C++ 11 Smart pointers : weak_ptr tutorial and example

unique_ptr is a smart pointer available in C++ 11.

Weak pointer is used along with shared pointers. You cannot create a standalone unique pointers.

Example of weak pointer:

#include<iostream>
#include <memory> 
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

class MyClass
{
  
  public:
  MyClass()
  {
  }
  
  ~MyClass()
  {
  }
  
  
};

int main() 
{ 
    shared_ptr<MyClass> p (new MyClass);
    cout<<"Reference Count = "<< p.use_count()<<endl;
     
    weak_ptr <MyClass> wp = p; // created a weak pointer
    cout<<"Reference Count = "<< p.use_count()<<endl;
    
    return 0; 
} 

Output:

Reference Count = 1
Reference Count = 1

As you can see form the output, the reference count is not increased. That is because we are using weak_ptr. If we have used shared_ptr, the reference counter will be incremented.

You can also use “wp” weak pointer to call the functions.

What is cyclic dependency problem with shared_pointer and how it can be solved using weak_ptr?

Cyclic dependancy will occur when you have two classes A and B where A has a reference to B which has a reference to A.

Example:

#include<iostream>
#include <memory> 
//for more C++ tutorial visit www.ProDeveloperTutorial.com
using namespace std;

class B;

class A
{
    shared_ptr<B> sP1; // use weak_ptr instead to avoid cyclic dependancy

public:
    A() {  cout << "A()" << endl; }
    ~A() { cout << "~A()" << endl; }

    void setShared(shared_ptr<B>& p)
    {
        sP1 = p;
    }
};

class B
{
    shared_ptr<A> sP1;

public:
    B() {  cout << "B()" << endl; }
    ~B() { cout << "~B()" << endl; }

    void setShared(shared_ptr<A>& p)
    {
        sP1 = p;
    }
};

int main()
{
    shared_ptr<A> aPtr(new A);
    shared_ptr<B> bPtr(new B);

    aPtr->setShared(bPtr);
    bPtr->setShared(aPtr);

    return 0;  
}

Output:

A()
B()

The cyclic dependancy can be avoided by using weak_ptr.