On Thu, Aug 30, 2018 at 7:43 PM, Henri Sivonen <hsivo...@mozilla.com> wrote:
>> What is then the point of SetCapacity anymore?
>
> To avoid multiple allocations during a sequence of Append()s. (This is
> documented on the header.)
At this point, it's probably relevant to mention that SetCapacity() in
situations other that ahead of a sequence of Append()s is most likely
wrong (and has been so since at least 2004; I didn't bother doing code
archeology further back than that).
SetCapacity() followed immediately by Truncate() is bad. SetCapacity()
allocates a buffer. Truncate() releases the buffer.
SetCapacity() followed immediately by AssignLiteral() of the
compatible character type ("" literal with nsACString and u"" literal
with nsAString) is bad. SetCapacity() allocates a buffer.
AssignLiteral() releases the buffer and makes the string point to the
literal in POD.
SetCapacity() followed immediately by Adopt() is bad. SetCapacity()
allocates a buffer. Adopt() releases the buffer and makes the string
point to the buffer passed to Adopt().
SetCapacity() followed immediately by Assign() is likely bad. If the
string that gets assigned points to a shareable buffer and doesn't
need to be copied, Assign() releases the buffer allocated by
SetCapacity().
Allocating an nsAuto[C]String and immediately calling SetCapacity()
with a constant argument is bad. If the requested capacity is smaller
than the inline buffer, it's a no-op. If the requested capacity is
larger, the inline buffer is wasted stack space. Instead of
SetCapacity(N), it makes sense to declare nsAuto[C]StringN<N+1> (with
awareness that a very large N may be a problem in terms of overflowing
the run-time stack).
(I've seen all of the above in our code base and have a patch coming up.)
--
Henri Sivonen
hsivo...@mozilla.com
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