const(X) member of Y

[deadalnix]

On Wednesday, 6 February 2013 at 18:15:53 UTC, Dan wrote:
>
> Start with:
>
> struct X {
>   char c[];
> }
>
> Assume you want value semantics - postblit provides this 
> capability. It appears with 2.061 'this(this) const' is now 
> supported. Previously, only 'this(this)' was recognized (i.e. 
> called) when expected. To get value semantics a developer must 
> choose between these two and for this case the goal is to 
> ensure 'c = c.dup;' so value semantics are preserved.
>
> Case 1: Go with 'this(this) { c = c.dup; }'. This works just 
> fine except for the case where you want a const(X) as a member 
> of some other class. For example: struct Y { const(X) x; }.
>
> Case 2: Go with 'this(this) const { c = c.dup; }'. This is not 
> possible because you can not change 'c' since the function is 
> const. Maxim Fomin pointed out a workaround.
>

Both are bugs, as const constructor are supposed to be able to 
set const variable once. See TDPL for reference on that.

> struct X {
>   char c[];
>   void _postblit_() { c = c.dup; }
>   this(this) const {
>     void delegate() dg = &_postblit_;
>     dg();
>   }
> }
>

This is also a bug as transitivity of const isn't respected.

Welcome in weirdland !