On Tue, Jan 17, 2012 at 08:25:28PM +0100, Timon Gehr wrote:
[...]
> In other words, non-static nested classes can reference non-static
> fields of the enclosing class. [...]
[...]
> void main() {
> auto a = new A;
> auto b = a.new B; // construct an 'A.B' with 'a' in implicit
> 'outer' field
> a.x = 100;
> b.echo(); // writes '100'
> }
>
> This is probably one of the more obscure features of D. =)
[...]
It totally makes sense though. In some of my past C++ projects, I've had
to use the inner class idiom quite often. Of course, it's not directly
supported by the language so I ended up writing lots of little nested
classes like this:
class outer {
...
class inner1 {
outer *ctxt;
...
inner1(outer *c) : ctxt(c) {}
};
...
class inner2 {
outer *ctxt;
...
inner2(outer *c) : ctxt(c) {}
};
...
void f() {
...
inner1 *helper1 = new inner1(this);
register_callback(helper1, ...);
...
inner2 *helper2 = new inner2(this);
register_callback(helper2, ...);
...
}
};
After a while, it just got really really tedious to keep writing the
same boilerplate code over and over again. In D, a lot of that
redundancy can be gotten rid of (no need for explicit outer pointers in
the inner classes, eliminate ctor parameters), just because (non-static)
inner classes automatically get an outer pointer, and you can just
instantiate them with:
auto helper1 = this.new inner1;
But D lets you do even better. Instead of creating an inner class, you
can just pass a delegate to do what needs to be done:
void f() {
...
register_callback((args) { this.state1++; }, ...);
register_callback((args) { this.state2++; }, ...);
...
}
Much more readable, and much less room for bugs to hide in.
T
--
"Maybe" is a strange word. When mom or dad says it it means "yes", but when my
big brothers say it it means "no"! -- PJ jr.
