Pointers and References in C/C++

Pointers and References in C/C++
An Introduction

Computer Science Department

bernstdh@jmu.edu

Motivation for Java Programmers

Some people say that Java is a "pointerless" language
In fact, Java uses references (which are related to pointers) but hides them from you
In C/C++ you don't have that luxury -- you have to worry about how and where variables are stored and how you can/should refer to them

Variables: Their Names, Values and Addresses

A Line of Code:
- int i;
What Happens:
- i is added to the name table
- i is designated a value type
- Memory is allocated for an int
- The memory is not initialized
Where is the Memory?
- Use &i
An Illustration:

Variables: Their Names, Values and Addresses (cont.)

Pointing to Memory

Pointing to Memory (cont.)

Using a Pointer:
- Pointers should contain memory addresses, but this is not enforced
An Example:
- ptr = &i;
An Illustration:

Pointing to Memory (cont.)

De-referencing a Pointer:
- Finds the value of the entity that is being pointed at
- Done with the * operator
An Example:
- int j; j = *ptr;
An Illustration:

Pointing to Memory (cont.)

Multiple Pointers to the Same Memory:
- Just as you can have two signs that point to the same airport, you can have two pointers that point to the same entity
An Example:
- int *otherptr; otherptr = &i;
An Illustration:

Pointing to Memory (cont.)

De-referencing Revisited:
- You can de-reference a pointer on the left-hand-side of an assignment operator
An Example:
- (*otherptr) = 9;
An Illustration:

Arrays

Java vs. C++:
- In Java, arrays are objects (and hence are reference types)
- What happens in C++?
An Example:
- int a[4];
An Illustration:

Arrays (cont.)

A Surprising Fact:
- In most contexts, an unsubscripted array name represents a pointer to the first element of the array
- So, in most contexts, a, &a and &a[0] all evaluate to the same thing
- The exceptions are when the array name passes the entire array (e.g., with sizeof and &)
An Example:
- cout << "The contents of a: " << a << "\n"; cout << "The address of a: " << &a << "\n"; cout << "The address of a[0]: " << &a[0] << "\n"; cout << "The address of a[1]: " << &a[1] << "\n"; cout << "The address of a[2]: " << &a[2] << "\n"; cout << "The address of a[3]: " << &a[3] << "\n";
A Note:
- In the last four, [] is evaluated before &

Arrays (cont.)

Arrays (cont.)

Pointers to Arrays:
- A pointer to an element of an int array is a pointer to an int
An Example:
- int *thirdptr; thirdptr = &a[0];
An Illustration:

Arrays (cont.)

Passing Arrays as Parameters:
- When an array is passed as an actual parameter it is converted to the corresponding pointer type
Be Careful:
- This means that the formal parameter (i.e., the parameter in the method declaration/definition) should be a pointer

Pointer Arithmetic

A Surprising Fact:
- Adding 1 to a pointer to an int does not change its contents by 1, it changes its contents by the size of the data type it is pointing to
An Example:
- thirdptr+1 is 0x22FF54
Using This Fact:
- *(thirdptr) is the same as a[0]
- *(thirdptr+1) is the same as a[1]
- *(thirdptr+2) is the same as a[2]
- *(thirdptr+3) is the same as a[3]

Arrays and Pointers

An Example

cppexamples/memory/IndexOf.cpp

References

Overview:
- A reference serves as an indirect handle to an entity
- References are declared using &
Differences Between Pointers and References:
- A pointer may not address an entity whereas a reference always does
- As a result, references must be initialized when declared
- A reference cannot be reassigned to refer to another entity

References (cont.)

An Example:
- int &r = i;
- r now refers to i so it has the same address and the same contents
An Illustration:

References (cont.)

Objects

An Encapsulation of a Simple Weight

cppexamples/memory/Weight.h

cppexamples/memory/Weight.cpp

Objects (cont.)

An Example:
- Weight w(6,11);
- Memory is allocated for a Weight object, its constructor is called, and the two member attributes are initialized
An Illustration:

There's Always More to Learn