip.. the code is more interesting]
Any help much appreciated.
Thanks -
Dom
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#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<int> vec;
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));
This will slice your temporary D object into an A object. ASet contains
objects of type A, and only type A.
if(!retval.second)
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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