Logical const

[Steven Schveighoffer]

On Tue, 30 Nov 2010 16:05:16 -0500, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> Steven Schveighoffer wrote:
>> On Tue, 30 Nov 2010 15:16:04 -0500, Walter Bright  
>> <newshound2 at digitalmars.com> wrote:
>>
>>> Steven Schveighoffer wrote:
>>>> The example that I gave does not seem to you like it would surprise  
>>>> someone?  I passed in a const object and it got modified, even though  
>>>> no casts were used.
>>>
>>> No, it doesn't surprise me. Const on one object does not apply to  
>>> another object.
>>  So this:
>>  void myfn(const(C) n)
>> {
>>    assert(n.x == 1);
>>    n.foo();
>>    assert(n.x == 5);
>> }
>>  Wouldn't be surprising to you?
>
> No. There are a lot of things a member "x" could be. The const only  
> applies to the instance data.

If you read my previous code, x is instance data.  I'll reiterate the  
class definition:

class C
{
    static C theCommonOne;
    int x;
    void foo() const {
       theCommonOne.x = 5; // theCommonOne could be this
    }
}

If you find the above unsurprising, you are in the minority.  I find it  
surprising, and invalid that anyone would write code this way.  People  
simply just don't do that normally.  It's just written to demonstrate a  
point that the compiler does not guarantee anything via const, it's  
guaranteed by convention.  The compiler simply helps you follow the  
convention.

-Steve