Embedded non-assignable containers

[Philippe Sigaud]

> To make a type "double linkable" the developer needs to mixin the
following mixin template:
>
>> mixin template DoubleLinkable()
>> {
>>   typeof(this) next;
>>   typeof(this) prev;
>> }
>
>
> The next and prev pointer can be access with the help of mixin by using
the following templates:
>
>> T* next(T, string name)(T node) pure nothrow const
>>
>> {
>>
>>   mixin("return &(node." ~ name ~ ".next);");
>>
>> }

These are free functions? Why not put them in the mixin templates?

> To use the above abstraction the developer just needs to do the following:
>
>> class Person
>> {
>>   int age;
>>   int weight;
>>   mixin DoubleLinkable people;
>> }

Oh, named mixins! Yet another D feature I totally forgot. Hey, those
reading this, who here knew you could do 'mixin someTemplate someName;' to
refer to the template scope by the given name?

> I am not a big fan of the template signature 'class DoubleLinkedList(T,
string name)' but I couldn't figure out a better way of allowing one object
to be embedded in multiple containers. Thoughts? Any idea how this can be
improved so that it is easier to use and read?

I didn't read the blog post, but any reason why you do not put all the list
machinery in a mixin template and refer directly to next/prev?

I'm on a pad, so typing is not so easy, but like this:

mixin template NodeImplementation()
{
    // should detect if typeof(this) is a reference type or a value type.
    // for a reference type, we should not use pointers.
    typeof(this) next() @property { return *_next;}
    typeof(this) prev() @property { return *_prev;}
/// + setters also
    private *typeof(this) _next,_prev;
}

struct Person()
{
    mixin NodeImplementation;
    string name;
    int age;
}

And DoublyLinkedList(Node) if (isNode!Node)

with

/**
A node is a type T that has .next and .prev fields
that return another T.
*/
isNode(T)
{
    enum isNode = __traits(compiles, { T p = T.init.prev; T n =
T.init.next;});
}
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