A C++ Crash Course
Part I
UW Association for Computing Machinery
http://www.cs.washington.edu/orgs/acm/tutorials
[email protected]
Questions & Feedback to Hannah C. Tang (hctang) and Albert J. Wong (awong)
What We’ll Cover
• C/C++ fundamentals
– Functions
– Primitive data types
• The stack
• C-style types
– Typedefs
– Structs
• Arrays (whole story)
– Arrays (working model)
– Pointers
• A practice program
• More C++-isms
– C++-style vs Java-style
references
– C++ gotchas
What We’re NOT Covering
• Topics related to C++ classes
– Multiple files
– The preprocessor
• C++ classes
– Inheritance and dynamic dispatch
• Memory management
– The heap
– Destructors
• Advanced topics
– Modifiers: const, static, and extern
– Operator overloading
– Templates
Goals of Java
Java, C, and C++, have different design goals.
Java
–
–
–
–
–
Simple
Consistent
Huge OO Focus
Cross platform via a virtual machine
Originally for embedded systems
Goals of C and C++
C and C++ are popular because they have met, with
reasonable success, their goals.
C
– Low level
– No Runtime Type info
– Easy implementation
C++
– Originally to add some OO functionality to C
– Attempt to be a higher-level language
– Now it’s a totally different language
A simple program snippet
public void printSum(void)
{
int x, y;
}
void printSum(void)
{
int x, y;
// … get user input …
// … get user input …
int sum = x + y;
int sum = x + y;
// … print sum …
// … print sum …
}
The simple program – Java version
class Calculator {
public void printSum(void) {
int x, y;
// … get user input …
App
int sum = x + y;
// … print sum …
}
printSum()
}
class App {
public static void main(String[] args) {
Calculator c = new Calculator;
c.printSum();
}
}
Calculator
The simple program – C++ version
void printSum(void) {
int x, y;
class Calculator {
public void printSum(void) {
int x, y;
// … get user input …
// … get user input …
int sum = x + y;
int sum = x + y;
// … print sum …
// … print sum …
}
}
}
int main(int argc,
const char * argv[]) {
printSum();
return 0;
}
class App {
public static void main(String[] args)
{
Calculator c = new Calculator;
c.printSum();
}
}
Procedural Programming
• Functions are free-floating “methods” disassociated from
any class
• Functions declarations can be separate from function
implementations
– The declaration provides the signature, which specifies the
name, return type, and parameter list of the function
• C is completely procedural
• C++ mixes object-oriented and procedural programming
Discussion Point I
Which of these programs can be written
procedurally? Object-orientedly?
• HelloWorld
• A traffic simulator
– Must simulate cars, roads, and the interactions between
these entities
• A calculator
– Accepts two numbers, then calculates the sum or
difference, depending on a user-selected operator
• An mp3 player
– Accepts a list of files, and plays them in the specified
order. Needs to support skins
• Come up with your own example
Function Syntax and Semantics
<ReturnType> functionName( … <parameter list> … );
int calculatePower(int base, int exponent);
• <ReturnType> can be any type except an array
• Class-scoped methods and free-floating
functions are basically the same, except …
Parameter Passing in Java – Part I
class Example {
public void moveToDiagonal(Point p) {
p.setY(p.getX());
}
public static void main( String[] args ) {
Point pt;
pt = new Point(3, 4);
moveToDiagonal(pt);
// What are the coordinates of pt now?
}
}
In Java, everything is a reference
x: 3 x: 3
y: 4 y: 3
pt
Point pt;
pt = new Point(3, 4)
p
moveToDiagonal(Point p) {
p.setY(p.getX());
}
In Java, modifying a method parameter means
modifying the original instance
… almost everything is a reference
Java atomic types:
• int
• double
• boolean
C++ atomic types:
• int
• double
• bool
• etc …
• etc …
In Java, modifying an atomically-typed parameter did
NOT modify the original instance.
In Java, atomic types are passed by copy. The same
semantics hold for C++ atomic types
C/C++ Function Parameters
• In C++, all function parameters are passed
by copy – even if they’re not of atomic type
• Why?
– First, a brief detour …
Detour: Functions & Memory
• Every function needs a place
to store its local variables.
Collectively, this storage is
i
Memory
called the stack
d2
location
• This storage (memory aka
d1
“RAM”), is a series of storage
spaces and their numerical
y
addresses
x
• Instead of using raw
addresses, we use variables
to attach a name to an
void aFunc(int x,
address
{
• All of the data/variables for a
double d1, d2;
particular function call are
int i;
located in a stack frame
}
int y)
Detour: Functions & Memory (cont)
• When a function is called, a new
stack frame is set aside
• Parameters and return values are
passed by copy (ie, they’re copied
into and out of the stack frame)
• When a function finishes, its stack
frame is reclaimed
void aFunc(int x, int y) {
double d1 = x + y;
}
int main(int argc,
const char * argv[]) {
int x = 7;
aFunc(1, 2);
aFunc(2, 3);
return 0;
}
d1
y
aFunc
x
x
7
main
C/C++ Function Parameters (cont.)
• In C++, all function parameters are passed by
copy – even if they’re not of atomic type
• Why?
– In C++, all variables exist on the stack by default
– In C++, parameters are copied into the callee’s stack
frame
– We’ll talk about Java parameter passing later (when
we talk compare C++ and Java references)
Discussion Point II
• Examine the code fragment below.
– Draw the stack frame(s) for some sample input.
– If you see any bugs, what are they? How would the
program behave?
void sillyRecursiveFunction(int i) {
if(i == 0) {
return;
}
else {
sillyRecursiveFunction(i – 1);
}
}
Arrays
<ArrayType> arrayName[ numElements ]
• Arrays are contiguous memory
locations, and its name refers
only to the address of the first
element
• Indexing into an array is the
same as adding an offset to the
address of the first element
• When declaring an array, its size
must be known at compile-time
myArray[5]
myArray[4]
myArray[3]
myArray[2]
myArray[1]
myArray[0]
or myArray
Arrays as function parameters
<ReturnType> funcName( ArrayType arrName[ ] )
int sumOfArray( int values[], int numValues )
• Arrays are not passed by copy. Instead,
the address of the first element is passed
to the function
– Note how array parameters and nonparameter arrays behave identically
Discussion Point III
• Why are arrays not passed by copy?
– Hint: the size of a stack frame is computed
long before the program is run (specifically, at
compile time)
Pointers
What if we had variables that contained addresses?
They could contain addresses of anything!
We could use these
variables in functions to
modify the caller’s data
(we could implement
Java’s parameterpassing semantics!)
x
(4104)
y
(4100)
Variable
name
n
(4096)
Address
Storage space
Pointers: vocabulary
• A pointer is a variable
which contains
addresses of other
variables
• Accessing the data at
the contained address
is called “dereferencing
a pointer” or “following
a pointer”
x
(4104)
y
(4100)
4096
n
(4096)
7
Pointer Syntax
Declaring Pointers
Using Pointers
Declaring a pointer:
<Type> * ptrName;
Dereferencing a pointer:
*ptrName
“Go to the address contained in the
“ptrName is a variable which
variable ptrName”
contains the address of
something of type <Type>”
Getting the address of a variable:
&aVar
“Get the address of aVar”
For example:
int * nPtr1, * nPtr2;
void aFunc(
int aParam,
int * ptrParam);
For example:
aFunc(myInt,
&anotherInt);
anInt = *myPtr * 4;
*dinner = 100;
Pointers: Putting it all together
The code
int * p;
int q;
p = &q
*p = 5;
Box Diagrams
“p’s type is int
pointer. q’s type
is int.”
“Assign 5 to where
p points (which is
q).”
p
q
5
Memory Layout
p contains the
address of an
int. q contains an
int.
Go to the address
that p contains, and
place a 5 there.
p (8200)
8196
q (8196)
5
Pointers: Putting it all together (cont.)
The code
Memory Layout
Box diagram
main
void doubleIt(int x,
int * p)
16
a
{
*p = 2 * x;
}
int main(int argc,
const char * argv[])
doubleIt
{
int a = 16;
x 9
doubleIt(9, &a);
return 0;
p
}
p
(8200)
8192
doubleIt
x
(8196)
9
a
(8192)
16
main
Pointer Arithmetic
Pointers are numbers, so you can do math on them!
int * p = &a;
p
(8200)
b
(8196)
a
(8192)
8192
9
16
*p = 200;
p
(8200)
b
(8196)
a
(8192)
8192
9
200
*(p+1) = 300;
p
(8200)
b
(8196)
a
(8192)
8192
300
200
Pointer p refers to an int, so adding 1 to p increments the address by
the size of one int. The C/C++ expression for this is sizeof(int)
Pointers and Arrays
Pointers and arrays are (almost) interchangeable
Given:
int myArray[5];
int * p = myArray;
These are equivalent:
•
•
•
•
•
•
*p
myArray[0]
*(p+0)
*myArray
p[0]
0[p]
myArray[4]
(9000)
myArray[3]
(8196)
myArray[2]
(8192)
myArray[1]
(8188)
myArray[0]
(8184)
p
(8180)
8184
Discussion Point IV
• How do pointers and arrays differ?
– Hint: how are pointers implemented in
memory? Arrays?
Exercise
• Get up and stretch!
• Do the worksheet exercise
• Then, write a program to do the following:
– Read some numbers from the user (up to a
max number of numbers)
– Calculate the average value of those numbers
– Print the user’s values which are greater than
the average
• Get up and stretch again!
Pointer Problems
• Pointers can refer to other variables, but:
– Create an additional variable
– Have an ugly syntax
–
–
Function Pointers
<ReturnType> (*ptrName)(arg type list );
•
•
•
•
Functions are pieces of code in memory
Pointers can point to functions.
This syntax is U-G-L-Y (the ugliest in C)
Notice that the name of the variable appears in
the middle of the statement!
• You do not have to dereference a function
pointer
Function pointers are not scary. They are useful!
Function Pointers - example
void foo(int i, char b);
void bar(int i, char b);
int main(void) {
void (*p)(int,char);
p = foo;
p(1, ‘c’);
// equivalent to foo(1, ’c’);
p = bar;
p(2, ‘b’); // equivalent to bar(2, ‘b’);
(*p)(2, ‘b’); // Exactly the same
return 0;
}
References
References are an additional name to an
existing memory location
If we wanted something called “ref” to refer to a variable x:
Pointer:
x
ref
Reference:
9
x
ref
9
Properties of References
Reference properties:
– Cannot be reassigned
– Must be assigned a referee at construction
Therefore:
– References cannot be NULL
– You cannot make an array of references.
Given what you know about references, can you
explain where these properties come from?
Reference Syntax
References
Pointers
Declaring a reference:
<Type> & refName =
referee;
Declaring a pointer:
<Type> * ptrName;
Usage:
int n;
int & referee = n;
void aFunc(
int aParam,
int & ptrParam);
Usage:
int n;
int * nPtr1 = &n;
void aFunc(
int aParam,
int * ptrParam);
aFunc(1, n);
aFunc(1, &n);
Discussion Point V
• What are the differences between Java
references and C++ references? What
about Java references and C++ pointers?
C-style struct
A struct is used to group related data items
struct student {
int id;
char name[80;]
};
Note that the it is optional
to name a struct
• To the programmer
– id and name are now related
– struct student creates a convenient grouping
• To the compiler
– Id and name have a fixed ordering (not offset) in memory
– Struct student is a first-class type that can be passed to
functions
struct Syntax
Declaring a Struct
Declaring a struct:
struct [optional name] {
<type> field1;
<type> field2;
…
} [instance list];
Examples:
struct Foo {
int field1;
char field2;
} foo,*foo_ptr;
struct Foo foo2;
struct { int a; } blah;
Access struct fields
Accessing a field in a struct:
foo.field1;
“gets field1 from the instance foo of
struct Foo”
Pointers syntax and structs
The * has lower precedence than the ‘.’ :
*foo_ptr.field1;
means
*(foo_ptr.field1);
Which won’t compile
Accessing a field in a struct pointer:
(*foo_ptr).field1;
foo_ptr->field1;
enum
An enum creates an enumerated type; they
are options with an associated value
enum PrimaryColors {
RED = 0,
GREEN,
BLUE
};
•
•
•
•
Note that the it is optional
to name an enum
By default, the first option is given the value 0
You can assign an option any integer
Subsequent options have the previous option’s value + 1
All enumeration values are in the same namespace
enum Syntax
Declaring an enum
Declaring a enum:
enum [optional name] {
OptionName [= int],
OptionName [= int],
…
} [instance list];
Example of an enum:
enum Color {
RED,
GREEN,
BLUE
} color, *color_ptr;
enum Color c;
void drawCircle
(enum Color c);
Enum quirks
Problems with Enums:
• Frail abstraction
• Treated as integers
• Can be assigned invalid values
• Flat namespace
Proper use guidelines:
• Avoid breaking abstraction
• Mangle name of enum into option
name (so ColorRed instead of Red)
Here is one sanctioned abstraction break
enum Color {
RED,
GREED,
BLUE,
NumberOfColors
};
union
An union creates an union type; all fields
share the same memory location
union Argument {
int intVal;
double doubleVal;
char charVal;
};
Note that the it is optional
to name a union
• Changing intVal changes doulbeVal and charVal!
• Can be used to create constrained-type containers
• Usually used in conjunction with an enum that says which
field is currently valid.
union Syntax
Declaring an enum
Declaring a enum:
union [optional name] {
<type> name1;
<type> name2;
…
} [instance list];
Example of a union:
union Argument {
int value;
char *string;
} arg1, *ptr;
union Argument arg2;
arg1.value = 3;
arg2.string = NULL;
Union quirks
Problems with Enums:
• Only assume that the last field
written two is valid.
• Don’t use to “save space.”
Proper use guidelines:
• Ensure you have another method
of knowing which field is currently
valid.
Typedef
Typedef is used to create an alias to a type
typedef
unsigned char
unsigned char mybyte;
byte mybyte;
byte;
• byte now represents an unsigned char
• Both definitions of mybyte are equivalent to the compiler.
• The second definition is preferred as it gives more info
Typedef – common uses
• Abstraction
– The user may easily change the type used to
represent a variable.
• Clarification
– More informative names for a type be given
– Variables that use the same type in different ways can
be separated easily
• Convenience
– Type names can get very long
– People like structs to look like real types
– Some type names (like function pointers or array
pointers) are really hard to read/write
Typedefs – structs/enums/unions
People often make a typedef of an
anonymous struct, enum, or union
typedef struct {
int id;
char name[80];
} Student;
struct Student {
int id;
char name[80];
};
Student st;
struct Student st;
These are almost the same.
However, anonymous
structs cannot refer to
themselves.
struct List {
int data;
struct List *next;
};
Discussion Point VI
• What advantages do named structs/unions
have over anonymous ones? Are enums
different?
– How would you try to pass anonymous
structs, enums, or unions to a function? Can
you?
C++ “Gotcha” I
Don’t use exceptions unless you
know what you’re doing!
• Uncaught C++ exceptions do not produce a stack trace.
• C++ does not automatically reclaim new’d resources
(more in a later tutorial)
void someFunc(void) {
throw “Exception!";
}
int main(int argc,
const char * argv[]) {
someFunc();
return 0;
}
$ ./myProg
Aborted
$
C++ “Gotcha” II
Don’t return pointers (or references) to
local variables!
double * aFunc(void) {
double d;
return &d;
}
int main(int argc,
const char * argv[]) {
double * pd = aFunc();
*pd = 3.14;
return 0;
}
Boom! (maybe)
C++ “Gotcha” III
Uninitialized pointers are bad!
int * i;
if( someCondition ) {
…
i = new int;
} else if( anotherCondition ) {
…
i = new int;
Does the phrase “null
}
*i = someVariable;
pointer exception”
sound familiar?
C++ “Gotcha” IV
Never use an array without knowing its size
int myArray[5];
• C++ arrays do not
know their own size.
– Always pass a size
variable with the array
– Always check the
bounds manually (C++
won’t do it for you!)
myArray[0]
myArray[1]
myArray[2]
myArray[3]
myArray[4]
=
=
=
=
=
85;
10;
2;
45;
393;
myArray[5] = 9;
myArray[-1] = 4;
No Error! Undefined Behavior!
What We Covered
• The procedural programming paradigm
• Functions and parameter passing
• The C/C++ memory model – Part I (the stack)
– Pointers
– Arrays
– C++-style References
• C type constructs
– Structs, enums, unions, typedefs
Any questions?
Acknowledgements & References
– Books:
– Essential C++ (C++ In-Depth Series), Stanley B. Lippman, 1999, 304
pgs.
– The C++ Primer, 3rd edition, Stanley B. Lippman, 1998, 1237 pgs.
– Effective C++, 2nd edition, Scott Meyers, 1997, 304 pgs.
– The C++ Language, 2nd Edition, Bjarne Stroustrup, 2000, 1019 pgs.
– Thinking in C++, 2nd Edition, Bruce Eckel, 2000, 814 pgs.
Also available online (for free):
http://mindview.net/Books/TICPP/ThinkingInCPP2e.html
• Nathan Ratliff
– Version 1 of the C++ tutorial
• Doug Zongker
– Version 1 of the handouts
• Hannah C. Tang & Albert J. Wong
– Wrote, proofread, and presented the current version of the tutorial and
handouts
It’s basically over now
The next few slides are here for completeness. You
do not need to know most of the following info.
The stuff on array, the majority of C developers
probably do not know this following info.
If you are not comfortable with the material on
pointers and arrays presented previously, just skip
the next slides.
If you are terminally curious, keep going.
Arrays (the whole story)
Arrays are not pointers. They are not first
class types either.
• Arrays know their size!
• Arrays forget their size after they get passed to a function!
• You CANNOT return arrays of any type
int foo(int ar[ ]) {
printf(“%d\n”, sizeof(ar));
}
int main(void) {
int ar[10];
printf(“%d\n”,sizeof(ar));
foo(ar);
return 0;
}
The output of this, assuming
a 4-byte int would be:
40
4
Pointers to Arrays
int (*ar)[3]
vs.
int *ar[3]
• The first is a pointer to an array of 3 integers.
• The second is a array of 3 elements, where each element
is an int-pointer.
• This is how multidimensional arrays work p
int a[3];
int *p = a;
p+1 == 8188
int (*p2)[3] = &a;
p2+1 == 8196
(*p2)[0] == p2[0][0] == 122
(*(p2+1))[0] == p2[1][0] == p2 == 8184
(8200)
p2
(8196)
&a[2]
(8192)
&a[1]
(8188)
&a[0]
(8184)
8184
8184
16
485
122
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The C++ Crash Course : Part I