[webkit-dev] String::operator+= considered harmful

Dirk Schulze dschulze at adobe.com
Tue Sep 4 16:27:38 PDT 2012

I thought we had efforts to make String::operator+= use StringBuilder somehow? I can remember that we had a discussion on webkit-dev and definitely on bugzilla about improving String::operator+= instead of replacing it with StringBuilder.


On Sep 4, 2012, at 4:22 PM, Adam Barth <abarth at webkit.org> wrote:

> As part of the work to deploy efficient string patterns [1] throughout
> WebKit, Benjamin and I noticed a bunch of very inefficient code that
> uses operator+= with Strings:
> String foo;
> for (...)
>  foo += [...];
> return foo;
> This pattern is particularly inefficient because += mallocs an
> entirely new buffer for the result and copies the the string into the
> new buffer.  That means that this pattern makes O(n) calls to malloc
> and does O(n^2) work.  A more efficient pattern is to use
> StringBuilder:
> StringBuilder foo;
> for (...)
>  foo.append([...]);
> return foo.toString();
> I'm in the process of going through WebCore and removing all callers
> of WTF::String::operator+=.  Once that's complete, my plan is to
> remove WTF::String::operator+= from WTFString.h.  Hopefully that will
> nudge contributors and reviewers towards more efficient string
> patterns.
> Removing operator+= will likely require changes to a number of
> port-specific files.  I'll do my best to remove these, but I might
> need some help from maintainers of individual ports.  If you're
> interested in helping out, please let me know.
> Many thanks,
> Adam
> [1] http://trac.webkit.org/wiki/EfficientStrings
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