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Re: Sequence container capacity after calling clear()

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In comp.lang.c++ Leigh Johnston <leigh@i42.co.uk> wrote:
> Currently the behaviour is unspecified and I know of at least one
> implementation that deallocates on vector<T>::clear().

One would think that such an implementation would get so many complaints
that they would fix that.

I don't think it's extremely rare for people to use std::vector as a
buffer to do some data manipulation. If said code is called very
frequently, then allocating a new array every single time would have
a really big impact on performance, so it's better to reuse the one
and same std::vector instance each time (by either making it 'static'
if it's inside the function, a member function of the class, if this
is a class, or a compilation-unit-local object.)

If this is so, then usually you want to clear the vector before
doing anything with it, so that it doesn't contain anything from the
previous run (assuming that you don't know in advance how many elements
there will be in the vector.)

If the vector implementation works as usual, this is a fast operation
because a 'clear()' will simply destroy the objects and set the size
to zero, but not actually free the memory. Thus it will work as a
static memory buffer.

However, if 'clear()' frees the memory, that means that there will be
a deallocation and an allocation every single time that the function is
called, having a big impact in performance. The only way to make sure
that this doesn't happen is to bypass std::vector and make your own
class which manages a dynamic array. The value of std::vector as a
useful tool has thus been diminished.

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Sequence container capacity after calling clear() Leigh Johnston <leigh@i42.co.uk> - 2013-03-23 16:27 -0600
  Re: Sequence container capacity after calling clear() Daniel Krügler <daniel.kruegler@googlemail.com> - 2013-03-23 22:05 -0700
    Re: Sequence container capacity after calling clear() James Kanze <james.kanze@googlemail.com> - 2013-03-27 13:28 -0600
  Re: Sequence container capacity after calling clear() Jason McKesson <jmckesson@googlemail.com> - 2013-03-23 22:05 -0700
  Re: Sequence container capacity after calling clear() Juha Nieminen <nospam@thanks.invalid> - 2013-03-24 23:36 -0700
  Re: Sequence container capacity after calling clear() Mathias Gaunard <loufoque@googlemail.com> - 2013-03-25 10:49 -0600
    Re: Sequence container capacity after calling clear() Leigh Johnston <leigh@i42.co.uk> - 2013-03-26 08:55 -0600

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