Re: Am I misusing std::vector?

From:

Antoun Kanawati <antounk@comcast.net>

Newsgroups:

comp.lang.c++.moderated

Date:

11 May 2006 18:23:40 -0400

Message-ID:

<B-2dndhgh-lAMv_ZRVn-sQ@comcast.com>

loose AT astron DOT nl wrote:

Hi,

I was surprised by the output of the program below. From what I
understand from the C++ STL documentation, vector<T>::resize(N), will
create N *newly constructed* objects of T. So, I expected *v[0].ip to
be equal to 0, and *v[sz-1].ip equal to -77. However, I discovered,
using a few different compilers (two different versions of gcc, and
icc), that both return -77. It turns out that only *one* object is
being constructed and this one object seems to be bitwise copied to the
other members. See my code below and try it for yourself.
Am I missing something here?

Try adding a copy constructor, and see what happens.

Regards,

Marcel Loose.

<code>
#include <vector>
#include <iostream>
using namespace std;

struct A
{
A(int i = 0) { ip = new int(i); }

A(const A &a) { ip = new int(*(a.ip)); }

  ~A() { delete ip; }
  int* ip;
};

int main()
{
  const unsigned sz = 1000000;
  vector<A> v;
  v.resize(sz);
  *v[sz-1].ip = -77;
  cout << "v.size() = " << v.size() << endl;
  cout << "v[0].ip = " << v[0].ip << "; " << "*v[0].ip = " <<
*v[0].ip << endl;
  cout << "v[sz-1].ip = " << v[sz-1].ip << "; " << "*v[sz-1].ip = "
<< *v[sz-1].ip << endl;
  return 0;
}
</code>

--
A. Kanawati
NO.antounk.SPAM@comcast.net

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