proposal: capacity variable for dynamic arrays

[Oskar Linde]

Ameer Armaly skrev:
> Right now dynamic arrays use length to resize and handle memmory allocation. 
> This is a bit limiting as far as the old memmory  reservation problem as 
> well as a few other things.  What I propose is adding a new variable called 
> capacity, which handles the actual memmory allocation and using length for 
> the accepted size of the array.  Here are a few code fragments to show how 
> it would work:
> 
> char[] string = new char[1024];
> assert(string.length==1024 && string.capacity=1024);
> string.length=0; // we've reserved memmory, but haven't freed it
> version(break) writefln(string[1..4]); // throws a bounds exception, since 
> length is zero
> string.capacity = 1300; // traditional resize with a different variable, 
> will be set to whatever the true capacity is
> string.length = 2048; // would still work, since length is greater than 
> capacity so it acts the same way.  However, capacity would be the actual 
> amount of memmory reserved by the malloc algorithm, in this case doubling 
> the size of the array.
> string.capacity = 0; // freed
> 
> In short, this  solves the problem of reserving memmory with little cost, 
> while maintaining backwards compatibility.  It could also be used in the 
> stream and socket code, where the actual capacity doesn't change but the 
> length does, removing the need for a separate size variable, something like 
> this:
> 
> char[] buf = new char[1024];
> buf.length = 0;
> Socket s;
> // set up the socket
> s.read(buf); // Length is set to the actual data, but capacity is the same. 
> This way the array is truely dynamic.
> 
> Thoughts?

Where would the capacity value be stored? Today the capacity is (as far 
as I understand) stored by the GC in correspondance to the the allocated 
  chunk of memory.

What could be done is having a virtual .capacity property that when set 
to a value makes sure the capacity is /at least/ the value set and 
returns the actual capacity when read. So for example after doing
arr.capacity = 1024;
arr.capacity could be something at least 1024, like probably 2047.

/Oskar