Generic collection/element function signatures in D2 versus D1

[Jacob Carlborg]

On 2010-09-07 17:37, Pelle wrote:
> On 09/07/2010 04:33 PM, Steven Schveighoffer wrote:
>> Yes, a valid return. Your function should be:
>>
>> void foo(void delegate(const(C) f) const
>>
>> It helps to understand that inout/const/immutable has NOTHING to do with
>> code generation, it only has to do with limiting what compiles. For this
>> reason, an inout function is compiled once, and works on all three
>> constancies (4 if you have a nested inout function). For the entire
>> function any inout variable is treated as a non-changeable value, just
>> like const. Then when you return, it's converted at the call site back
>> to the constancy with which it was called. If the return value is void,
>> then there's nothing to convert, and no reason to use inout over const.
>>
>> I'll repeat -- there is no benefit to inout if you are not returning
>> anything.
>>
>> -Steve
>
> That's not an equivalent function signature. Or maybe it is, but look at
> this (sorry it's so long):
>
> class C {
> int x;
> this(int y) { x = y; }
>
> inout(int*) foo() inout {
> return &x;
> }
> void bar(void delegate(int*) f) {
> f(&x);
> }
> void bar(void delegate(const(int*)) f) const {
> f(&x);
> }
> void bar(void delegate(immutable(int*)) f) immutable {
> f(&x);
> }
> }
>
> void main() {
>
> immutable(int)* wah;
> void wahwah(immutable(int*) x) {
> wah = x;
> }
> auto c = new immutable(C)(10);
> wahwah(c.foo); // why is this privilegied with inout
> c.bar(&wahwah); // and this not?
>
> writeln(*wah);
>
> }
>
> Can't use void delegate(const(int*)) there.

That won't work, you can't have inout on return of a function without 
having inout for a least one parameter. The compiler can't know what to 
resolve inout to, mutable, immutable or const.

-- 
/Jacob Carlborg