• bitwise (34/34) Jan 25 2017 Compiling the code below gives these errors:
• Adam D. Ruppe (13/15) Jan 25 2017 It isn't the inout that's getting you, it is the const object in
• bitwise (21/36) Jan 25 2017 This was intentional, because I thought that due to transitivity,

bitwise <bitwise.pvt gmail.com> writes:

Compiling the code below gives these errors:

main.d(92): Error: 
cast(inout(int))this.list.data[cast(uint)(this.pos + i)] is not 
an lvalue
main.d(101): Error: template instance 
main.Iterator!(const(List!int)) error instantiating
main.d(108): instantiated from here: first!(const(List!int))

struct Iterator(C)
{
     C list;
     int pos;
	
     alias T = typeof(list.data[0]);

     this(C list, int pos) {
         this.list = list;
         this.pos = pos;
     }

     ref inout(T) opIndex(int i) inout {
         return list.data[pos + i];
     }
}

class List(T)
{
     T[] data = new T[1];

     auto first(this This)() {
         return Iterator!This(this, 0);
     }
}

int main(string[] argv)
{
     const(List!int) c;
     auto it = c.first;
}

Is it not possible to return a ref from an inout function?

Adam D. Ruppe <destructionator gmail.com> writes:

On Wednesday, 25 January 2017 at 20:42:52 UTC, bitwise wrote:
 Is it not possible to return a ref from an inout function?

It isn't the inout that's getting you, it is the const object in 
main().

     const(List!int) c;

Make that mutable and it works. Why? Cuz the `C list` in the 
iterator keeps that const with it.... I'm not sure why that kills 
it though, the error tells me it is an internal cast that is 
breaking things but I don't see why that logically would.

This might arguably be a bug, but you could work around it by 
checking for that const This and offering a different method that 
just returns const instead of ref.

     T[] data = new T[1];

BTW what do you think this line does? I ask because most people 
who use it don't get what they expect out of it....

bitwise <bitwise.pvt gmail.com> writes:

On Wednesday, 25 January 2017 at 21:04:50 UTC, Adam D. Ruppe 
wrote:
 On Wednesday, 25 January 2017 at 20:42:52 UTC, bitwise wrote:
 Is it not possible to return a ref from an inout function?

 It isn't the inout that's getting you, it is the const object 
 in main().

     const(List!int) c;

 Make that mutable and it works. Why? Cuz the `C list` in the 
 iterator keeps that const with it.... I'm not sure why that 
 kills it though, the error tells me it is an internal cast that 
 is breaking things but I don't see why that logically would.

This was intentional, because I thought that due to transitivity, 
'alias T' would also be const, and opIndex would return const(T) 
when created from a const(List!T).

I just tried the following though, and it outputs 'int' rather 
than 'const(T)' for a const(List!T):

alias T = typeof(list.data[0]);
pragma(msg, T.stringof);

This is not what I thought would happen.

I actually changed it up like this, and things seem to work:

alias T = CopyTypeQualifiers!(C, typeof(list.data[0]));

ref inout(T) opIndex(int i) inout{
     return list.data[pos + i];
}

 This might arguably be a bug, but you could work around it by 
 checking for that const This and offering a different method 
 that just returns const instead of ref.

Given what the solution was, I think the error message could be 
improved, but I'm not sure what the right approach would be.

     T[] data = new T[1];

 BTW what do you think this line does? I ask because most people 
 who use it don't get what they expect out of it....

Should be an array of 'T' with all elements set to T.init right?
This was just an example - for some reason, I thought it would 
make things clearer.

   Thanks