opApply ref and "const" iterators

[Denis Koroskin]

On Tue, 29 Sep 2009 02:58:01 +0400, gzp <galap at freemail.hu> wrote:

> Hello,
>
> Iám new to D so i might be all wrong. Given the code:
>

Hello and welcome to the D world!

> class Foo {
> 	int opApply( int delegate(ref real) dg ) {...} //A.
> 	int opApply( int delegate(real) dg ) {...} // B,
> }
>
> ...
> 	Foo foo = new Foo;
> 	foreach( a; foo ) {...}  // 1.
> 	foreach( ref a; foo ) {...} // 2,
>
> Is it normal that, the A. version (with ref) is called for both cases

I believe questions like this belong to D.learn newsgroup.
I always wondered why is it so, too, but I never thought as being a bug  
about it.
Perhaps, someone else may explain a reason behind it. You may create a  
bugzilla ticket if no one will give an answer.

> and if the A is eliminated, only the 2. foreach loop will compile ?

I believe none of them will compile.

> The second case works as i expected but what about the first case - the  
> B. function won't be called ever ?
>
> Furthermore, i wanted to create a const iterator
>
> class Foo {
> 	int opApply( int delegate(ref real) dg ) {...} //A.
> 	const int opApply( int delegate(real) dg ) {...} // B,
> }
>
> ...
> 	Foo foo = new Foo;
>         const Foo f2 = foo;
> 	foreach( a; f2 ) {...}  // 1.
> 	foreach( ref a; foo ) {...} // 2,
>
> It won't compile. (dmd 2.032) (I expected that, for the 1. loop the B  
> version and for the 2. loop the A version of the functions'd  be called)
>
> So what is the D way to create const iterator ?

Once again, a ref version (A) is called, which is not marked as const and  
as such your code doesn't compile. Try marking it const, too, and it will  
pass.