Re: [boost] boost] Attn metaprogrammers! [Better Enums] More complete release - feedback requested

Subject: Re: [boost] boost] Attn metaprogrammers! [Better Enums] More complete release - feedback requested
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2015-06-09 10:08:46

Gavin Lambert wrote:
> On 9/06/2015 03:01, Anton Bachin wrote:
>> That's right, for now I am simply doing a linear scan through an array of
>> strings. Changing the complexity to sub-linear is on my list of things to do,
>> and I've considered those approaches (compile-time binary search, hashing, and
>> trie).
>
> Don't forget to measure performance.  Given that enums tend to have a 
> very low N, when I made a superficially similar static map with a 
> runtime string index scan it turned out to be faster to just do the 
> linear scan than to try to compute hashes or do more complex searches.

In my case I was processing a file with key-value data; I needed to
convert the key string to an enum.  There were about 25 keys that I
was interested in.  Initially I had a sequence of if statements each
doing a std::string::operator==(const char*).  I changed that to a
switch statement on the first character, followed by std::string::
operator==(const char*) for each of the keys with that initial letter.
This increased the speed of that code 50X, and the speed of the
overall file processing 2X.  It was definitely worthwhile.  There is
an important special case - when the string is not one of the valid
keys.  If this is frequent in a particular application, a linear scan
will do especially badly.

Regards,  Phil.