Re: stl <vector> clear and erase.
Am 29.04.2012 22:09, schrieb Peter Liedermann:
My free profiler "Very Sleepy", which has been of great use to me,
points me to the following, which is not my code and therefore
presumed to be STL-implementation. (VC++ 11, presumably Dinkum)
void clear()
0.16s { // erase all
0.08s erase(begin(), end());
0.00s }
Question: Why does it perform this erase as part of clear(), can I
keep it from doing so and how? I see no point in erasing for a vector
<unsigned>. As far as I remember what I have read, there should not
be any erasing in this case. The (logical) length of the vector
should be adjusted (constant complexity) and the old contents are left
as garbage.
The implementation you are observing, is conforming, because the
standard describes above semantics. Nonetheless you are right: A good
quality-implementation can skip the iteration here and can simply adjust
the internal length representation via an O(1) step. I'm pretty sure
that gcc 4.8 does this, for example, if the value type of the vector has
a trivial destructor.
Could I be better off using old C arrays?
Any hint greatly appreciated, this is really a hotspot in my program.
This is really an quality-of-implementation issue, you should ask your
vendor for improvements, bringing the hot-spot situation as an example.
HTH & Greetings from Bremen,
Daniel Kr?gler
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