972167 - when copy constructing an nsAutoTArray it does not use its in-built buffer

Status: RESOLVED FIXED

(Core :: XPCOM, defect)

Description

[Cameron McCormack (:heycam)]

(Found in bug 972078.)  I find when I copy construct an nsAutoTArray from another nsAutoTArray that it does not end up invoking either of nsAutoBaseBase's two constructors, and thus doesn't call Init().  This means that mCapacity remains at 0 instead of the fixed internal buffer size, and will mean that the AppendElements call will always allocate memory even if there was enough space in the internal buffer.

Comment 1

[Cameron McCormack (:heycam)]

Attachment: patch

Don't really know if this is the right thing, or why the existing explicit copy constructor on nsAutoTArray doesn't get called.  But it does fix the problem I was having.

Comment 2

[[e-mail]]

Attachment: Proposed patch

I don't think it's the right thing.

Comments in nsTArray.h correctly point out that nsTAutoArray's implicit copy constructor will use nsAutoArrayBase's copy constructor. Unfortunately the provided constructor is not a copy constructor because its argument is of the wrong type and so it doesn't get used.

How your patch worked was that it was an explicit constructor, so it can only use an explicit superclass constructor, in this case the default constructor, although you should have constructed Base explicitly using
explicit nsAutoTArray(const nsAutoTArray<E, N>& other) : Base(other) {}

Comment 3

[[e-mail]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/bba551f00ace

Comment 4

[[e-mail]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/c4393e9e9e21

Because the constructor was itself broken, although MSVC doesn't care.

Comment 5

[Cameron McCormack (:heycam)]

Thanks for finding the correct fix Neil.

Comment 6

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/bba551f00ace
https://hg.mozilla.org/mozilla-central/rev/c4393e9e9e21