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i found something wrong code.

int i = 0;
i = i++;

is it standard c++? peoples say i == 1 in most compilers but anothers
not the same. if so, may be it's not standard code, however i want know
positively.

ljh131 wrote:
> i found something wrong code.
>
> int i = 0;
> i = i++;
>
> is it standard c++? peoples say i == 1 in most compilers but anothers
> not the same. if so, may be it's not standard code, however i want know
> positively.
>
It's undefined behavior. You are attempting to change a variable
twice between sequence points (once for the assignment and once
for the side-effect of ++).

ljh131 wrote:
> i found something wrong code.
>
> int i = 0;
> i = i++;
>
> is it standard c++?

Yes, in the sense that it will compile. However, the behaviour of
that code is undefined, according to the Standard. So, the usual
advice is: don't do that.

> peoples say i == 1 in most compilers but anothers
> not the same. if so, may be it's not standard code, however i want know
> positively.
>

If the behaviour is undefined, there is nothing we can say about the
outcome of executing that code from the Standard C++ point of view.

V

"ljh131" <> schrieb im Newsbeitrag
news: oups.com...
>i found something wrong code.
>
> int i = 0;
> i = i++;
>
> is it standard c++? peoples say i == 1 in most compilers but anothers
> not the same. if so, may be it's not standard code, however i want know
> positively.

Yes, it is standard C++, but the behaviour is not defined by the standard.
In the statement 'i = i++', there is no sequence point and i is modified
more than once. Many compilers might create code such that finaly i == 1,
but it is easy to imagine a sequence of code, that results in i == 0:

1. Evaluate the expression i
2. Increment i
3. Assign the result of step 1 to i

So remember what they tell the kids in TV -- "Don't do that at work"

HTH
Heinz

In article <4374d35c$0$21945$>,
Heinz Ozwirk <> wrote:
>"ljh131" <> schrieb im Newsbeitrag
>news: roups.com...
>>i found something wrong code.
>>
>> int i = 0;
>> i = i++;
>>
>> is it standard c++? peoples say i == 1 in most compilers but anothers
>> not the same. if so, may be it's not standard code, however i want know
>> positively.
>
>Yes, it is standard C++, but the behaviour is not defined by the standard.
>In the statement 'i = i++', there is no sequence point and i is modified
>more than once. Many compilers might create code such that finaly i == 1,
>but it is easy to imagine a sequence of code, that results in i == 0:
>
> 1. Evaluate the expression i
> 2. Increment i
> 3. Assign the result of step 1 to i
>
>So remember what they tell the kids in TV -- "Don't do that at work"

Right, the problem is whether the ++ side-effect takes place
before or after the assignment. But we don't get only those
two choices (which is bad enough) and hence as you say it is
undefined behavior.
--
Greg Comeau / Celebrating 20 years of Comeauity!
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