can casting away constness lead to undefined behavior?

Question

const int a = 10
int *p = (int*) &a;
*p = 20;
printf("a = %d", a);

Is it possible to output either 10 or 20, depending on the compiler?

I gave your question a title that summarizes its contents. Don't use such generic titles as you did, they don't help anybody. — Jens Gustedt, Aug 02 '13 at 06:46
[C/C++ changing the value of a const](http://stackoverflow.com/questions/583076/c-c-changing-the-value-of-a-const) — Grijesh Chauhan, Aug 02 '13 at 07:12
[Can we modify the value of a const variable?](http://stackoverflow.com/questions/12245333/can-we-modify-the-value-of-a-const-variable) — Grijesh Chauhan, Aug 02 '13 at 07:13

score 15 · Answer 1 · answered Aug 02 '13 at 06:28

15

Is it possible to output either 10 or 20, depending on the compiler?

Yes, or even nasal demons can appear. The behavior of this program is undefined, the code is ill-formed, because modifying a const object is a constraint violation.

answered Aug 02 '13 at 06:28

2

+ for `constraint violation` its called const-constraint - I like correct terminologies. – Grijesh Chauhan Aug 02 '13 at 07:15
@GrijeshChauhan Unrelated: thanks for the dupe links. – Aug 02 '13 at 07:17
@H2CO3 I have read these answers many times. I had same confusion in past. actually question is good and obvious one. – Grijesh Chauhan Aug 02 '13 at 07:19

score 11 · Answer 2 · answered Aug 02 '13 at 06:28

11

As it's written, your code has undefined behavior, so yes, you could get 10 or 20 or anything else (e.g., an access violation).

answered Aug 02 '13 at 06:28

Jerry Coffin

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score 6 · Answer 3 · answered Aug 02 '13 at 06:30

It's undefined behavior:

C11 6.7.3 Type qualifiers

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.

score 6 · Answer 4 · answered Aug 02 '13 at 06:30

Yes it is undefined behaviour, and I think this is where is tells about it.

C99 Section 6.7.3 Paragraph 5

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.115)

score 3 · Answer 5 · edited Aug 02 '13 at 07:16

3

When you do *p=20, you are trying to change the value of a constant, which is not allowed.

edited Aug 02 '13 at 07:16

Grijesh Chauhan

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answered Aug 02 '13 at 06:26

Aakash Anuj

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can casting away constness lead to undefined behavior?

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