Associative Array of structs

[Jarrett Billingsley]

On Tue, Dec 2, 2008 at 7:34 PM, Zane <zane.sims at gmail.com> wrote:
> I tried writing to an associative array and got the exception:
>
> tango.core.Exception.ArrayBoundsException at test(15): Array index out of bounds
>
> the program below causes the exception:
>
> module test;
>
> import tango.io.Stdout;
> import tango.io.Console;
>
> struct S {uint i;}
>
> int main()
> {
>        S[char[]] s;
>
>        s["test"].i = 1;
>
>        Stdout.format("Done!").flush;
>
>        return 0;
> }
>
> But the program below works fine:
>
> module test;
>
> import tango.io.Stdout;
> import tango.io.Console;
>
> struct S {uint i;}
>
> int main()
> {
>        S[char[]] s;
>
>        S t;
>        t.i = 1;
>
>        s["test"] = t;
>
>        Stdout.format("Done!").flush;
>
>        return 0;
> }
>
> Am I supposed to be able to do both methods, or is the second the only possible way?  Some explanation would be very helpful.

Only the second is legal.  The first is doing something like this:

auto temp = &s["test"];
temp.i = 1;

Of course, you haven't added anything to s yet, so s["test"] fails
with an out-of-bounds error.

It's a little awkward, and in fact long ago, AAs used to implicitly
add items when accessing undefined indices (but many people found that
unintuitive and the behavior was changed to what it is now).

It's not too much work to make an AA wrapper struct which overloads
opIndex[Assign] and allows you to do this, by adding key-value pairs
if they don't already exist.