On Monday 18 January 2016 18:47:42 Matthew Woehlke wrote:
> Not really. There is a huge difference between a relocatable type (many,
> if not most), and a trivially copyable type (a lot fewer). Operations
> such as growing the buffer can be vastly more expensive if the item type
> is not relocatable vs. if it is. However, C++ itself currently does not
> provide any support for relocation.
>
> I want to say e.g. std::string is relocatable, but it is certainly not
> trivially copyable. Now, imagine a std::vector<std::string> where both
> the vector and strings are very large. When the vector needs to resize,
> it will have to do a deep copy of ever single item, allocating and
> freeing lots of memory in the process. Compare to QVector<std::string>
> (assume Q_DECLARE_TYPEINFO declares std::string relocatable), which...
> will do a realloc(). Worst case, that's 1 alloc, 1 free, and 1 block
> copy... and 0 ctor/dtor calls.
>
> That's a significant difference.
>
> (Even using QString instead of std::string does not help much... the
> QVector still just does a realloc, while std::vector must perform 2*N
> atomic operations.)
a) std::string is not a movable type (at least I get a heap corruption when
marking it as such in GCC 5.3)
b)
#include <QVector>
#include <string>
int main() {
QVector<std::string> l;
int oldC = l.capacity();
for (int i = 0; i < 10000000; ++i) {
char buf[std::numeric_limits<int>::digits + 1];
sprintf(buf, "%d", i);
l.push_back(buf);
int newC = l.capacity();
if (newC != oldC)
qDebug("%d", newC);
oldC = newC;
}
}
$ time ./test
<snip>
real 0m1.598s
user 0m1.400s
sys 0m0.192s
#include <QVector>
#include <string>
int main() {
std::vector<std::string> l;
int oldC = l.capacity();
for (int i = 0; i < 10000000; ++i) {
char buf[std::numeric_limits<int>::digits + 1];
sprintf(buf, "%d", i);
l.push_back(buf);
int newC = l.capacity();
if (newC != oldC)
qDebug("%d", newC);
oldC = newC;
}
}
$ time ./test
<snip>
real 0m1.481s
user 0m1.264s
sys 0m0.212s
#include <QVector>
#include <string>
int main() {
QVector<QByteArray> l;
int oldC = l.capacity();
for (int i = 0; i < 10000000; ++i) {
char buf[std::numeric_limits<int>::digits + 1];
sprintf(buf, "%d", i);
l.push_back(buf);
int newC = l.capacity();
if (newC != oldC)
qDebug("%d", newC);
oldC = newC;
}
}
$ time ./test
<snip>
real 0m1.769s
user 0m1.572s
sys 0m0.192s
best of three runs, core i7-2720QM, GCC 5.3
HTH,
Marc
--
Marc Mutz <marc.m...@kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt Experts
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