How to: Create and Use weak_ptr Instances

Sometimes an object must store a way to access the underlying object of a shared_ptr without causing the reference count to be incremented. Typically, this situation occurs when you have cyclic references between shared_ptr instances.

The best design is to avoid shared ownership of pointers whenever you can. However, if you must have shared ownership of shared_ptr instances, avoid cyclic references between them. When cyclic references are unavoidable, or even preferable for some reason, use weak_ptr to give one or more of the owners a weak reference to another shared_ptr. By using a weak_ptr, you can create a shared_ptr that joins to an existing set of related instances, but only if the underlying memory resource is still valid. A weak_ptr itself does not participate in the reference counting, and therefore, it cannot prevent the reference count from going to zero. However, you can use a weak_ptr to try to obtain a new copy of the shared_ptr with which it was initialized. If the memory has already been deleted, a bad_weak_ptr exception is thrown. If the memory is still valid, the new shared pointer increments the reference count and guarantees that the memory will be valid as long as the shared_ptr variable stays in scope.

The following code example shows a case where weak_ptr is used to ensure proper deletion of objects that have circular dependencies. As you examine the example, assume that it was created only after alternative solutions were considered. The Controller objects represent some aspect of a machine process, and they operate independently. Each controller must be able to query the status of the other controllers at any time, and each one contains a private vector<weak_ptr<Controller>> for this purpose. Each vector contains a circular reference, and therefore, weak_ptr instances are used instead of shared_ptr.

#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

class Controller 
{
public:
    int Num;
    wstring Status;
    vector<weak_ptr<Controller>> others;
    explicit Controller(int i) : Num(i) , Status(L"On")
    {
        wcout << L"Creating Controller" << Num << endl;
    }

    ~Controller()
    {
        wcout << L"Destroying Controller" << Num << endl;
    }

    // Demonstrates how to test whether the   
    // pointed-to memory still exists or not.  
    void CheckStatuses() const
    {
        for_each(others.begin(), others.end(), [] (weak_ptr<Controller> wp)
        {
            try
            {
                auto p = wp.lock();
                wcout << L"Status of " << p->Num << " = " << p->Status << endl;
            }

            catch (bad_weak_ptr b)
            {
                wcout << L"Null object" << endl;
            }                
        });
    }
};

void RunTest()
{
    vector<shared_ptr<Controller>> v;

    v.push_back(shared_ptr<Controller>(new Controller(0)));
    v.push_back(shared_ptr<Controller>(new Controller(1)));
    v.push_back(shared_ptr<Controller>(new Controller(2)));
    v.push_back(shared_ptr<Controller>(new Controller(3)));
    v.push_back(shared_ptr<Controller>(new Controller(4)));

    // Each controller depends on all others not being deleted.  
    // Give each controller a pointer to all the others.  
    for (int i = 0 ; i < v.size(); ++i)
    {
        for_each(v.begin(), v.end(), [v,i] (shared_ptr<Controller> p)
        {
            if(p->Num != i)
            {
                v[i]->others.push_back(weak_ptr<Controller>(p));
                wcout << L"push_back to v[" << i << "]: " << p->Num << endl;
            }
        });        
    }

    for_each(v.begin(), v.end(), [](shared_ptr<Controller>& p)
    {
        wcout << L"use_count = " << p.use_count() << endl;
        p->CheckStatuses();
    });
}

int main()
{    
    RunTest(); 
    wcout << L"Press any key" << endl;
    char ch;
    cin.getline(&ch, 1);
}
Creating Controller0
Creating Controller1
Creating Controller2
Creating Controller3
Creating Controller4
push_back to v[0]: 1
push_back to v[0]: 2
push_back to v[0]: 3
push_back to v[0]: 4
push_back to v[1]: 0
push_back to v[1]: 2
push_back to v[1]: 3
push_back to v[1]: 4
push_back to v[2]: 0
push_back to v[2]: 1
push_back to v[2]: 3
push_back to v[2]: 4
push_back to v[3]: 0
push_back to v[3]: 1
push_back to v[3]: 2
push_back to v[3]: 4
push_back to v[4]: 0
push_back to v[4]: 1
push_back to v[4]: 2
push_back to v[4]: 3
use_count = 1
Status of 1 = On
Status of 2 = On
Status of 3 = On
Status of 4 = On
use_count = 1
Status of 0 = On
Status of 2 = On
Status of 3 = On
Status of 4 = On
use_count = 1
Status of 0 = On
Status of 1 = On
Status of 3 = On
Status of 4 = On
use_count = 1
Status of 0 = On
Status of 1 = On
Status of 2 = On
Status of 4 = On
use_count = 1
Status of 0 = On
Status of 1 = On
Status of 2 = On
Status of 3 = On
Destroying Controller0
Destroying Controller1
Destroying Controller2
Destroying Controller3
Destroying Controller4
Press any key

As an experiment, modify the vector others to be a vector<shared_ptr<Controller>>, and then in the output, notice that no destructors are invoked when TestRun returns.

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