return value of += operator (probably an easy q for somebody)

[Guest]

Consider the following small program

#include <stdio.h

#define printvars printf("a = %d, b = %d, c = %d\n", a, b, c)

int main(

int a = 1, b = 2, c = 3
printvar
int x = a + (b += 3)
printf("x = %d\n", x)
printvar
return 0


on line 3, I'm saying I want the variable b to be incremented by 3, and then the result added to a and the result of that stored in x
But is it according to the C++ standard that the expression '(b += 3)' should actually RETURN 'b+3' aswell as just doing the action of incrementing 'b' by 3? Or does it just happen to on MSVC

Thanks

 

[Simon Trew]

int main()
{
int a = 1, b = 2, c = 3;
int x = a + (b += 3);
printf("x = %d\n", x);
return 0;
}

on line 3, I'm saying I want the variable b to be incremented by 3, and

then the result added to a and the result of that stored in x.

But is it according to the C++ standard that the expression '(b += 3)'

should actually RETURN 'b+3' aswell as just doing the action of incrementing
'b' by 3? Or does it just happen to on MSVC?

I'm not going to quote from the standard, but the expression "b + 3" should
actually act as if returning a reference to b, i.e. its signature would look
like this

int & operator +=(int other);

So yes, the new value of b will be added to a, and this is portable.

Note that you are not guaranteed the order of evaluation of operands, so if
"a" was an alias for b then all bets would be off:

int b = 2, &a = b; // a is an alias for b
int x = a + (b += 3); // this is not portable as "a" might be evaluated
before or after "(b += 3)".

 

[James Curran]

um... Exactly what is that getting you that

b+=3;
x = a + b;

does not.

The two versions should generate exactly the same machine code, but with
mine, NO ONE will ever have to ask on a newsgroup, what it does.....

--
Truth,
James Curran
Home: www.noveltheory.com Work: www.njtheater.com
Blog: www.honestillusion.com Day Job: www.partsearch.com
(note new day job!)

Consider the following small program:

#include <stdio.h>

#define printvars printf("a = %d, b = %d, c = %d\n", a, b, c);

int main()
{
int a = 1, b = 2, c = 3;
printvars
int x = a + (b += 3);
printf("x = %d\n", x);
printvars
return 0;
}

on line 3, I'm saying I want the variable b to be incremented by 3, and

then the result added to a and the result of that stored in x.

But is it according to the C++ standard that the expression '(b += 3)'

should actually RETURN 'b+3' aswell as just doing the action of incrementing
'b' by 3? Or does it just happen to on MSVC?

 

[songie D]

But... your version's two lines. Mine's one. That makes mine 'better'.
I've a compulsion for wanting to write code that's as confusing
to others as possible. Does this make me a sick pervert?

 

[Simon Trew]

But... your version's two lines. Mine's one. That makes mine 'better'.
I've a compulsion for wanting to write code that's as confusing
to others as possible. Does this make me a sick pervert?

Yes.

 

[David Webber]

Yes.

Not to worry. Write it

b+=3; x = a + b;

and you too have only one line of code. :-(

Dave
--
David Webber
Author MOZART the music processor for Windows -
http://www.mozart.co.uk
For discussion/support see
http://www.mozart.co.uk/mzusers/mailinglist.htm