How to create map of scoped_ptr in c++

Question

I have the following code and line 2 gives me an error during compilation. Is it possible to create a map of scoped pointers or do I have to use shared pointers instead?

map<int, scoped_ptr> mp;
mp[1] = scoped_ptr(new obj());

error:

boost::scoped_ptr<T>& boost::scoped_ptr<T>::operator=(const boost::scoped_ptr<T>&) [with T = ]’ is private

if you don't have access to c++11, use boost::ptr_container. it does what you need. — Richard Hodges, Nov 09 '15 at 11:40

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

1

You can't, boost::scoped_ptr is non-copyable by design (emphasis mine):

The scoped_ptr template is a simple solution for simple needs. It supplies a basic "resource acquisition is initialization" facility, without shared-ownership or transfer-of-ownership semantics. Both its name and enforcement of semantics (by being noncopyable) signal its intent to retain ownership solely within the current scope.

<...>

scoped_ptr cannot be used in C++ Standard Library containers. Use shared_ptr if you need a smart pointer that can.

You can, however, emplace shared_ptrs into a container, since in this case no copying is performed:

std::list<boost::scoped_ptr<MyClass>> list;
list.emplace_back(new MyClass());

edited Jun 20 '20 at 09:12

Community

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answered Nov 09 '15 at 11:29

SingerOfTheFall

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does this mean that scoped pointers can't be used with containers? – mkmostafa Nov 09 '15 at 11:31
1

@mlmostafa, yes, it's explicitly stated in the documentation that you can't use it in standard library containers – SingerOfTheFall Nov 09 '15 at 11:33
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@SingerOfTheFall I think you need to clarify this more. They _can_ be used, you just need to be careful to move/emplace the `scoped_ptr`s, not copy them. – user1095108 Nov 09 '15 at 11:51

score 0 · Answer 2 · answered Nov 09 '15 at 12:26

boost::scoped_ptr is non-copyable, but you still can swap it.

Here is a trick :

// Example program
#include <iostream>
#include <string>
#include <map>
#include <boost/scoped_ptr.hpp>

int main()
{
  std::map<int, boost::scoped_ptr<int>> myMap;

  int * test = new int();
  *test = 589;

  boost::scoped_ptr<int> myScoped(test);

  boost::swap(myMap[1], myScoped);

  std::cout << *myMap[1];

}

Gives :

See on C++ Shell : http://cpp.sh/3zm3

However, I advise you not to do it.

How to create map of scoped_ptr in c++

2 Answers2