Re: std::vector slow?
On Nov 8, 11:14 am, David Klein <dkle...@gmail.com> wrote:
I wrote a simple template class Vec and when I compile with
optimization under Visual Studio 2005, std::vector takes 56% more
time. I would have thought that std::vector is much more optimized
than anything I could roll myself. Am I mis-using std::vector somehow?
Or is it really that inefficient?
....
Vec(int size) : mNElem(0), mPtr(NULL)
{
if (size < 0) throw "Size < 0\n";
if (size > 0)
{
mPtr = new T [size];
if (mPtr == NULL)throw "Vec allocation failed\n";
mNElem = size;
}
}
....
int main(int argc, char* argv[])
{
....
vector<int> y(n);
for (int i=0; i<n; i++) {
y[i] = i;
}
....
Your Vec doesn't initialize its elements on construction if they are
built-in types, whereas std::vector does. Try replacing the fragment
above with the following:
vector<int> y;
y.reserve(n);
for (int i=0; i<n; i++) {
y.push_back(i);
}
Also note that the default copying that Vec does is wrong -- I assume
you omitted the correct implementation for demonstration purposes. If
it had proper copying, copying uninitialized values is undefined
behaviour.
Yechezkel Mett
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