How to release pointer from boost::shared_ptr?

Question

Can boost::shared_ptr release the stored pointer without deleting it?

I can see no release function exists in the documentation, also in the FAQ is explained why it

Answer 1

I'm not entirely sure if your question is about achieving this, but if you want behaviour from a shared_ptr, where, if you release the value from one shared_ptr, all the other shared pointers to the same value become a nullptr, then you can put a unique_ptr in a shared_ptr to achieve that behaviour.

void print(std::string name, std::shared_ptr>& ptr)
{
    if(ptr == nullptr || *ptr == nullptr)
    {
        std::cout << name << " points to nullptr" << std::endl;
    }
    else
    {
        std::cout << name << " points to value " << *(*ptr) << std::endl;
    }
}

int main()
{
    std::shared_ptr> original;
    original = std::make_shared>(std::make_unique(50));

    std::shared_ptr> shared_original = original;

    std::shared_ptr> thief = nullptr;

    print(std::string("original"), original);
    print(std::string("shared_original"), shared_original);
    print(std::string("thief"), thief);

    thief = std::make_shared>(original->release());

    print(std::string("original"), original);
    print(std::string("shared_original"), shared_original);
    print(std::string("thief"), thief);

    return 0;
}

Output:

original points to value 50
shared_original points to value 50
thief points to nullptr
original points to nullptr
shared_original points to nullptr
thief points to value 50

This behaviour allows you to share a resource (like an array), then later reuse that resource while invalidating all the shared references to this resource.