API : constness ?

Erik Max Francis max at alcyone.com
Tue Jun 1 20:42:03 CEST 2004


Andrea Griffini wrote:

> You can declare a constant pointer (you can't change
> the pointer) to constant data (you can't change the
> data). But still those properties are properties of
> the pointer, not of the data. I know it may be
> surprising at a first sight, but the code generated
> by the compiler is not allowed to assume that the
> "pointed to" data will not change; the reason is
> that the limit of "you can't change the data" is
> indeed just related to the pointer... in other words
> the limit is only that you can't change the data
> 
>             ==> USING THAT POINTER <==
> 
> By no means you're stating the data is really constant.

You're eliminating an important distinction here, which is the
difference between a pointer-to-const parameter, and data that is
actually const.  You're talking about a pointer-to-const parameter, I'm
talking about data that's actually const.  The original poster wrote
code that created data that was actually const, and then passed it to a
function that he wanted to take a pointer-to-const parameter.

> Note also that casting away const-ness from a pointer
> and changing the data is something that must be
> supported if the data is not really constant.

Yes, that is correct.

> In other words:
> 
>    void foo(const int *x)
>    {
>      int *y = (int *)x;
>      ++(*y);
>    }
> 
>    int main()
>    {
>      static int a = 3;
>      foo(&a);
>      // Here a will be 4
>      ...
>    }
> 
> The above code is perfectly legal; looking at main()
> and at the declaration of foo the compiler cannot
> decide to put "a" in read-only memory.

Yes.  And if a were declared

	static const int a = 3;

it would be illegal.  That is the case that I was discussing.

-- 
 __ Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
/  \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ But who shall dwell in these worlds if they be inhabited?
    -- Johannes Kepler



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