andrew@teslab.lab.OZ (Andrew Phillips) (11/28/90)
In "The C++ Programming Language" by Bjarne Stroustrup, on page 204,
I found the following which I believe to be a bug according to the
rules of C and C++:
last = last->next = new ...
Since the order of evaluation of subexpressions is undefined it is
unclear whether last in last->next refers to the old value of last or
the newly assigned value of last. Or am I missing something?
BTW I was surprised to find an even more obvious bug on the next page:
last = (f==last) ? 0 : f->next;
which should have been:
if (f == last) last = NULL; else last->next = f->next;
although I believe this was fixed in a later printing.
Also I was surprised to see that what is the first expression in a
for statement in C can be a statement in C++. Presumably this was
just added for the convenience of being able to declare a variable in
the for loop, so that you can say:
for (int i = 0; i < max; ++i) ....
But I tried several different kinds of statements in a for loop with
the C++ compiler I use and only some work. For example this works:
for ( {int i = 0} ; ) ;
but this doesn't:
for ( for (;;) ; ; ) ;
Thanks in advance for any replies to these questions, especially the
first one.
Andrew.
--
Andrew Phillips (andrew@teslab.lab.oz.au) Phone +61 (Aust) 2 (Sydney) 289 8712fuchs@it.uka.de (Harald Fuchs) (11/30/90)
andrew@teslab.lab.OZ (Andrew Phillips) writes: >In "The C++ Programming Language" by Bjarne Stroustrup, on page 204, >I found the following which I believe to be a bug according to the >rules of C and C++: > last = last->next = new ... >Since the order of evaluation of subexpressions is undefined it is >unclear whether last in last->next refers to the old value of last or >the newly assigned value of last. Or am I missing something? Yes. You are missing the fact that the assignment operator groups right-to-left. So the execution order is: 1. eval new... 2. eval last->next 3. assign (1) to (2) 4. eval last 5. assign the result of (3) (i.e. (1)) to (4) -- Harald Fuchs <fuchs@it.uka.de> <fuchs%it.uka.de@relay.cs.net> ... <fuchs@telematik.informatik.uni-karlsruhe.dbp.de> *gulp*
andrew@teslab.lab.OZ (Andrew Phillips) (12/04/90)
In article <1160@teslab.lab.OZ> andrew@teslab.lab.oz.au (I) wrote: >I found the following which I believe to be a bug according to the >rules of C and C++: > > last = last->next = new ... Thanks to the many people who replied that assignment operators are right associative. However, this does not address the question. Right-associativity means that a=b=c is interpreted as a=(b=c) and not (a=b)=c. But does this mean that the compiler can't interpret it as b=(a=c) [where I'm using brackets to show order of evaluation, even though brackets do not do this in C/C++]? Could someone give a reference to a standard document as to why the above is OK. Well do you think (I do) that this is wrong: last->next = last = ... since the first (right hand side :-) assignment to "last" is a side effect, and so the compiler can use either value of "last" (the old or the new) when evaluating "last->next"? > last = (f==last) ? 0 : f->next; > >which should have been: > > if (f == last) last = NULL; else last->next = f->next; ^^^^^^ (this is important bit) I'm sorry that I mentioned this bug as several people misunderstood what I was saying. You really need to look at several pages of code in (an old version of) the book to see why it is a bug. I now know for a fact that this bug WAS fixed in a later printing. > for ( for (;;) ; ; ) ; This is valid C++ syntax and I've been told does work with at least some compilers. Andrew. -- Andrew Phillips (andrew@teslab.lab.oz.au) Phone +61 (Aust) 2 (Sydney) 289 8712
jimad@microsoft.UUCP (Jim ADCOCK) (12/11/90)
The ability to declare a variable in the for loop was left out of one preliminary version of the C++ reference manual, with the result that some compilers implemented during that brief time frame do not allow declaration of the loop variable inside the for loop. C++ compilers implemented before and after that time period do support declaration of the loop variable inside the for statement. Such use is common within the C++ community, and supported by the current working papers of the ANSI-C++ committee.