Stupid const question

[QuoVadis]

Hi all. Why does this code cause a compiler error ("you cannot assign to a variable that is const"). I thought the int wasn't const?

int a = 72;
int* const pint2 = &a;
*pint2++;

Cheers

[kikirikou]

As i can see the problem is not the int* const pint2 = &a; because you can pass a non cast variable into a cast.Your problem is here *pint2++; ,pint2 is const so you can't modify it. cheers :)

[QuoVadis]

Yes, as you say, pint2 is const but *pint2 isn't (or so i thought).

[kikirikou]

Look there are 3 ways you can use your const

Ι)char *const cp;
ΙΙ)char const* pc;
ΙΙΙ)const char* pc2;


The first is a const pointer to a char ,its address doesn't change ,only its value.
The second is a pointer to a const char //You can change its address but not its value
The third is the same as the second one.

So you belong in the second type,you try to change the value which is contained in a .This is not allowed!

[kbw]

It's not a stupid question.

pint2 is a pointer to a constant int. You're trying to assign it a pointer to non-constant int (&a).

You may find this useful.
http://www.cplusplus.com/forum/beginner/10602/

[QuoVadis]

How do I belong 'in the second type'. My declaration has the same form as the first one.

[QuoVadis]

kbw, thanks. However, unless I've misunderstood you, the compiler has no problem at all with these two lines:

int a = 72;
int* const pint2 = &a;

Only the last one:

*pint2++;

[kikirikou]

Look if you write it this way is correct :

int a = 72,b=60;
const int* pint2 = &a;
*pint2++;

Because is a const pointer to changeable value!

[QuoVadis]

Yes, thanks, but what I was after is *why* doesn't the code in my original post compile?

[kikirikou]

The only change you can do in your original code is this :

int a = 72,b=70;
int* const pint2=&a;
*pint2=b;

because is a pointer to a const value.You can not change its value but you can change where it shows!!!Here *pint2=b; i make him to show a new value!

[QuoVadis]

Thanks for your help. I'm confused because I have read that a declaration like this one

int* const pint2 = whatever;

is declaring a *const* pointer to a *non-const* int.

[QuoVadis]

I've just got the real answer to this from another forum. The reason my code doesn't compile was down to operator precedence. In this line:

*pint2++;

The ++ operator is applied before the * operator meaning it therefore is applied to the pointer not the int.

[kikirikou]

Dum spiro spero