Again a simple question about std::vector

[ciccio]

Hi,

Again I am wondering about something.

If you use the swap function of an std::vector, what does it do?

Does it swap each element separately by means of copying to a tmp
variable, or does it just swap some pointers to memory blocks.

Regards

[Victor Bazarov]

ciccio wrote:

Again I am wondering about something.

If you use the swap function of an std::vector, what does it do?

Does it swap each element separately by means of copying to a tmp
variable, or does it just swap some pointers to memory blocks.

In most implementations, the latter. Of course it also has to
swap the size if it's cached (and it probably is), and other data
members.

V
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[Jerry Coffin]

In article <ft**********@ikaria.belnet.be>, no************@spam.com
says...

Hi,

Again I am wondering about something.

If you use the swap function of an std::vector, what does it do?

Does it swap each element separately by means of copying to a tmp
variable, or does it just swap some pointers to memory blocks.

It's pretty much required to do the latter. It's required to have
constant complexity, and it's can't throw an exception (a swap() can
only throw an exception if the container's Compare() function throws the
exception, and a vector doesn't have a Compare() function).

Contrary to the implication elsethread, there's no real difference
between std::swap<vector, vectorand vector::swap(vector). According to
section 23.2.4.4, the standard library is required to contain a
specialization of std::swap for vector so that if v1 and v2 both refer
to vectors, std:swap(v1, v2) is equivalent to v1.swap(v2).

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Lionel B]

On Thu, 03 Apr 2008 08:14:18 -0600, Jerry Coffin wrote:

In article <ft**********@ikaria.belnet.be>, no************@spam.com
says...

>Hi,

Again I am wondering about something.

If you use the swap function of an std::vector, what does it do?

Does it swap each element separately by means of copying to a tmp
variable, or does it just swap some pointers to memory blocks.

[...]

Contrary to the implication elsethread, there's no real difference
between std::swap<vector, vectorand vector::swap(vector). According to
section 23.2.4.4, the standard library is required to contain a
specialization of std::swap for vector so that if v1 and v2 both refer
to vectors, std:swap(v1, v2) is equivalent to v1.swap(v2).

Right, I wasn't aware that it was actually a requirement. I guess that
applies to every container, then (I can't think why it shouldn't).

--
Lionel B

[James Kanze]

On 3 avr, 16:14, Jerry Coffin <[e-mail]:

In article <ft2ck5$[e-mail]>, [e-mail]
says...

If you use the swap function of an std::vector, what does it do?

Does it swap each element separately by means of copying to a tmp
variable, or does it just swap some pointers to memory blocks.

It's pretty much required to do the latter. It's required to have
constant complexity, and it's can't throw an exception (a swap() can
only throw an exception if the container's Compare() function throws the
exception, and a vector doesn't have a Compare() function).

I've been wondering about this. What about the allocators? If
I understand the philosophy behind them, all memory that was
allocated must be freed by an allocator of the same type, which
compares equal to the one used to allocate. The same type is
automatic, since the type of the allocator is part of the type
of the template specialization. But maintaining the second
condition means somehow swapping the allocators as well, at
least if they don't compare equal. And off hand, I don't see
any requirement that allocators are "Swappable", nor that they
cannot throw if you attempt to swap them with std::swap.

Is this an oversight in the standard, or is there something I've
missed?

--
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[Jerry Coffin]

In article <ft**********@south.jnrs.ja.net>, [e-mail] says...

[ ... ]

Contrary to the implication elsethread, there's no real difference
between std::swap<vector, vectorand vector::swap(vector). According to
section 23.2.4.4, the standard library is required to contain a
specialization of std::swap for vector so that if v1 and v2 both refer
to vectors, std:swap(v1, v2) is equivalent to v1.swap(v2).

Right, I wasn't aware that it was actually a requirement. I guess that
applies to every container, then (I can't think why it shouldn't).

I believe that's correct, yes.

--
Later,
Jerry.

The universe is a figment of its own imagination.