Smart Pointers

Smart Pointers

• There are 4 types of Smart Pointers: Unique Pointer unique_ptr, Shared Pointers shared_ptr, Weak Pointers weak_ptr, Auto Pointers auto_ptr.

• Smart pointers are objects.

• Smart pointers are implemented as c++ template classes (like vector templates) which can be instantiated.

• Smart pointers can ONLY point to heap-allocated memory.

• Smart pointers call delete when no longer needed.

• Smart pointers adhere to RAII (Resource Acquisition Is Initialization) principles. First RAII objects get allocated on the stack. RA (Resource Acquisition) object will acquire some resource, such as opening a file allocating memory that acquires a lock, an so forth. RAII (Initialization), means the resources acquired an object initialization time (which happens in the objects constructor). At some point the need to relinquish that resource that was required and that happens in the destructor.

unique_ptr

• Unique Pointers (unique_ptr) - simple smart pointer, very efficient! unique_ptr<T>

  • points to an object of type T on the heap.

  • it's unique - there can only be ONE unique_ptr<T> pointing to the object on the heap

  • owns what it points to

  • cannot be assigned or copied

  • cannot be moved

  • when the pointer is destroyed, what it points to is automatically destroyed

// unique_ptr - creating, initializing and using
{
    std::unique_ptr<int> p1 { new int {100} }; 
    std::cout << *p1 << std::endl;  // 100
    *p1 = 200;
    std::cout << *p1 << std::endl;  // 200
}  // automatically deleted

// unique_ptr - some other useful methods
{
    std::unique_ptr<int> p1 { new int {100} };
    std::cout << *p1.get() << std::endl;  // 0x564388
    *p1.reset();  // p1 is now nullptr
    if (p1)  // checks if pointer p1 is initialized
    std::cout << *p1 << std::endl;  // won't execute
}  // automatically deleted

// unique_ptr - user defined classes
{
    std::unique_ptr<Account> p1 { new Account {"Larry} }; // unique_ptr manage Account object on the heap
    std::cout << *p1 << std::endl;  // display account
    p1->deposit(1000);
    p1->deposit(500);
} // automatically deleted

// unique_ptr - vectors and move
{
    std::vector<std::unique_ptr<int>> vec; // unique_ptr manage Account object on the heap
    std::unique_ptr<int> ptr { new int{100} };
    vec.push_back(ptr);  // Error - copy not allowed
    vec.push_back(std::move(ptr));  // giving ownership of pointer to vector and ptr will be set to null pointer
} // automatically deleted

// unique_ptr - make_unique function (C++14) returns a unique pointer of the specified type and allows it to pass initialization values into the constructor for the managed object
{
    std::unique_ptr<int> p1 = make_unique<int>(100);  // 
    std::unique_ptr<Account> p2 = make_unique<Account>("Curly", 5000);  // display account
    auto p3 = = make_unique<Player>("Hero", 100, 100);
} // automatically deleted

• Shared Pointers (shared_ptr)

• Weak Pointers (weak_ptr)

• Auto Pointers (auto_ptr)

How To Create A Smart Pointer

• #include <memory>

// smart pointer 'ptr' will manage an object of type `Some_Class` that's been allocated on the heap.
// after smart pointer object is create you can use the dereference operator, the member select operator, or any of the other smart pointer member methods.
{
std::smart_pointer_type<Some_Class> ptr = ...

    ptr -> method();
    std::cout << (*ptr) << endl;
}
// pointer will auto delete when no longer needed

Can't Be Done Using Smart Pointers

• Can't do pointer arithmetic on smart pointers like you can with raw pointers.