Logical const

[Max Samukha]

On 11/29/2010 11:22 PM, Max Samukha wrote:
> On 11/29/2010 08:58 PM, Steven Schveighoffer wrote:
>> Most likely not. How do you say that the 'draw' function switches the
>> widget to a different parameterized type? With const, you can just slap
>> a const on the end of the function.
>>
>> Here is some example of what I mean:
>>
>> class Widget
>> {
>> mutable Window location;
>> int x, y, width, height;
>> void draw() const { location.drawRectangle(x, y, width, height); }
>> }
>>
>> where Window.drawRectangle is not a const function.
>>
>> -Steve
>
> I think we could try to devise a library solution. For example, the
> problem could be solved with a templated struct + "alias this", provided
> bugs in "alias this" are fixed. The following code works even with the
> current implementation (dmd 2.050):
>
> struct Mutable(T)
> {
> T value;
>
> @property ref Unqual!T mutable() const
> {
> auto p = cast(Unqual!T*)&value;
> return *p;
> }
> alias mutable this;
>
> ref opAssign(U)(auto ref U v) const
> {
> return mutable() = v;
> }
> }
>
> class Window
> {
> void drawRectangle(int x, int y, int width, int height)
> {
> writeln("Draw");
> }
> }
>
> class C
> {
> int x, y, width, height;
> Mutable!Window location;
>
> this()
> {
> location = new Window;
> }
>
> void draw() const { location.drawRectangle(x, y, width, height); }
> }
>
> void main()
> {
> const C c = new C;
> c.draw();
> }
>
> (opAssign shouldn't be necessary. The compiler should properly forward
> the assignment operation to the value returned by "mutable")
>

"C" is a remnant of the test I used locally. Should be renamed to Widget.

p temporary in "mutable()" should not be necessary but compilation fails 
without it. Also, can a cast from lvalue result in lvalue wherever this 
makes sense?