Replacing characters in C++ Pointer Character string?

Question

Suppose I have this in C++:

char *p = "Apple";

I can't do this:

p[1] = 'w';

But why can I do this?

p = "OrangeTorange";

There is about 100 of these on here already. I'll be back with a definite duplicate. — Mats Petersson, Aug 15 '13 at 17:39
@MatsPetersson: That is not a duplicate. He clearly knows that he cannot write to the literal, but he is asking if he can point to a different literal (although he might not know that he is asking it). Not sure why -3 here... the user might not know what the difference is, but the question is proper. — David Rodríguez - dribeas, Aug 15 '13 at 17:48
Thanks David. @Mats, I actually google-d and checked stackoverflow before asking. I understand that related questions were asked but I couldn't find which exactly answered this. — user1343318, Aug 15 '13 at 17:59

score 3 · Accepted Answer · edited May 23 '17 at 12:05

As p points to constant string literal so if you do: p[1] = 'w'; then you are trying to modifying string literal that is read only constant and its illegal operation (Undefined behavior).

Whereas in expression p = "OrangeTorange"; you modify value of p variable that is pointer to a char. And assigning new address value to p is a valid operation, now p start pointing to new string literal.

To add further, Suppose if p points an array then p[1] = 'w'; is not a invalid operation consider below example code:

char str[] = "Apple";
char* p = str; // p points to a array 
p[1] = 'w';    // valid expression, not str[1] = 'w' is well valid. 
p = "OrangeTorange";   // is valid
// str = "OrangeTorange"; is NOT valid as `str` is not a pointer but array name

Here both operations asked are valid!

Note: Two declarations char *str and char str[] are different. To understand it read: What does sizeof(&arr) return?

score 3 · Answer 2 · answered Aug 15 '13 at 17:39

3

p[1] = 'w' is attempting to modify a string literal, which is illegal. p = "OrangeTorange" is just assigning a different string literal to p, which is fine.

answered Aug 15 '13 at 17:39

Mud

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score 0 · Answer 3 · answered Aug 15 '13 at 17:40

You can't modify the original string because it's read-only data. You can modify the pointer to point to a different string because the pointer is modifiable. That is, p = "OrangeTorange" is not modifying the original string, only changing where the pointer p points to.

Replacing characters in C++ Pointer Character string?

3 Answers3