ambiguty caused by compiler

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C++

/*on dev-cpp output=10
 visual c++ output=12
 why*/
#include<stdio.h>
#include<conio.h>
int main()
{
   int i,j;
   i=1;
   j=(++i)+(++i)+(++i);
   printf("%d",j);
   getch();
}

[edit]Code block added, HTML encoded - OriginalGriff[/edit]

Posted 17-Mar-13 3:44am

Sagar Srivastava

Updated 17-Mar-13 4:34am

OriginalGriff

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nv3 17-Mar-13 11:03am

There is an old saying: Crap in - crap out. I admit, OriginalGriff's explanation is more detailed :-)

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Shmuel Zang · Accepted Answer · 2013-03-17T05:11:00

It seems like the compilers handle it differently. Let's divide the statement and, try to figure it out.

First, you use the Prefix Increment Operator - which means that the increment operation should occur before the use of the value of i. But, how much before?

It seems like the first compiler increments the values directly before they are used. So, as we have two + operators, the first two increments are performed before the first + operator and the third increment is performed before the second + operator. What gives us: 3 + 3 + 4 = 10.

The second compiler performs the whole of the increments at the beginning of the statement. What gives us: 4 + 4 + 4 = 12.

OriginalGriff · Accepted Answer · 2013-03-17T04:41:00

The difference is because the specification does not formally say when the pre-increment should occur: the compiler can decide for itself whether it wants to do the pre-increment immediately before the use or at the beginning of the statement, to the benefit of it's execution module.
Clearly, the two compilers have different ideas of how to do it!

You really shouldn't do that - divide it into a couple of lines and let the copiler sort out any optimisations. It is not clear to read that exactly what you should get - one school of thought could be that it "should" be:

2 + 3 + 4 = 9

not 12 or 10!

ambiguty caused by compiler

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