Path: utzoo!telly!attcan!uunet!tut.cis.ohio-state.edu!UUNET.UU.NET!kewill!bryan
From: kewill!bryan@UUNET.UU.NET (Bryan Boreham)
Newsgroups: gnu.g++
Subject: Assignment to "this" causes vptr to be set.
Message-ID: <8906171903.AA16063@kewill.uucp>
Date: 17 Jun 89 19:03:47 GMT
Sender: daemon@tut.cis.ohio-state.edu
Distribution: gnu
Organization: GNUs Not Usenet
Lines: 68


// This is derived from a problem that I came across when porting ET++
// to g++ 1.35.1-.

#include <stdio.h>

class Base 
{
public:
  Base();
  Base(int t) { }
  virtual void print() { printf("I'm a Base.\n"); }
};

class Derived : public Base
{
public:
  Derived(int t) : (t) { }
  void print() { printf("I'm a Derived.\n"); }
};

void MakeDer(Base **d)
{
  *d = new Derived(3);
}

Base::Base() 
{
  Base* tmp_this;

  MakeDer(&tmp_this);
  tmp_this->print();	 // I'm a Derived .
  this = tmp_this;	 // now it stuffs the vptr in
  this->print();	 // I'm a Base.
}

int main()
{
  Base* b = new Base;
  b->print();		 // I'm a Base.
  delete b;
  return 1;
}


The idea is that Base should actually construct a Derived when called
by new. ET++ uses a technique like this to make things like Font,
Bitmap etc., window-system independant, but they used cfront, which
apparently lets them call MakeDer(&this). I don't have access to a
cfront compiler, so I can't check what this does.

Anyway, g++ won't allow "&this", so I tried code like the above. Many
wierd things happened, and I traced it to the fact that g++ stuffs in
the vptr for Base after the first assignment to "this". If the
assignment is in an if statement, the vptr gets set after the whole if.

I am currently working round this by saving the value of the vptr as
it comes from the desired class, and re-setting it after g++ has
clobbered it, but I'd like to know what should really be happening.
Perhaps there should be a way to take the address of "this"? Or a way
to stop the vptr assignment? Maybe I missed the whole point.

Bryan Boreham		bryan@kewill.uucp  or  kewill!bryan@uunet.UU.NET
Software Engineer	{Atlantic Ocean} ... !mcvax!ukc!root44!kewill!bryan
Kewill Systems PLC	Work: (+44) 932 248 328
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