Is there a way to make a class variable visible but constant to outsiders, but changeable (mutable) to the class itself?

[Mike Parker via Digitalmars-d-learn]

On Sunday, 22 May 2016 at 19:29:59 UTC, Meta wrote:

> Const *is* necessary to prevent _myVar being written to through 
> code like:
>
> f.myVar = 4;
>
> Of course this isn't necessary for value types, but for 
> reference types.
>


I was referring specifically to marking the function const, not 
the return type. Marking the return type const is highly 
context-dependent. It's perfectly reasonable to return a 
non-const class reference from a getter property. As long as the 
internal reference is private, it isn't going to be overwritten 
externally without a setter property. I don't see how it could be 
considered necessary. For a pointer, sure, to prevent 
*(bar.fooVer) = Foo(10). But for class references it's only 
necessary if you don't want the returned instances members to be 
modified.

> It's also useful for value types, IMO, for preventing someone 
> from doing this:
>
> f.myVar = 4;
>
> And wondering why the code has no effect.

The compiler already gives an error message describing the 
problem:

  Error: function mod.Foo.myVar () is not callable using argument 
types (int)

How does const help here?