Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!usc!rutgers!att!tellab5!wheaton!obdient!gill!paul
From: paul@gill.UUCP (Paul Nordstrom)
Newsgroups: comp.lang.c++
Subject: type compatibility
Keywords: compilers
Message-ID: <754@gill.UUCP>
Date: 26 Nov 89 17:52:18 GMT
Organization: Gill & Co., L.P., Chicago, IL
Lines: 62

While attempting to port InterViews to the Oregon C++ compiler, I came
across the following code (simplified drastically):


typedef const void* TextData;

static TextData SELF = (void*)-1;

class Text {
public:
    Text(TextData context = SELF);
protected:
    TextData context;
    int size;
};

class Whitespace : public Text {
public:
    Whitespace(int size, TextData context = SELF);
    virtual Text* Copy();
};

Text* Whitespace::Copy () {
#ifdef OREGON
  if ( context == this )
     return new Whitespace(size, SELF);
  else
     return new Whitespace(size, context);
#else
     return new Whitespace(size, context==this ? SELF : context);
#endif
}


The portion of code that is compiled when OREGON is defined compiles
(presumably) correctly under Oregon C++ (1.2Ag).  The #else side is
the code is as distributed in the Interviews release currently on
uunet.  Obviously (he says blithely :-) ) the two sections of code
are equivalent.  The Oregon compiler ("occ") yields the following
error message on the original code:


   27 *         return new Whitespace(size, context==this ? SELF :
context);
                                                          ^203
*** 203: Error: Operands are of differing or incompatible type


In case the spacing gets garbled, the ^203 is pointing to the
question mark in the line of code, indicating that that is where the
compiler thinks the error is.  Assuming that I am correct that the
two pieces of code are in fact equivalent, clearly occ has a bug of
some kind: one piece compiles, one does not.  My question is: are
both pieces of code correct, or both incorrect.  The issue I see is:
should the compiler allow the test "context==this" when context is a
void* and this is a Whitespace* ?  Any comments, anyone?


-- 
Paul Nordstrom
Gill & Co., L.P.
uunet!gill!paul