explain outpt

[meera]

void main()
{
int a=10;
cout<<++a<<a;
}
the above code gives the output as 20, 10. please explain

[z05DSL3A]

Are you sure it outputs 20, 10?
It looks like it would output 1111.

[anders43]

strictly speaking the above code is illegal

there is no sequence point in between to determine whether '++a' or 'a' should be evaluated first therefore the return value and what cout prints is undefined.

[z05DSL3A]

I haven't given this much thought before (and I may be wrong) but isn't

std::cout << ++a << a;

just a tidy way of writing

std::cout.operator<<(++a).operator <<(a);

and therfore it will have sequence points because you are calling the operator functions?

[jsmith]

Yes, but it is still up to the compiler to decide when to post increment.

The code is not illegal; it's output is just undefined.

[Zaita]

I don't see how it would be undefined?

Execution order would be evaluated left to write, and the pre-increment before function call. So the output is well defined.
The order would be
++a
cout << a;
cout << a;

if it was cout << a << ++a; then it'd be
cout << a;
++a;
cout << a;

What's undefined about that?

[Bazzy]

Not on all the operators the operands are for sure evaluated from left to right

eg: on VC++ cout << ++a << a; and cout << a << ++a; display the same output but on MingW don't

[Zaita]

hmmm. Is there anything in the standard relating to that?

It seems as though VC++ is doing some instruction re-ordering thereby giving undefined results. In this case, it seems very un-intuitive.

What about doing cout << (++a) << (a); etc?

[z05DSL3A]

It was kind of making sense until I tried
std::cout << a << a-- << std::endl;

[helios]

jsmith is right. The moment at which the increment is done is undefined:
VC++:
b=++a + ++a + a++ + a++;
is the same as
++a;
++a;
b=a+a+a+a;
++a;
++a;

GCC:
b=++a + ++a + a++ + a++;
is the same as
++a;
b=a;
++a;
b+=a+a;
++a;
b+=a;
++a;

EDIT:
Grey Wolf: I get "10 10" for a=10.

[z05DSL3A]

VC gives 9 10 for a=10.

It's late (here), I'm off to sleep...

[helios]

Okay, I was wrong.
Both VC++ and GCC (or at least MinGW) seem to be doing
++a;
++a;
b=a+a+a+a;
++a;
++a;

Well, my point stands. There some compiler that does
++a;
b=a;
++a;
b+=a+a;
++a;
b+=a;
++a;

[anders43]

behavior undefined -> illegal in my book.