Re: std::vector: reserve required?

From:

red floyd <no.spam.here@example.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 04 Jul 2008 09:11:30 -0700

Message-ID:

<iVrbk.865$zv7.165@flpi143.ffdc.sbc.com>

Mike -- Email Ignored wrote:

On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:

On Jul 4, 7:54 am, callumurquh...@googlemail.com wrote:

http://www.cplusplus.com/reference/stl/vector/operator[].html

If the OP did not concentrate on the vec.reserve(en), the document you
show could be used to help with the problem. But because the OP does
expect some behavior from vec.reserve(en), the problem is not with
operator[].

Ali

Another test using:
      vec.reserve(en);
      for (int jj = 0; jj < en; ++jj)
         vec[jj] = jj;
      cout << "vec.size() = " << vec.size() << endl;

prevents the crash, but vec.size() returns zero, showing that
in this case, reserve() really does not work.

No, reserve() does work. You misunderstand how it does.
vector<>::reserve changes the CAPACITY -- that is, you can use
push_back() or resize() up to the amount you've reserved without
the vector reallocating. It does not change the SIZE of the vector.

Try this:

#include <iostream>
#include <ostream>
#include <vector>

using namespace std;

int main()
{
     vector<int> v;

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

     v.reserve(200);

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

     v.resize(200);

     cout << "size = " << v.size() << "\n"
          << "capacity = " << v.capacity << endl;

     return 0;
}