const version for foreach/opApply

[Steven Schveighoffer]

On Fri, 08 Jun 2012 12:33:20 -0400, Matthias Walter  
<xammy at xammy.homelinux.net> wrote:

> Hi,
>
> trying to traverse the entries of a std.bitmanip.BitArray I stumbled
> upon the following problem:
>
> The original code is as follows:
>
> int opApply(scope int delegate(ref bool) dg)
> {
>   int result;
>   for (size_t i = 0; i < len; i++)
>   {
>     bool b = opIndex(i);
>     result = dg(b);
>     this[i] = b;
>     if (result)
>       break;
>   }
>   return result;
> }
>
> In case I want to accept const(BitArray) objects, it shall look like the
> following (maybe using "ref const(bool)" for the delegate parameter?):
>
> int opApply(scope int delegate(bool) dg) const
> {
>   int result;
>   for (size_t i = 0; i < len; i++)
>   {
>     bool b = opIndex(i);
>     result = dg(b);
>     if (result)
>       break;
>   }
>   return result;
> }
>
> Can one glue both things together into a single routine (using inout
> magic or whatever)?

Not currently, because we have an issue of inout having two different  
meanings.

When a function's parameters or return values are adorned with inout,  
inout becomes sort of a wildcard.

When inside a function, however, inout turns into a restrictive form of  
const.

But what happens with a delegate parameter that *takes* an inout  
parameter?  For example:

int opApply(scope int delegate(ref inout(bool)) dg) inout

Is inout(bool) a wildcard for the delegate?  Or is part of the inout  
wildcard of the entire function?  Inside opApply, should it be the  
restrictive const type?

Currently, it is a wildcard for the delegate, and so it doesn't really  
help, once you go into opApply, you can't temporarily remove the inout  
type.

Note that for your particular instance I'd *recommend* two different  
versions, since the const version can be vastly optimized (passing a bool  
by value vs. by reference is much more efficient, and then there is no  
need to re-store the value after the delegate).

There are quite a few ideas being floated around as to how to make inout  
work for opApply, but for now, you should avoid it.

-Steve