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From: gt3930b@prism.gatech.EDU (NICHOLS,STEVEN MALCOLM)
Newsgroups: comp.lang.c++
Subject: value of 'this' in constructor
Message-ID: <12680@hydra.gatech.EDU>
Date: 16 Aug 90 22:10:10 GMT
Sender: gt3930b@prism.gatech.EDU
Distribution: na
Organization: Georgia Institute of Technology
Lines: 50


	Consider the following piece of code.  From p. 284 of "Using
C++", and also p. 164 of "The C++ Programming Language", I am lead to 
believe that the value of the pointer 'this', on entry to a constructor,
will be 0 if the instance of the class has been dynamically allocated.
If the class has been statically allocated, the value of 'this' will be
non-zero.  However, I see non-zero values for this for both static and dynamic
instances of the class test in the code fragment that follows. 
	I see this behavior for both ZORTECH v. 2.06 on a 386 based PC, and
a similar output for the same code compiled using Oregon C++ on a VMS based
DEC micro-vax.  Since this is happening for *two* compilers, I conclude that
I have misunderstood Eckel and Stroustrup when they described this feature.
	If the error is in my understanding, or is well known, please e-mail
me at gt3930b@prism.gatech.edu; if it is a bona fide bug, or a non-trivial
language issue, please post for others to read.


// the code sample, set up for ZORTECH
#include <stream.hpp>

class test
{
   test* this_on_entry;
public:
   test() { this_on_entry = this; }
   void show() {cout << "this_on_entry = " << (unsigned)this_on_entry << "\n"; }
};

main()
{
   test static_test;
   test* dynam_test;

      dynam_test = new test;
      cout << "static:";
      static_test.show();
      cout << "dynamic:";
      dynam_test->show();
}



// the output from ZORTECH
static:this_on_entry = 9128
dynamic:this_on_entry = 9170


thanks for the help,
steve nichols.
gt3930b@prism.gatech.edu