How to find VPTR in C++ assembly code?

Question

class Base {
 public:
  Base() {}
  virtual void Get() { }
};

class Derivered : public Base {
 public:
  virtual void Get() { }
};

int main() {
  Base* base = new Derivered();
  base->Get();
  return 0;
}

I use gcc 5.4.0 to compile the code, and use objdump -S a.out to disassemble binary file. I want to find Base's vptr, but only display an unknown address 0x80487d4. The max address number is 0x80487b7, I cann't understand. command list: g++ test.cpp -O0; objdump -S a.out

080486fe <_ZN4BaseC1Ev>:
 80486fe:   55                      push   %ebp
 80486ff:   89 e5                   mov    %esp,%ebp
 8048701:   ba d4 87 04 08          mov    $0x80487d4,%edx
 8048706:   8b 45 08                mov    0x8(%ebp),%eax
 8048709:   89 10                   mov    %edx,(%eax)

Have you read this [article](http://phrack.org/issues/56/8.html) ? — perror, Aug 29 '17 at 11:57
If you turn optimizations, its easy for the compiler to *Devirtualize* the function call, inline it, and even eliminate it, as in this context, — WhiZTiM, Aug 29 '17 at 11:57
Which processor? Assembly language is processor dependent. Linux can run on an ARM processor (and thus have different assembly language). — Thomas Matthews, Aug 29 '17 at 15:57

score 0 · Answer 1 · answered Aug 29 '17 at 17:56

080486fe <_ZN4BaseC1Ev>:
  80486fe:   55                      push   %ebp
  80486ff:   89 e5                   mov    %esp,%ebp
  8048701:   ba d4 87 04 08          mov    $0x80487d4,%edx
  8048706:   8b 45 08                mov    0x8(%ebp),%eax
  8048709:   89 10                   mov    %edx,(%eax)

Is...

push %ebp             ;- save frame pointer
mov %esp, %ebp        ;- mov esp-> ebp -ebp is frame pointer
mov $0x80487d4, %edx  ; load vptr address into edx
mov 0x8(%ebp), %eax   ; ld eax with address of this
mov %edx,(%eax)       ; store vptr in this byte 0

How to find VPTR in C++ assembly code?

1 Answers1