why use shared_from_this() ?

Why would you need to use shared_from_this on an object? As the name implies, if you want to pass a shared pointer to *this (pointer to current object), then you use shared_from_this.

What happens if you don't? Well, a scenario could happen where you wrap *this pointer in a smart pointer from a member function, expecting that to work just fine. But what happens when you do that is you create another shared_ptr to *this with reference count of 1. When this member function exits, the shared_ptr gets destroyed, and your *this pointer is deleted, causing the destructor of the object to be called.

An example will make this clearer:

#include <iostream>
#include <boost/shared_ptr.hpp>

class Bar;
void foo(const boost::shared_ptr<Bar>& ptr);

class Bar
{
public:
   void process()
   {
       foo(boost::shared_ptr<Bar>(this));
   }
   void print() const {std::cout << "I am still alive!!" << std::endl; }

   ~Bar() { std::cout << "Uh oh... I am dead!!" << std::endl; }
};

void foo(const boost::shared_ptr<Bar>& ptr)
{
   ptr->print();
}

int main()
{
   boost::shared_ptr<Bar> sPtr(new Bar());
   sPtr->process();
   sPtr->print();
   std::cout << "Just before leavnig main " << std::endl;
   sPtr->print();
   return 0;
}

When you run this code (fix any errors first), it'll crash. The reason being, when the function process exits, the temporary shared_ptr resets its refcount to 0, and the *this object gets deleted. Then when main tries to delete the object again upon exit, it crashes.

To fix this, we use shared_from_this(), which will simply make sure that we get a smart pointer *this instead of a raw pointer *this. Read more about it in the boost docs

#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>

class Bar;
void foo(const boost::shared_ptr<Bar>& ptr);

class Bar : public boost::enable_shared_from_this<Bar>
{
public:
   void process()
   {
       foo(shared_from_this());
   }
   void print() const {std::cout << "I am still alive!!" << std::endl; }

   ~Bar() { std::cout << "Uh oh... I am dead!!" << std::endl; }
};

void foo(const boost::shared_ptr<Bar>& ptr)
{
   ptr->print();
}

int main()
{
   boost::shared_ptr<Bar> sPtr(new Bar());
   sPtr->process();
   sPtr->print();
   std::cout << "Just before leavnig main " << std::endl;
   sPtr->print();
   return 0;
}