A case for valueless AA's?

[Steven Schveighoffer]

On Tue, 29 Mar 2011 18:42:59 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> Piotr Szturmaj:
>
>> void[0][string] aa;
>> aa["key"] = [];
>> assert("key" in aa);
>
> Nice. I have done a benchmark, and it seems a void[0][int] uses the same  
> amount of memory as a int[int]. I have not tried a void[0][string].

Each node in the hash has a pointer (for the next node), a hash (size_t)  
and the key and value.  Given the granularity of allocation (16 bytes),  
anything less than 16 bytes will be 16 bytes.

If the above trick works correctly, a void[0][string] should be the same  
size as int[int]. (8 bytes for pointer and hash, 8 bytes for string key,  
and hopefully 0 bytes for the value).

BTW, you should not use [] in your code, as this calls a runtime array  
allocation function.  Better to use something else, not sure what though.   
Maybe an enum?

-Steve