Comprehensive Guide to C++ Reference and Pointer

Basics

De-referencing

add * before a pointer to expose the object’s value it’s pointing

• varN, varN’s value
• ptrN, varN’s address
• *ptrN, de-reference ptrN, varN’s value
• (&ptrN, ptrN’s address, which has little meaning)

Symbol to access a member of a class or class object (instance)

• a->b: b is the member of the object that pointer a refers to
• a.b: b is the member of the object, or the reference of an object a
• a::b: b is the member of the class, or namespace a

Declaration

Pointer Declaration

Syntax: varType* varName

Declaration

//declare a c++ pointer to an integer
int* ptrx; 

Initialization

// a pointer pointing to address of varN, varN type should be int
int varN = 9;
int* ptrN;
ptrN = &varN;

short-hand:

int* ptrN = &varN;  

Reference Declaration

Syntax: varType& varName

int main()
{
    int& invalidRef;   // error: references must be initialized

    int x = 5;
    int& ref = x; // okay: reference to int is bound to int variable

    return 0;
}

When a reference is initialized with an object (or function), we say it is bound to that object (or function). The process by which such a reference is bound is called reference binding

references must be bound to a modifiable variable

Function Pass Argument

Both of these methods are prevented copying the variable, and capable of modifying the variable value that is passed in; if a variable is expensive to copy, e.g. struct, class

By Reference

• Modify the variable, can’t pass in an un-modifiable variable like const

  Syntax varType& varName

  void printValue(int& y)
  {
      std::cout << y << '\n';
  }

• Prevent modifying the variable, while having the benefit of not making a copy

  Syntax const varType& varName

  void printValue(const int& y)
  {
      std::cout << y << '\n';
  }

By Address (via pointer)

Syntax varType* varName, to access the variable, de-reference using *varName

void printByAddress(const std::string* ptr) // The function parameter is a pointer that holds the address of str
{
    std::cout << *ptr << '\n'; // print the value via the dereferenced pointer
}

Function Return Argument

Variable Pointer

works almost identically to return by reference, except a pointer to an object is returned instead of a reference to an object

The major advantage of return by address over return by reference is that we can have the function return nullptr if there is no valid object to return

Variable Reference

Never return a reference to a local variable or some such, because it won’t be there to be referenced.

Syntax varType& funName(args)

References

Stack Overflow - When do I use a dot, arrow, or double colon to refer to members of a class in C++?

Runestone Academy - Pointers

Learn C++ - Lvalue reference

Learn C++ - Return by reference and return by address

Learn C++ - Pass by lvalue reference

Learn C++ - Pass by address