NotNull pointers

[Alex Rønne Petersen]

On 29-08-2011 22:22, Walter Bright wrote:
> For the latest dmd,
> https://github.com/D-Programming-Language/dmd/commit/1193f7828b444056c943742daae0a5ccf262272e
> ,
> I've implemented the ability to disable default initialization. This
> makes it practical to implement a library based "NotNull" type without a
> special syntax for it. The rationale for this is (rather than making it
> builtin) one can now build any type that is a restricted subset of
> another type. I suspect that a non-null type is just scratching the
> surface of this ability.
>
> Here's a draft prototype of the NotNull type:
>
> import std.c.stdio;
>
> struct NotNull(P) if (is(typeof({P p = null;})))
> {
> P p;
>
> this() @disable;
>
> this(P q)
> {
> assert(q);
> p = q;
> }
>
> NotNull opAssign(P q)
> {
> assert(q);
> p = q;
> return this;
> }
>
> alias p this;
> }
>
> void main()
> {
> int x;
> NotNull!(int*) s = &x;
> *s = 3;
> printf("test1 %d\n", *s);
> s++;
> printf("test2 %d\n", s[-1]);
> }
>
> What it does is:
> 1. disallows instantiation with any type that null doesn't implicitly
> convert to
> 2. disables default construction (this is the new feature)
> 3. intercepts construction and assignment to enforce not-nullness
> 4. uses alias this to forward the rest to the wrapped pointer type
>
> For example, try this:
>
> void main()
> {
> NotNull!(int*) s;
> // Error: variable test.main.s initializer required for type NotNull!(int*)
> }
>
> I do think that the "this() @disable;" is an ugly syntax, and I cringe
> when seeing it. But I can't think of anything better. It does make
> logical sense given the existence of default construction syntax and the
> @disable, so in a sense it is just connecting existing dots, which has a
> compelling value.

This is great! I've often had to use classes where I wanted to use 
structs because the default state would result in bad behavior.

Thanks!

- Alex