What are the different types of pointers and references in C++ ? Why are they useful?

Question

Explain as simply as possible ?

Answer 1

Let T be a class in this tutorial Pointers in C++ can be divided into 3 types :

1) Raw pointers :

T a;  
T * _ptr = &a; 

They hold a memory address to a location in memory. Use with caution , as programs become complex hard to keep track.

Pointers with const data or address { Read backwards }

T a ; 
const T * ptr1 = &a ; 
T const * ptr1 = &a ;

Pointer to a data type T which is a const. Meaning you cannot change the data type using the pointer. ie *ptr1 = 19 ; will not work. But you can move the pointer. ie ptr1++ , ptr1-- ; etc will work. Read backwards : pointer to type T which is const

  T * const ptr2 ;

A const pointer to a data type T . Meaning you cannot move the pointer but you can change the value pointed to by the pointer. ie *ptr2 = 19 will work but ptr2++ ; ptr2-- etc will not work. Read backwards : const pointer to a type T

const T * const ptr3 ; 

A const pointer to a const data type T . Meaning you cannot either move the pointer nor can you change the data type pointer to be the pointer. ie . ptr3-- ; ptr3++ ; *ptr3 = 19; will not work

3) Smart Pointers : { #include <memory> }

Shared Pointer:

  T a ; 
     //shared_ptr<T> shptr(new T) ; not recommended but works 
     shared_ptr<T> shptr = make_shared<T>(); // faster + exception safe

     std::cout << shptr.use_count() ; // 1 //  gives the number of " 
things " pointing to it. 
     T * temp = shptr.get(); // gives a pointer to object

     // shared_pointer used like a regular pointer to call member functions
      shptr->memFn();
     (*shptr).memFn(); 

    //
     shptr.reset() ; // frees the object pointed to be the ptr 
     shptr = nullptr ; // frees the object 
     shptr = make_shared<T>() ; // frees the original object and points to new object

Implemented using reference counting to keep track of how many " things " point to the object pointed to by the pointer. When this count goes to 0 , the object is automatically deleted , ie objected is deleted when all the share_ptr pointing to the object goes out of scope. This gets rid of the headache of having to delete objects which you have allocated using new.

Weak Pointer : Helps deal with cyclic reference which arises when using Shared Pointer If you have two objects pointed to by two shared pointers and there is an internal shared pointer pointing to each others shared pointer then there will be a cyclic reference and the object will not be deleted when shared pointers go out of scope. To solve this , change the internal member from a shared_ptr to weak_ptr. Note : To access the element pointed to by a weak pointer use lock() , this returns a weak_ptr.

T a ; 
shared_ptr<T> shr = make_shared<T>() ; 
weak_ptr<T> wk = shr ; // initialize a weak_ptr from a shared_ptr 
wk.lock()->memFn() ; // use lock to get a shared_ptr 
//   ^^^ Can lead to exception if the shared ptr has gone out of scope
if(!wk.expired()) wk.lock()->memFn() ;
// Check if shared ptr has gone out of scope before access

See : When is std::weak_ptr useful?

Unique Pointer : Light weight smart pointer with exclusive ownership. Use when pointer points to unique objects without sharing the objects between the pointers.

unique_ptr<T> uptr(new T);
uptr->memFn(); 

//T * ptr = uptr.release(); // uptr becomes null and object is pointed to by ptr
uptr.reset() ; // deletes the object pointed to by uptr 

To change the object pointed to by the unique ptr , use move semantics

unique_ptr<T> uptr1(new T);
unique_ptr<T> uptr2(new T);
uptr2 = std::move(uptr1); 
// object pointed by uptr2 is deleted and 
// object pointed by uptr1 is pointed to by uptr2
// uptr1 becomes null 

References : They can essentially be though of as const pointers, ie a pointer which is const and cannot be moved with better syntax.

See : What are the differences between a pointer variable and a reference variable in C++?

r-value reference : reference to a temporary object   
l-value reference : reference to an object whose address can be obtained
const reference : reference to a data type which is const and cannot be modified 

Reference : https://www.youtube.com/channel/UCEOGtxYTB6vo6MQ-WQ9W_nQ