Re: Iterator or for loop?
On Dec 22, 12:53 am, Ian Collins <ian-n...@hotmail.com> wrote:
On 12/22/10 01:46 PM, Chris Gordon-Smith wrote:
On Tue, 21 Dec 2010 15:12:49 -0800 (PST), Joshua Maurice<joshuamaur...@gmail.com> wrote:
On Dec 21, 3:09 pm, Andrea Crotti<andrea.crott...@gmail.com> wrote:
The "standard" way to iterate over a container should be:
std::vector<int>::iterator iter;
for (iter=var.begin(); iter!=var.end(); ++iter) {
...
}
I read (I think in in one of Herb Sutter's books) that this entails
calculating var.end() every time round the loop, and that using
a local variable set to var.end() can be more efficient. I wondered
whether compilers would be smart enough to optimise this without
any special user coding.
For std::vector, end() almost certainly optimises a way to a constant
(end doesn't have to be calculated in the loop).
For std::vector, end() certainly isn't a constant. It will
change anytime you grow the vector.
A really good compiler certainly could determine that it was
a loop invariant (if you are actually growing the vector in the
loop, you're likely to have problems with iter as well), but the
analysis isn't trivial, and in the cases I've actually measured,
it does make a (very small) difference.
With regards to the initial question, int the implementations of
std::vector that I've actually looked at, size is implemented:
size_t size() const { return end() - begin(); }
So using indexes and size() won't help anything.
I'd use whatever is most reasonable, until the profiler said
otherwise. For an experienced C++ programmer, that sounds like
the iterator version to me.
--
James Kanze