Head Const

[Nick Treleaven via Digitalmars-d]

On Tuesday, 16 February 2016 at 10:32:45 UTC, Timon Gehr wrote:
> struct S{
>     int[] a;
>     void foo()headconst{
>         a[0]=1; // ok
>         // arr.length=2 // error
>     }
> }
>
> void main(){
>     headconst(S) s([0,2,3]);
>     s.foo();
>     assert(s.a==[1,2,3]);
> }
>
> How to do this in the library?

Maybe make foo a free function:

struct S{
     int[] a;
}

void foo(HConst!S self){
     self.a[0]=1; // ok
     self.arr.length=2 // no effect
}

Haven't tried, but maybe HConst can allow access to fields of S 
by value, not ref. That could prevent mutation like C++ const, 
but it wouldn't disallow mutation of the temporary copy. Not sure 
if that's good enough. I suppose the compiler could warn about 
unused temporary mutation.