I cant understand the output

Hi all.

It been a really long time ive spent time on this board and ive moved on from C and im trying to get back into it. However my memory had been very rusty.

Could someone please help understand why i get 3 3 1 here please?

Code:

---

#include <stdio.h>

int main ( int argc, char *argv[] ) {
    int x = 1;
    printf ( "%d %d %d\n", ++x, x, x++ );
    return 0;
}

---

Many thanks

Undefined behavior.

The behaviour is undefined because you attempt to modify the same object more than once between consecutive sequence points (or you read it and also modify it separately). So, the reason why you get this output is that the compiler decided it was a nice thing to do. It might decide otherwise tomorrow, or another compiler might disagree.

Also, you should be aware that even if you didn't try to do this to x, the order of evaluation of function arguments is unspecified, so you still shouldn't be depending on such an ordering.

Quote:

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Originally Posted by laserlight

The behaviour is undefined because you attempt to modify the same object more than once between consecutive sequence points (or you read it and also modify it separately). So, the reason why you get this output is that the compiler decided it was a nice thing to do. It might decide otherwise tomorrow, or another compiler might disagree.

Also, you should be aware that even if you didn't try to do this to x, the order of evaluation of function arguments is unspecified, so you still shouldn't be depending on such an ordering.

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Many thanks laserlight. Could you please elaborate on this? What do you mean by "consecutive sequence points". How does it make it undefined when the same object being modified. Surely i can update object as many times i like?

Order of evaluation - cppreference.com

Code:

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int x = 1; printf( "%d %d %d\n", ++x, x, x++ );

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Quote:

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why i get 3 3 1 here

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"there is absolutely no guarantee that this order would be used in any other environment, in the same environment with different options selected, or even in the same environment with the exact same options selected."

"The above invokes undefined behavior by referencing both ‘x’ and ‘x++’ in the argument list. It is not defined in which order the arguments are evaluated. Different compilers may choose different orders. A single compiler can also choose different orders at different times."

"Therefore, it is not recommended... This means that there’s absolutely no temporal ordering in this process. The arguments can be evaluated in any order, and the process of their evaluation can be intertwined in any way."

"If you look at the assembly instructions generated by the compiler, you can see exactly what order the compiler is choosing to use."

Thanks very much all this is clear now :)