CS 2304: Pointers, References, and Memory McQuain 2012, Gusukuma - - PowerPoint PPT Presentation

CS2304: C++ for Java Programmers

CS 2304: Pointers, References, and Memory

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Memory

• Where your variables are stored
• Where your code is stored

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Types of Memory

• Stack Memory
• Static memory, no management required
• Heap Memory
• Dynamic memory, management required
• Use new and delete to manage the memory
• Code memory
• Where code is stored
• (not actual name)

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers

• Pointer
• A variable whose value is a memory address
• Internal representation is an integer
• Another primitive type (sort of)
• Pointee
• A value in memory whose address is stored in a pointer; the pointee is the

target of the pointer

• Valid pointees
• Arrays, variables/objects, and functions (the latter is known as a function pointer)
• NULL (value of pointer is actually 0)

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers in Graphic Detail

McQuain 2012, Gusukuma 2015

int x = 42, y = 99; int* p1 = &x; int* p2 = &y; int& r1 = x; int& r2 = y; int** p3 = &p2;

Address Name:value 0001 x:42 0002 y:92 0003 p1:0001 0004 p2:0002 0005 r1:0001 0006 r2:0002 0007 p3:0004

CS2304: C++ for Java Programmers

Pointer/Reference Operators

• Dependent on where they occur in code
• * - dereference operator
• int c = *myIntegerPointer;
• * - pointer declaration (not a name)
• int* myIntegerPointer;
• & - Address of operator
• int* myIntegerPointer = &myInteger;
• & - reference declaration (not a name)
• int& myIntegerReference = myInteger;
• [] – subscript operator

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers n’ stuff

int a = 27; //aRef is a REFERENCE to a int& aRef = a; //aRef2 is also a REFERENCE to a int& aRef2 = aRef; //aPt is a pointer to where a is int* aPt = &a; //aRef3 is ALSO a reference to a int& aRef3 = *aPt; int aCopy = aRef3;

• Reference
• Treated like a variable,

except it modifies the

• riginal variable
• What’s the effect of

“aRef2++;”

• a == 28; //true
• aRef == 28; //true
• aRef2 == 28; //true
• aPt[0] == 28; //true
• *aPt == 28; //true
• aCopy == 27; //true

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers n’ stuff (cont)

//declaration of an array int foo[5] = {16,2,77,40,8};

int* fooPt = foo;//WHAAATTT?! int b = fooPt[2]; //What’s b’s value? int& bRef = fooPt[2]; bRef = 36;//What’s the contents of foo?

• Arrays are pointers allocated on

stack memory (slight lie, but 95% true)

• (the other 5%)

http://www.geeksforgeeks.org/g- fact-5/

• References can be made to array

elements

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers to Pointers

• Can be convoluted
• int a = 27;
• int* aPt = &a;
• int** aPtPt = &aPt;
• Can be 2-D array

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Pointers Pointers

• Pointer
• Value of a place in memory, that you can access
• Reference
• Essentially a pointer, but is treated like a variable as opposed to a pointer, and

modifies the original variable

• Rhyming Couplets

type* points the way *var is what it says type& is a sneaky change &var is a pointer exchange

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Dynamic Memory Allocation

int theArrayStatic[12];//completely static, legal int x = 0; cin >> x; int theArrayDynamic[x];//dynamic, illegal int* theArrayDynamic = new int[x];//dynamic, legal

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

When to Dynamically Allocate Memory?

• When?
• When the amount of memory necessary can’t be determined BEFORE run

time e.g. dynamically sizing containers

• When you need something to last beyond its scope
• Note: More often than not, you SHOULD NOT make functions that allocate memory

dynamically and instead make use of references/return by reference via function arguments

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Passing Streams to Functions

• If you pass a stream to a function, you MUST pass it by reference
• Passing a stream by value results in undefined behavior

McQuain 2012, Gusukuma 2015

void foo(ifstream input); // wrong void foo(ifstream& input); // right

CS2304: C++ for Java Programmers

Return by Pointer/Reference

• Remember: function arguments are passed by value (passed by copy)
• References are like pointers, but are syntactically treated like normal

variables

• Using return by pointer/reference, you’re not copying an ENTIRE
• bject

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Return by Pointer/Reference – Java Misconceptions

• http://www.tutorialspoint.com/compile_java_online.php
• Java passes by value
• Java NEVER gives you an object, it gives you a pointer to an object
• Hence the new operator
• Java does NOT pass by reference

McQuain 2012, Gusukuma 2015

CS2304: C++ for Java Programmers

Return by Pointer/Reference

• void L07_PointersExample02_0(Dog* myDog);
• void L07_PointersExample02_1(Dog& myDog);
• Both of these examples can MODIFY the original Dog object
• Alternative to creating an object inside the function

McQuain 2012, Gusukuma 2015