More with Pointers

Overview

• Objects in C++ share many similarities with Java,
  but are considered more powerful:

  • In C++, objects can be on the stack.

  • C++ allows multiple inheritance.

    • no need for the interface keyword.

  • Templates act like generics, except they are a compile-time construct, not run time.

• Remember, C++ is compiled to machine code; Java compiles to byte code!

Review

Pointers to Stack Memory

int num {32};	Declare an int variable on the stack and set to 32.
int *ptr {};	Declare a variable ptr to store the memory address of where an int resides.
ptr = #	&num gets the memory address of num. Stores the memory address of num in ptr.
*ptr = 64;	Go to where ptr is pointing (*) and store 64 there. Since ptr is pointing to num, num now has 64.

Pointer to Heap Memory

int *a;	Create a variable a on the stack that will store the memory address of an integer.
a = new int{};	Allocate an int in memory and store the memory address of that int in a.
*a = 5;	Manipulate the new int just like before.
delete a;	Must deallocate because the int is in the heap!
a = nullptr;	After delete, reassign the pointer to nullptr (best practice).

Arrays with Pointer Arithmetic

Declaration	Description
int ary[100];	Create an array of 100 integers the stack.
int *pAry {ary};	Create a pointer to the array. Note: & was not used.

Given the above declarations, the following four statements are equivalent.

Square Brackets	Pointer Arithmetic
ary[10] = 50;	*(ary + 10) = 50;
pAry[10] = 50;	*(pAry + 10) = 50;

Arrays

• In Java, arrays are actually objects that store length and other information.

• In C++, an array is a sequence list of data. (Nothing else is stored.)

• These are valid declarations in C++, but not in Java. Why?

  int x[20];    // Create 20 integers
  Button b[20]; // Create 20 Buttons

Pointers and Arrays

• In C++, pointers and arrays have a close relationship.

• Instead of int x[20];
  we can can write
  int *pX = new int[20];
  to allow for dynamic allocation.

  • Usage of the array (e.g., x[3] = 5; and pX[3] = 5) are identical in both cases.

  • To deallocate, use delete[] pX;

Constants

Pointers and Const-Correctness

A good rule of thumb is to use const wherever possible. Pointers have two types of const-ness.

• A pointer to constant data
  const int * ptr;
int const * ptr;
  You cannot change the value at the address this pointer is pointing to.

• A pointer with a const memory address
  int * const ptr;
  You cannot change what this pointer points to.

Pointers and Const-Correctness

A good rule of thumb is to use const wherever possible. Pointers have two types of const-ness.

• A const pointer to const data
  const int * const ptr;
int const * const ptr;

Casting Pointers

In C++, a pointer can point to anything.

• Primitives
• Arrays
• Other pointers
• Objects
• Functions
• Something nonspecific (void *).
  Void pointers are more useful in C but should be avoided in C++.

Casting

C++ is incredibly powerful, because of casting.

int num {300};
int* const pInt {&num}; // pointer is const
float* const pFloat = reinterpret_cast<float*>(pInt);
*pFloat = 3.14159; // undefined behavior
std::cout << "*pFloat = " << *pFloat;
std::cout << "\n    num = " << num << '\n';

• Yes, that is “legal.”

• The value of num is undefined!

Pointers to Objects

Pointers to instances of structs and classes work the same as with primitive types (e.g., int).

struct Dog
{
  std::string name;
  std::string breed;
  int yearsOld;
};

int main()
{
  Dog myPet {"Bella", "German Shepherd", 7};
  Dog *pFriend = new Dog {"Rocky", "Poodle", 2};

  std::cout << myPet.name << std::endl;
  std::cout << (*pFriend).name << std::endl;

  // using member-access arrow
  std::cout << pFriend->name << std::endl;

  return 0;
}

• In Java, a pointer is called a reference to an object:

  String str = new String("Hello!");

• In C++, a pointer stores the memory location of something else. For objects:

  std::string *str = new std::string("Hello!");

• In Java, pointers only exist for objects.

• In C++, a pointer may point to anything.

Function Pointers

• A pointer is a variable that stores the address of something.

• A function pointer stores the address of a function.

• The function can be called by dereferencing the function pointer.

• Function pointers can be passed as parameters to other functions.

  • Commonly used in callbacks.

• Functions can return function pointers.

#include <iostream>

int foo() { return 4; }

int main()
{
  // Display functions memory address.
  // Cast to void pointer to ensure the address is properly displayed.
  std::cout << reinterpret_cast<void*>(main) << std::endl;
  std::cout << reinterpret_cast<void*>(foo) << std::endl;

  // Create a pointer that points to foo()
  int (*fcnPtr)() = foo;

  // Call foo() using pointer
  std::cout << "The return value of foo() is "
    << (*fcnPtr)()
    << std::endl;

  return 0;
}

Lab 4: Pointers

Let’s talk about Lab 4, which is more practice with pointers.