On Mon, Jun 07, 2021 at 03:26:27PM +0000, someone via Digitalmars-d-learn wrote:
> Consider the following code:
>
> ```d
> class classComputer {
[...]
> }
>
> class classComputers {
>
> classComputers lhs;
> classComputers rhs;
>
> int opApply(int delegate(classComputers) dg) { /// boilerplate code to
> handle the class's default collection
>
> if (lhs && lhs.opApply(dg)) return 1;
> if (dg(this)) return 1;
> if (rhs && rhs.opApply(dg)) return 1;
> return 0;
>
> }
>
> public classComputer[] computers; /// how can I tag this as the default
> property ?
alias computers this;
>
> }
>
> void main (
>
> ) {
>
> classComputers lobjComputers = new classComputers;
> lobjComputers.computers ~= new classComputer("dell");
> lobjComputers.computers ~= new classComputer("ibm");
> lobjComputers.computers ~= new classComputer("apple");
> lobjComputers.computers[1].name = r"lenovo";
>
> foreach(lobjComputer; lobjComputers.computers) {
> writeln(lobjComputer.name); }
>
> ///foreach(lobjComputer; lobjComputers) { writeln(lobjComputer.name); }
> /// with default property (if possible)
>
> }
> ```
>
> The above code works correctly, however, avoiding the redundancy of
> lobjComputers.computers will be a plus.
>
> Also tell me if the collection is implemented the right way, it is my
> first code using the opApply() delegate which I don't deeply
> understand for the time being.
It's very simple. Whenever some non-array object appears on the right
side of a foreach() statement, the compiler looks for a method on the
object called .opApply. If it exists, the loop body is passed to that
method as a delegate. IOW:
// This:
foreach (item; myCollection) {
/* loop body here */
}
// Gets translated to this:
myCollection.opApply((item) { /* loop body here */ });
Given that, your .opApply method doesn't really do what you want. It
should instead be written like this:
// N.B.: dg is NOT the type of the collection, but the
// individual item you want to iterate over.
int opApply(int delegate(classComputer) dg)
{
// Loop over the computers in the current node first
foreach (computer; computers) {
// Pass single item to loop body.
auto ret = dg(computer);
// N.B.: do NOT assume a non-zero return value
// will always be 1; it may not be if your loop
// body contains `break` or `continue`.
if (ret) return ret;
}
// Now recurse child nodes
if (lhs) {
auto ret = lhs.opApply(dg);
if (ret) return ret; // again, don't assume it will be 1
}
if (rhs) {
auto ret = rhs.opApply(dg);
if (ret) return ret; // again, don't assume it will be 1
}
return 0;
}
T
--
Which is worse: ignorance or apathy? Who knows? Who cares? -- Erich Schubert
