Bug 128644 – Remove some unintended copies in ranged for loops

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKit Misc.

[Brent Fulgham]

Once I noticed the handful of errors in Bug 128578, I did some more searching and turned up a few more.

Per http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2049.pdf, the ranged iterator syntax:

    for( type-specifier-seq simple-declarator : expression )
        statement

is syntactically equivalent to

    typedef decltype(expression) C;
    auto&& rng(expression);
    for (auto begin(std::For<C>::begin(rng)), end(std::For<C>::end(rng)); begin != end; ++ begin) {
        type-specifier-seq simple-declarator(∗begin);
        statement
    }

The issue here is that the type of auto (instead of 'auto&' or 'const auto&') causes us to make a copy of each element as we pass through the loop.

When the container is just a  set of pointers, it's not much of an issue. But when we have iterate over reference-counted types, strings, or other larger objects we incur unnecessary costs.

[Brent Fulgham]

Created attachment 223932 [details]
Patch

[Brent Fulgham]

Committed r163959: <http://trac.webkit.org/changeset/163959>