[webkit-dev] Custom String class over STL string class

wei wang apunix at gmail.com
Thu Jan 15 01:48:11 PST 2009

Thank you for replying.
Well I have another question on using string in WebKit. std::string employs
its own memory management policy, but I've found that StringImpl uses
'malloc' and 'free' directly. It doesn't use copy-on-write strategy, even in
its copy constructor. So how is StringImpl's performance of memory
management compared with std::string.

Thanks again.

On Wed, Jan 14, 2009 at 1:21 AM, Darin Adler <darin at apple.com> wrote:

> On Jan 12, 2009, at 4:53 PM, wei wang wrote:
>  WebKit uses its own string classes instead of using STL string, I would
>> like to ask why you want to implement your own string classes? Although
>> webkit uses UTF-16 internally, we can also implement our own string traits
>> to manipulate UTF-16 char using STL string.
>> What is your driving force? Are there some advantages of own string
>> classes over STL string? Thank you.
> I think you're asking this question backwards given the history of WebKit.
> Originally, WebKit was started from the KHTML and KJS projects. Those
> projects had multiple string classes in their code:
>    QString, the Qt string class
>    DOMString, used in the HTML DOM
>    KJS::UString, used in the JavaScript interpreter
> Over time, we've simplified these, eliminated QString, and are still trying
> to reduce the number of classes.
> If your question is, "Why don't we switch to std::string?", then the burden
> of proof goes in the other direction. The burden is on the person proposing
> the switch.
> The various string classes have many features that std::string does not
> have, but it's not as if someone has done this analysis and has a list of
> them on hand. I can think of four differences quickl:
>    WebCore::String is immutable, std::string is not
>    WebCore::String stores a hash code to make it efficient to use the
> string repeatedly as a hash table key
>    WebCore::String stores only a length, not a capacity, so is smaller than
> a string class that also stores a capacity for efficient resizing
>    WebCore::String uses high-speed single-thread reference counting
> But I'm not sure these are the most important differences; there may be
> other more critical ones. Since std::string is part of the standard library,
> it's implementation and quality of implementation are likely to be different
> on different platforms.
>    -- Darin
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