Re: "A quick poll about order of =?windows-1252?Q?evaluation=85=22?=

From:

JiiPee <no@notvalid.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 02 Dec 2014 18:45:23 +0000

Message-ID:

<eNnfw.43598$X16.14698@fx39.am4>

On 02/12/2014 16:35, Bo Persson wrote:

On 2014-12-02 01:02, Christopher Pisz wrote:

On 12/1/2014 5:53 PM, Christopher Pisz wrote:

On 12/1/2014 5:51 PM, Christopher Pisz wrote:

On 12/1/2014 2:54 PM, Lynn McGuire wrote:

"A quick poll about order of evaluation?"

http://herbsutter.com/2014/12/01/a-quick-poll-about-order-of-evaluation/

"Consider this program fragment:"

" std::vector<int> v = { 0, 0 };
int i = 0;
v[i++] = i++;
std::cout << v[0] << v[1] << endl; "

"My question is not what it might print under today?s C++ rules. The
third line runs afoul of two different categories of undefined and
unspecified behavior."

"Rather, my question is what you would like the result to be. Please
let
me know."

My answer is 02.

Lynn

Does not compile.
If it did, I am not sure what the problem would be.

I expect the output to be 01 and when I rewrote your snippet to a
fully
compilable program it indeed was 01.

// Standard Includes
#include <iostream>
#include <vector>

int main()
{
     std::vector<int> v;
     v.push_back(0);
     v.push_back(0);

     int i = 0;
     v[i++] = i++;

     std::cout << v[0] << v[1] << std::endl;

     return 0;
}

Is there some undefined behavior here that I am not aware of?

Derp, I mean 10. 5'o clock dyslexia.

Oh, I see, you are getting at the postfix occurring after the statement
rather than after the lhs or rhs of the statement is evaluated. Didn't
know that was undefined and made it 20 years without running into that.
I suppose that's a product of not using postfix in favor of code that
looks like it does what it actually does.

So you obviously haven't used some obscure hardware, where
simultaneously reading and writing the same memory address would lock
the system up.

(can't remember its name right now).

BTW, v[i+1] = i+2; is the same number of characters, and a lot more
obvious.

Whatever it is, in my opinion this kind of confusing code is not a good
practice. better to create temporary variables and use them instead -- I
mean one of them should be set using a temp variable before this and
then used.