Consider the program below. It has been simplified from a complex case. It fails on deleting the previous allocated memory, unless I remove the virtual destructor in the Obj class. I don't understand why the two addresses from the output of the program differ, only if the virtual destructor is present.
// GCC 4.4
#include <iostream>
using namespace std;
class Arena {
public:
void* alloc(size_t s) {
char* p = new char[s];
cout << "Allocated memory address starts at: " << (void*)p << '\n';
return p;
}
void free(void* p) {
cout << "The memory to be deallocated starts at: " << p << '\n';
delete [] static_cast<char*> (p); // the program fails here
}
};
struct Obj {
void* operator new[](size_t s, Arena& a) {
return a.alloc(s);
}
virtual ~Obj() {} // if I remove this everything works as expected
void destroy(size_t n, Arena* a) {
for (size_t i = 0; i < n; i++)
this[n - i - 1].~Obj();
if (a)
a->free(this);
}
};
int main(int argc, char** argv) {
Arena a;
Obj* p = new(a) Obj[5]();
p->destroy(5, &a);
return 0;
}
This is the output of the program in my implementation when the virtual destructor is present:
Allocated memory address starts at: 0x8895008 The memory to be deallocated starts at: 0x889500c
RUN FAILED (exit value 1)
Please don't ask what the program it's supposed to do. As I said it comes from a more complex case where Arena is an interface for various types of memory. In this example the memory is just allocated and deallocated from the heap.
