Creating Event call back for all instance of a class

EventCallBackForAllObjectsOfAClass.cpp

#include <iostream>
#include <unordered_set>

class TestEvent {
    private:
        static std::unordered_set<TestEvent*> objects;
        std::string m_title;
    public:
        TestEvent(const std::string& title):m_title (title)  
        { 
            objects.insert(this); 
            
        }
        
        ~TestEvent() 
        { 
            objects.erase(this) ; 
        }
        
        template< typename FN > static void ApplyOnAllInstances( FN function_call_back )
        { 
            for( TestEvent* obj : objects ) 
            {
                function_call_back(obj) ; 
            }
        }
        
        void onConnect()
        {
            std::cout<<"Connecting '"<<m_title<<"'"<<std::endl;
        }
        
        void onDisconnect()
        {
            std::cout<<"Disconnecting '"<<m_title<<"'"<<std::endl;
        }
};

std::unordered_set<TestEvent*> TestEvent::objects;

int main()
{
    TestEvent* t1 = new TestEvent("Instance 1");
    TestEvent* t2 = new TestEvent("Instance 2");
    TestEvent* t3 = new TestEvent("Instance 3");
    delete t3;
    TestEvent::ApplyOnAllInstances([](TestEvent* t){t->onConnect();});
    TestEvent::ApplyOnAllInstances([](TestEvent* t){t->onDisconnect();});
    return 0;
}