Invalidate all shared ptrs toward a specific managed object

Question

Is it possible, in C++11, to have an object managed by several std::shared_ptrs. I want to delete the object via one std::shared_ptr and have the other shared_ptrs invalidated (set empty or null), is this possible? If not, what is the best method to inform all other "references" (in a liberal use of the word) that the object is no longer valid?

Answer 1

To do this, the other shared_ptrs have to be replaced with weak_ptrs. The shared_ptr that does the deletion is the one actually manages the lifetime of the object in this scenario. It's worthwhile at this point to figure out if you really need shared ownership semantics. In general, if you find yourself trying to do something the interface doesn't let you do, that's an indication that you need something with different semantics.

Alternatively, if you really can't manage the object's lifetime from one place, you can use shared_ptr<unique_ptr<T>>, but this is more cumbersome (not to mention slower) and is better to avoid. Here you would delete the object by resetting the inner unique_ptr.

Answer 2

Here is a good example of weak_ptr and to be informed when all other "references" is no longer valid.

#include <iostream>
#include <memory>

std::weak_ptr<int> gw;

void f()
{
    std::cout << "use_count == " << gw.use_count() << ": ";
    if (auto spt = gw.lock()) 
    { // Has to be copied into a shared_ptr before usage
        std::cout << *spt << "\n";
    }
    else 
    {
        std::cout << "gw is expired\n";
    }
}

int main()
{
    {
        std::shared_ptr<int> sp = std::make_shared<int>(42);
        gw = sp;
        f();
    }
    f();
}

Output:
use_count == 1: 42
use_count == 0: gw is expired