https://www.reddit.com/r/cpp_questions/comments/8d5ory/when_to_use_a_smart_pointer_a_raw_pointer_or_a/dxklo2r/

pointers.cpp

#include <iostream>
#include <memory>

void test_unique() {
        std::unique_ptr<int> foo = std::make_unique<int>(4);
        std::cout<<*foo<<std::endl;
        //std::unique_ptr<int> bar = foo;
}

void test_shared() {
        {
                std::shared_ptr<int> foo = std::make_shared<int>(9);
                std::cout<<*foo<<std::endl; //prints 9
                std::shared_ptr<int> bar = foo; //increases the reference counter to 2
                foo = nullptr; //decreases the reference counter to 1
                (*bar)++; // increments the value in bar
                std::cout<<*bar<<std::endl; //prints 10
        }
        //when bar goes out of scope the memory is freed and the pointer deleted
}

void test_weak() {
        std::weak_ptr<int> weak;
        {
                std::shared_ptr<int> foo = std::make_shared<int>(-2);
                weak = foo; //does not increase the reference counter
                //in order to use the weak pointer it must be copied into a shared pointer
                if (std::shared_ptr<int> bar = weak.lock()) { //this increases the reference coutner to 2
                        std::cout<<*bar<<std::endl;
                } else { // and leaving scope decreases it to 1
                        std::cout<<"The pointer is expired"<<std::endl;
                }
        }
        if (std::shared_ptr<int> bar = weak.lock()) { // the shared pointer is expired at this point
                std::cout<<*bar<<std::endl;
        } else {
                //so this occurs
                std::cout<<"The pointer is expired"<<std::endl;
        }
}

int main() {
        std::cout<<"Unique Pointers"<<std::endl;
        test_unique();
        std::cout<<"Shared Pointers"<<std::endl;
        test_shared();
        std::cout<<"Weak Pointers"<<std::endl;
        test_weak();

        return 0;
}