dynamic array .length vs .reserve - what's the difference?

[bachmeier]

On Thursday, 30 July 2020 at 15:58:28 UTC, wjoe wrote:
> I just stumbled upon code like this:
>
> struct Foo(T)
> {
>     T[] b;
>
>     this(int n)
>     {
>         b.reserve(n);
>         b.length = n;
>     }
> }
>
> .reserve looks redundant.
>
> The docs are explaining .length nicely, however lack any 
> specifics about reserve.

Here's a simple example to see that reserve and length serve 
different purposes. In particular, reserve doesn't fill any 
elements but allocates enough memory, while setting length puts 
values into the elements:

import std.stdio;

void main() {
	double[] x;
     writeln(x.length);
     writeln(x.capacity);
     x.reserve(50);
     writeln(x.length);
     writeln(x.capacity);
     x ~= 1.5;
     writeln(x);

     double[] y;
     y.length = 50;
     writeln(y.length);
     writeln(y.capacity);
     y ~= 1.5;
     writeln(y);
}

https://run.dlang.io/is/1EWf5R