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

vtable.cpp

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#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;

}