Re: A question about '=='' in C++!

On 21 Sep 2007 05:15:35 GMT, "Guofu Chen" <gchen@[EMAIL PROTECTED]
> wrote
in comp.lang.c.moderated:

> Can anyone tell me the different between

The C language does not define anything about C++ at all.  The place
to ask this question is comp.lang.c++ or comp.lang.c++ moderated.
Nevertheless, since it is here and moderation is slow, I will provide
some insight into what would happen if this were C, and no guarantees
that C++ is the same.

> (1)
> int i, j;
> i = j ==1;

This is undefined behavior, since you are testing to see if the value
of 'j' is equal to 1, but 'j' is uninitialized so using its value,
even to compare to a constant like 1 is undefined.

> cout << i;
> cout << j;

Here you are again accessing the value of 'j', which is still
uninitialized.  Due to the undefined behavior, there is no correct or
incorrect output.

> and
> (2)
> int i, j;
> i = j = 1;

The statement above sets the value of both 'i' and 'j' to 1.  What is
puzzling about it?

> cout << i;
> cout << j;
> 
> 
> Why the result of  (1) is
> 
> 0 0

Undefined behavior can produce any result.

> while the result of (2) is
> 
> 1 1

Because you set the value of both 'i' and 'j' to 1.  Technically
speaking, the statement:

   i = j = 1;

....sets 'j' to 1 and then sets 'i' to the value of 'j'.

> Thanks! 

It seems as though you have some confusion on the meaning of the
assignment operator ('=') and the comparison operator ('==').  Best
consult your text book or other language reference.

And post C++ questions to a C++ newsgroup in the future.

-- 
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.para****ft.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html
-- 
comp.lang.c.moderated - moderation address: clcm@[EMAIL PROTECTED]
 -- you must
have an appropriate newsgroups line in your header for your mail to be
seen,
or the newsgroup name in square brackets in the subject line.  Sorry.