Behavior of c++ virtual calls inside constructor.

README.md

Description

Behavior of c++ virtual calls inside constructor.

main.cc

#include <cstdio>
#include <cstddef>
#include <cstdint>

class Base {
 public:
  Base() { }

  Base(void *) {
    // Get vtable
    if (!baseVTab) {
      baseVTab = *((intptr_t *) this);
    }
  }

  // LOL
  Base(int);

  virtual const char *getName() {
    return "Base";
  }

 static intptr_t baseVTab;
};


class Sub : public Base {
 public:

  Sub(void *) : Base((void *) 0) {
    // Get vtable
    if (!subVTab) {
      subVTab = *((intptr_t *) this);
    }
  }

  // Another LOL
  Sub(int);

  virtual const char *getName() {
    return "Sub";
  }

  static intptr_t subVTab;
};

Base::Base(int) {
  *((intptr_t *) this) = Sub::subVTab;
  printf("[@%p] Base ctor, this->getName() = %s\n", this, getName());
  printf("[@%p] Base ctor, ((Sub *) this)->getName() = %s\n",
      this, ((Sub *) this)->getName());
}

Sub::Sub(int) {
  *((intptr_t *) this) = Sub::baseVTab;
  printf("[@%p] Sub ctor, this->getName() = %s\n", this, getName());
  printf("[@%p] Sub ctor, ((Base *) this)->getName() = %s\n",
      this, ((Base *) this)->getName());
}

intptr_t Base::baseVTab = 0;
intptr_t Sub::subVTab = 0;

int main() {
  {
    // Get vtables
    Sub s((void *) 0);
  }

  Base b((int) 0);
  Sub s((int) 0);
  return 0;
}

output

[@000000000022FE50] Base ctor, this->getName() = Base
[@000000000022FE50] Base ctor, ((Sub *) this)->getName() = Sub
[@000000000022FE40] Sub ctor, this->getName() = Sub
[@000000000022FE40] Sub ctor, ((Base *) this)->getName() = Base