empty vs. cont.begin() == cont.end()

[gelbeiche]

Is
( cont.begin() == cont.end() )
essentially equivalent to writing
( cont.empty() )
for a STL container ?

[CrayzeeWulf]

gelbeiche wrote:

Is
( cont.begin() == cont.end() )
essentially equivalent to writing
( cont.empty() )
for a STL container ?

Yes. According to the standard the operational semantics of the related
expressions are:

a.size() => a.end() - a.begin()
a.empty() => a.size() == 0

q.e.d
--
CrayzeeWulf

[Mike Wahler]

"gelbeiche" <bo*****@fliege npilz.de> wrote in message
news:m3******** ****@redrat.qua rk.de...

Is
( cont.begin() == cont.end() )
essentially equivalent to writing
( cont.empty() )
for a STL container ?

Yes, but there's a good possiblity that using 'empty()'
could be (fractionally) faster because it might be
optimized to simply report the value of a flag instead
of performing a comparison. However, such a 'performance'
difference would likley not be detectable in most situations.
I personally prefer to use 'empty()' for the simple reason that
it's imo more descriptive than 'begin() == end()' (and it's
less typing, too. :-) )

-Mike

[Jeff Flinn]

CrayzeeWulf wrote:

gelbeiche wrote:

Is
( cont.begin() == cont.end() )
essentially equivalent to writing
( cont.empty() )
for a STL container ?
Yes. According to the standard the operational semantics of the
related expressions are:

std::map<int,in t> a;
a.size() => a.end() - a.begin()
Doesn't compute! Not all containers provide random access iterators.
a.empty() => a.size() == 0

!q.e.d

When you want to know if a container is empty, use c::empty. Assume that the
implementer will use the most efficient implementation for the container.

Jeff Flinn

[CrayzeeWulf]

Jeff Flinn wrote:

Doesn't compute! Not all containers provide random access iterators.

True. That is why they are operational semantics only and not necessarily
the way to implement the corresponding methods. However, if "a.end() -
a.begin()" is allowed, then it is logically equivalent to "a.empty()" .

Thanks,
--
CrayzeeWulf

[CrayzeeWulf]

Jeff Flinn wrote:

Doesn't compute! Not all containers provide random access iterators.

True. That is why they are operational semantics only and not necessarily
the way to implement the corresponding methods. However, if "a.end() -
a.begin()" is allowed, then it is logically equivalent to "a.empty()" .

Note that the original post was about "a.begin() == a.end()" which will work
for all iterators. As stated in ISO/IEC 14882:2003(E) Section 23.1(para#7):

"begin() returns an iterator referring to the first element in the
container. end() returns an iterator which is the past-the-end value for
the container. If the container is empty, then begin() == end();"

Thanks,
--
CrayzeeWulf