Modifying const string

[kunalnandi]

Hi,

const char chArr[] = "Hello World!!!";
char *const pch = &chArr[0];

*pch = 'A'; //Allowed
chArr[0] = 'B' //Not-Allowed

Why it possible to change the Value at &chArr[0] using *pch pointing to &chArr[0] even though chArr[0] is readonly..

Thanks!

[MK27]

Because pch is a const pointer to a non-const object. If you have compiler warnings enabled, you should get something like "initialization discards qualifiers from pointer target type" for pch (referring to the fact that pch should point to a char, but chArr[0] is a const char).

Here's the difference: notice you will also get an error for this, post initialization:

pch = NULL;

Because pch is a const pointer. This is not the same thing as a pointer to a const value.

char *const pch; // const pointer to non-const type
const char *pch; // non-const pointer to const type
const char *const pch; // const pointer to const type

The last two will give you an error for *pch = 'A', and get rid of the discarded qualifier warning, because the types match correctly.

BTW, the normal way to initialize a pointer to a char array is:

char *pch = &chArr[0]; // confusing and unnecessary
char *pch = chArr; // better

[grumpy]

Because pch is a const pointer to a non-const char. Its value can't be changed, but what it points to can. Move the const to the left of the asterix to make it a pointer to const char.

In your code, the assignment of *pch has undefined behaviour according to the standard. Among other things, that says programmers should avoid such things, but a compiler is not required to issue a diagnostic (aka error message) in response. Several good modern compilers, however, will issue a warning if you configure to higher warning levels.

It is a historical anomaly in the C language that the initialisation of pch in your code is not formally a compiler-diagnosable error. In the days before the C standard, when the const keyword did not even exist, allowing an assignment of the form "char *p = "some string literal" was introduced to cater for programmer laziness. A lot of lazy but vocal programmers used that feature in their code so, when const was later introduced (and a string literal was specified to have a const attribute), an exception had to be written into the language in order to avoid breaking a lot of existing code.