#define Query

shyam_oec · #1 01-Feb-2008, 08:22

CPP / C++ / C Code:

#define pro(X) (X*X)
main()
{

int i=3,j,k;
j=pro(i++);
k=pro(++i);
printf("\n%d %d",j,k);
getch();
}

output is 9 49
can any one explain why we get second output as 49.i guessed it to be 25,but after running program i was socked!

fakepoo · #2 01-Feb-2008, 08:33

Because it is a macro, the compiler actually replaces the code with the macro. So, what really is happening is:

CPP / C++ / C Code:

main()
{ int i=3,j,k;
  //j=pro(i++);
  j = (i++ * i++); // i get incremented twice after this statement

  //k=pro(++i);
  k = (++i * ++i); // i gets incremented twice before this statement
  
  printf("\n%d %d",j,k);
  getch();
}

Notice that i gets incremented four times, not twice.

davekw7x · #3 01-Feb-2008, 08:38

Quote:

Originally Posted by shyam_oec

can any one explain...!

If you just make the macro substitution yourself, the first statement looks like

CPP / C++ / C Code:

    j = i++ * i++;

and the second one looks like

CPP / C++ / C Code:

    k = ++i * ++i;

The program indulges in undefined behavior. Any time you have an expression with side effects, and you refer to the same variable more than once, the result is undefined behavior.

You could, perhaps, infer what a particular compiler does with the code by looking at the output, but the results are actually undefined. The order of evaluation of terms in an arithmetic expression is undefined. This is very explicit in the C Standard language specification. (So, for example, in either of the two statements, the value of the variable may be incremented twice before the individual values are multiplied. Or not.)

This means that you can't depend on any other compiler to give the same result. Not even a different version of the same compiler. Saying that you "expected" some particular result is just, well, non-productive. Undefined is undefined. Period. Full stop.

You can read more at places like this: http://c-faq.com/expr/index.html

Look, particularly at http://c-faq.com/expr/evalorder2.html

Regards,

Dave

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#define Query

Re: #define Query

Re: #define Query