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.
