Re: reduce c++ vector size

sukhmeet <sukhmet@gmail.com> wrote in
news:jqh2da$dam$1@adenine.netfront.net:


It's a bit hard to believe. Reserve() is working fine with MSVC and g++,
at least for avoiding allocation of 32 entries instead of 10. Probably
you are using it wrong.

Coming to think of that, your initial claim that pushing an element into
an empty std::vector jumps its capacity to 32 is also a bit hard to
believe. What C++ implementation is doing this?

If you wan't to get rid of any excess reserved memory at any time point,
you can always construct a new vector of the right size, once you know
it. This will cost an extra copy of the data though.

#include <iostream>
#include <vector>

int main() {

    std::vector<int> v;
    v.reserve(10);
    for(int i=0; i<3; ++i) {
        v.push_back(i);
    }
    std::cout << "size==" << v.size() <<
     ", capacity==" << v.capacity() << "\n";

    std::vector<int> w(v.begin(), v.end());
    v.swap(w);

    std::cout << "size==" << v.size() << ",
     capacity==" << v.capacity() << "\n";

}

Output:
size==3, capacity==10
size==3, capacity==3