Immutable member functions on mutable objects

[Steven Schveighoffer]

On Sun, 29 Nov 2009 07:23:07 -0500, Tomek Sowiñski <just at ask.me> wrote:

> I've got a problem calling an immutable getter on an "ordinary" object.
>
> struct A {
>     float _pole;
>     float pole() immutable {
>         return _pole;
>     }
> }
>
> void main() {
>     A a;
>     auto x = a.pole;   // Ouch!
> }
>
> Error: function hello.A.pole () immutable is not callable using argument  
> types ()
>
> There's no problem when pole is const. I assume the problem is the  
> hidden "this" parameter (is it? the message is a bit confusing). Then  
> again, A is implicitly convertible to immutable(A) so there shouldn't be  
> a problem, no? Maybe a compiler bug?
>
> BTW, can someone explain what's exactly the difference between a const  
> and immutable member function? The D page says only about the latter.


Don't forget that:

struct A {
float pole() immutable {...}
}

is essentially syntax sugar for:

struct A {}

float pole(immutable ref A this) {...}

It is a common misconception that const and immutable are *function*  
decorators.  They are actually decorators for the hidden 'this' reference.

When you look at it that way, it becomes hopefully much clearer how to  
deal with const and immutable.

-Steve