Question

I have read that one must not store std::auto_ptr in std::vector and that boost::ptr_vector could be used instead. I have been able to do so, I don’t know however how to use ptr_vector, when I don’t want to store pointers, but a struct, which has a pointer member.

In this example, I want to open some files and store the associated ofstream object with some additional data, for later use. I would like to replace the file field of struct data with a smart pointer. Since the vector<data> v should be the owner, I think that a shared_ptr would work, but wouldn’t be appropriate.

What should I replace the naked pointer file with?

#include <iostream>
#include <fstream>
#include <vector>

struct data {
  std::string filename;
  std::ofstream* file;

  data(const std::string filename, std::ofstream* file)
    : filename(filename), file(file)
  {
  }
};

std::vector<data> open_files()
{
  std::vector<data> v;
  v.push_back(data("foo", new std::ofstream("foo")));
  return v;
}

int main()
{
  std::vector<data> v = open_files();

  /* use the files */
  *(v[0].file) << "foo";

  delete v[0].file;  // either rely on dtor to close(), or call it manually
}

Update:
I feel I have done a sub optimal job in describing my problem, let me try with another example. Also I am looking for a C++03 solution:

#include <memory>
#include <vector>
#include <boost/ptr_container/ptr_vector.hpp>

struct T {
  std::auto_ptr<int> a;
};

int main()
{
  // instead of
  // std::vector<std::auto_ptr<int> > v;
  // use
  boost::ptr_vector<int> v;

  // what to use instead of
  // std::vector<T> w;
}

Answer 1

Concerning your data class, I would suggest using an std::unique_ptr<std::ofstream>. This is not to save you from an accidental memory leak, since you are deleting the pointer in the constructor, but rather to make the ownership explicit. A user of your code would have to know what data is doing with the pointer it takes in the constructor:

std::ofstream ofs;
{
   data d1("crash", &ofs);
} // error! d1 will attempt to delete stack allocated object

std::ofstream* pOfs = new std::ofstream(....);
data d2("crash again", pOfs);
delete pOFs; // user thinks data makes a deep copy

However, with unique_ptr the intend is clear, hence it is harder to make mistakes:

data d3("OK", std::unique_ptr<std::ofstream>(new std::ofstream(....)));

std::unique_ptr<std::ofstream> pOfs2(new std::ofstream(....));
data d4("OK", pOfs2); // safe, pOfs's contents have been safely moved

// we can check pOfs2 after the move
if (pOfs2) { /*  */ }