Monkey patching a C++ class by modifying its VTABLE.

vtable.cpp

#include <stdio.h>
#include <stdint.h>

class A {
public:
  virtual void doThing() {
    printf("I'm an A\n");
  }
};

void dynamicOverride(A* a) {
  printf("I'm an imposter!\n");
}

int main(int argc, char *argv[]) {
  A* a = new A();

  // grab the vtable for the A class
  void** vtable = *(void***)a;

  a->doThing();
  // out: "I'm an A"

  void (A::* ptr)() = &A::doThing;
  void* offset = *(void**)&ptr;
  vtable[((uintptr_t)offset)/sizeof(void*)] = (void*)&dynamicOverride;

  a->doThing();
  // out: "I'm an imposter!"

  // affects future instances as well
  A* a2 = new A();
  a->doThing();
  // out: "I'm an imposter!"

  return 0;
}