• =?ISO-8859-1?Q?=22Sz=2E_Horv=E1t=22?= Oct 09 2006
• "Matthew" <matthew hat.stlsoft.dot.org> Oct 10 2006
• Derek Parnell <derek nomail.afraid.org> Oct 10 2006
• Derek Parnell <derek nomail.afraid.org> Oct 10 2006
• "Matthew" <matthew hat.stlsoft.dot.org> Oct 10 2006
• "Matthew" <matthew hat.stlsoft.dot.org> Oct 10 2006
• Sean Kelly <sean f4.ca> Oct 10 2006
• Sean Kelly <sean f4.ca> Oct 11 2006
• Derek Parnell <derek nomail.afraid.org> Oct 10 2006

Okay, I know that this is a stupid question, but I am curious:

Should the following be legal?

#include <iostream>

using namespace std;

int main() {
     int i = 0;
     (++i)++;            // <-- the question is about this
     cout << i << endl;
     return 0;
}

It does compile with gcc but not with dmc 8.49. I thought that (++i) 
should be an lvalue. If it is legal then why does the posfix operator 
have a higher precedence than the prefix operator?

It's a bug in DMC++

"Sz. Horvát" <szhorvat-nospamplease gmail.com> wrote in message
news:egdsnm$jvm$1 digitaldaemon.com...
 Okay, I know that this is a stupid question, but I am curious:

 Should the following be legal?

 #include <iostream>

 using namespace std;

 int main() {
      int i = 0;
      (++i)++;            // <-- the question is about this
      cout << i << endl;
      return 0;
 }

 It does compile with gcc but not with dmc 8.49. I thought that (++i)
 should be an lvalue. If it is legal then why does the posfix operator
 have a higher precedence than the prefix operator?

On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote: 
 "Sz. Horvát" <szhorvat-nospamplease gmail.com> wrote in message
 news:egdsnm$jvm$1 digitaldaemon.com...
 Okay, I know that this is a stupid question, but I am curious:

 Should the following be legal?

 #include <iostream>

 using namespace std;

 int main() {
      int i = 0;
      (++i)++;            // <-- the question is about this
      cout << i << endl;
      return 0;
 }

 It does compile with gcc but not with dmc 8.49. I thought that (++i)
 should be an lvalue. If it is legal then why does the posfix operator
 have a higher precedence than the prefix operator?


 It's a bug in DMC++


Why do you think this is a bug?

Is '++' identical in meaning to '+='? And if so, then an assignment is
implied in '++' and thus it needs a memory address.

(++i) is an expression that adds one to 'i' and evaluates to the resulting
value and not a memory address. If '(++i)++' was valid, then '(j=i)++'
should also be valid. And it would be if '++' was not identical to '+='. 

I'm pretty sure that '++' means 'add one to the variable operand and return
the result' and it does *not* mean 'if the operand is a variable then add
one to it and return the result otherwise just add one to the operand's
value and return the result'.

Where is the exact definition of '++' as I'm just going off memory here so
it could be highly suspect ;-)

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
11/10/2006 9:21:53 AM

On Wed, 11 Oct 2006 09:29:17 +1000, Derek Parnell wrote:

 On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote: 
 It's a bug in DMC++




Further more, if '(++i)++' was legal then (3)++ should also be legal.

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
11/10/2006 10:02:33 AM

"Derek Parnell" <derek nomail.afraid.org> wrote in message
news:pykh8vjb00oz$.1vipkpdvx9igh$.dlg 40tude.net...
 On Wed, 11 Oct 2006 09:29:17 +1000, Derek Parnell wrote:

 On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote:
 It's a bug in DMC++




 Further more, if '(++i)++' was legal then (3)++ should also be legal.


No. i may (and in this case must) be an lvalue. 3 is not an lvalue.

"Derek Parnell" <derek nomail.afraid.org> wrote in message
news:texhpcyj3e5c.x2tpmo3z3l60.dlg 40tude.net...
 On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote:
 "Sz. Horvát" <szhorvat-nospamplease gmail.com> wrote in message
 news:egdsnm$jvm$1 digitaldaemon.com...
 Okay, I know that this is a stupid question, but I am curious:

 Should the following be legal?

 #include <iostream>

 using namespace std;

 int main() {
      int i = 0;
      (++i)++;            // <-- the question is about this
      cout << i << endl;
      return 0;
 }

 It does compile with gcc but not with dmc 8.49. I thought that (++i)
 should be an lvalue. If it is legal then why does the posfix operator
 have a higher precedence than the prefix operator?


 It's a bug in DMC++


 Why do you think this is a bug?


C++-98: 5.3.2;1 - "The value is the new value of the operand: it is an
lvalue"

Or, from common experience, when one implements operator ++, one does the
following:

  class X
  {
  public:
    X &operator ++();  // prefix. Return ref (to self)
    X operator ++(int);  // postfix. Return copy
  };

Thus

    X    x;

    (++(++(++(++(++x)))));    // legal

and even

     ++++++++++x;    // legal

Matthew wrote:
 "Derek Parnell" <derek nomail.afraid.org> wrote in message
 news:texhpcyj3e5c.x2tpmo3z3l60.dlg 40tude.net...
 On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote:
 "Sz. Horvát" <szhorvat-nospamplease gmail.com> wrote in message
 news:egdsnm$jvm$1 digitaldaemon.com...
 Okay, I know that this is a stupid question, but I am curious:

 Should the following be legal?

 #include <iostream>

 using namespace std;

 int main() {
      int i = 0;
      (++i)++;            // <-- the question is about this
      cout << i << endl;
      return 0;
 }

 It does compile with gcc but not with dmc 8.49. I thought that (++i)
 should be an lvalue. If it is legal then why does the posfix operator
 have a higher precedence than the prefix operator?






 C++-98: 5.3.2;1 - "The value is the new value of the operand: it is an
 lvalue"
 
 Or, from common experience, when one implements operator ++, one does the
 following:
 
   class X
   {
   public:
     X &operator ++();  // prefix. Return ref (to self)
     X operator ++(int);  // postfix. Return copy
   };
 
 Thus
 
     X    x;
 
     (++(++(++(++(++x)))));    // legal
 
 and even
 
      ++++++++++x;    // legal


Are you sure?  From 5.1.4 (2003 revision):

     Between the previous and next sequence point a scalar object shall
     have its stored value modified at most once by the evaluation
     of an expression. Furthermore, the prior value shall be accessed
     only to determine the value to be stored.  The requirements of this
     paragraph shall be met for each allowable ordering of the
     subexpressions of a full expression; otherwise the behavior is
     undefined.

         i = ++i + 1; // the behavior is unspecified

So by my reading, modifying x twice in the same expression--no matter 
how it's done--has an undefined result.  Wouldn't this make your 
examples above illegal?


Sean

Sean Kelly wrote:
 Wouldn't this make your examples above illegal?


I suppose I should clarify.  By illegal I meant "undefined" -- the 
compiler obviously isn't required to flag such code as an error.


Sean

On Wed, 11 Oct 2006 10:12:00 +1000, Matthew wrote:

 "Derek Parnell" <derek nomail.afraid.org> wrote in message
 news:texhpcyj3e5c.x2tpmo3z3l60.dlg 40tude.net...
 On Wed, 11 Oct 2006 07:37:50 +1000, Matthew wrote:
 It's a bug in DMC++


 Why do you think this is a bug?


 C++-98: 5.3.2;1 - "The value is the new value of the operand: it is an
 lvalue"


Thanks. My memory was faulty yet again :-)

Is there any practical usage for this idiom? It seems to me that if one
increments a variable that is immediately used as a lvalue, the effect of
the increment is lost. Unless there are side-effects for an Object, which
sounds like a dubious practice anyhow. 

   (++i) = j; // why bother with the increment?

And something like '(++i)++' is just got to be an exercise in obfuscation,
no?

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
11/10/2006 11:01:00 AM