a=a++ wondering answer of visual studio?

Question

Possible Duplicate:
is i=i++ truly a undefined behavior?

i just want too explain ++ and -- to my students and show them some code about them in visual studio 2010 I just test this code on it

int main(){
   int a=3;
   int b=3;
   a=a++;
   cout<<a<<endl;
   cout<<b++<<endl;
}

I expect that both of cout print 3 but the first cout print 4!!!! I test it in g++ and both of couts print 3... what's wrong???

@Neal: I think Tomalak is setting out to be the unsung hero of many, many future Turbo C++ programmers who don't even know who saved them from certain doom. — Kerrek SB, Dec 07 '11 at 17:57
@KerrekSB: Haha precisely :) [though _sung_ would be preferable] — Lightness Races in Orbit, Dec 07 '11 at 18:07

score 7 · Accepted Answer · answered Dec 07 '11 at 17:53

7

The behavior of a=a++ is undefined. If you'd like to increment a, use a++ instead.

answered Dec 07 '11 at 17:53

Phonon

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score 4 · Answer 2 · answered Dec 07 '11 at 17:55

4

http://www.slideshare.net/olvemaudal/deep-c

Read about sequence points.

answered Dec 07 '11 at 17:55

emesx

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score 1 · Answer 3 · answered Dec 07 '11 at 17:53

1

a=a++; is not well defined. Don't use it.

answered Dec 07 '11 at 17:53

Dan

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11

It's not even poorly defined. – Lightness Races in Orbit Dec 07 '11 at 17:55

score 1 · Answer 4 · answered Dec 07 '11 at 17:55

1

You are only allowed do one assignment within one sequence point in C++ IIRC. So this is undefined. The following presentation discusses this issue in deep http://www.slideshare.net/olvemaudal/deep-c .

answered Dec 07 '11 at 17:55

Nils

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a=a++ wondering answer of visual studio?

4 Answers4