The analogue of "fill-pointer" in D

[Dennis Ritchie via Digitalmars-d-learn]

On Monday, 18 May 2015 at 17:14:46 UTC, Steven Schveighoffer 
wrote:
> capacity is analogous to the number of elements in the vector 
> (as returned by array-dimension according to 
> https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node162.html).
>
> arr.length is analogous to the fill pointer.
>
> example:
>
> int[] arr = new int[](5);
> assert(arr.capacity > 5);
> assert(arr.length == 5);
>
> arr.reserve(100); // expand arr memory block to be able to hold 
> *at least* 100 ints
>
> assert(arr.capacity >= 100);
> assert(arr.length == 5);
>
> auto ptr = arr.ptr; // for later assert
>
> arr ~= 1; // increment length by 1, 'fill in' tail of array 
> with '1'
>
> // this should demonstrate how it works
> assert(arr.length == 6); // new fill pointer
> assert(arr.capacity >= 100); // capacity unchanged
> assert(arr.ptr is ptr); // array still lives in same memory 
> block
>
> Apologies for not translating to lisp, I don't know it.
>
> -Steve

Thank you. This is what you need!