getting started with c++ id1218 lecture 082009-11-23 christian schulte [email protected] software and...

GETTING STARTED WITH C++

ID1218 Lecture 08 2009-11-23

Christian [email protected]

Software and Computer SystemsSchool of Information and Communication

TechnologyKTH – Royal Institute of Technology

Stockholm, Sweden

mailto:[email protected]

L08, 2009-11-23ID1218, Christian Schulte

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Overview

Functions Pointers Arrays Objects


3

Reading Suggestions

All of you thorough reading of chapters 0 to 4 take a peek at chapter 11

Functions

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ID1218, Christian Schulte


5

Function Definition

Function definition contains return type function name parameter list body

Function can be called given function name and appropriate parameters

Function can only be defined once


6

Function Example

int maxx(int x, int y) { return (x > y) ? x : y;}


7

Function Invocation

Print maximum of two numberscout << maxx(43,27) << endl;

Uses call-by-value Parameters need not be of type int

automatic cast long int possibly with warning


8

Function Overloading

The same function name can be used multiply with different types in parameter list

double maxx(double x, double y) {

return (x>y) ? x : y;

} Complicated set of rules describe which

definition is chosen Overloading with different return type only

not allowed


9

Function Declarations

Every function needs to be declared prior to invocation

declared, but not defined! can be declared multiply

A function declaration is done by giving a function prototype

int maxx(int, int);or

int maxx(int a, int b);


10

Default Parameters

Default values can be given for formal parameters

if function has declaration, in declaration if function has only definition, in definition only allowed as last formal parameters

Assume that maxx is often used with 0 as second argument

int maxx(int, int = 0);

Then the following invocation is legal

int z = maxx(-45); In this case, definition remains unchanged


11

Inline Functions

Overhead of function call be significant: maxx is good example

Give function definition an inline directive

inline int maxx(int x, int y) {

…

} Invocation of function is replaced by body Definition must be textually before first invocation

Compilers will also inline other, small functions


12

Separate Compilation

Structure larger programs into separate files

each file contains some functions: better structure

each file can be compiled independently: save compilation time during development

Header file contains declarations and definitions of inline

functions file name extensions: .h, .hh

Implementation file contains definition of functions

(implementation)


13

Header File maxx.h

/* * Declaration of maximum function */

int maxx(int, int);


14

Implementation File maxx.cpp#include "maxx.h"

int maxx(int x, int y) { return (x > y) ? x : y;}


15

Main File main.cpp

#include <iostream>#include "maxx.h"

int main() { std::cout << maxx(47,23) << std::endl;

return 0;}


16

Putting Everything Together

Compile each implementation file independently

g++ -c main.cppg++ -c maxx.cpp

creates the files main.o and maxx.o contain object code but require linking

Put everything together (linking)g++ main.o maxx.o –o main.exe


17

Include Each Header at Most Once Remember: inline functions must be defined

not only declared before usage Remember: at most one definition!

what if header is included from other header files possibly having multiple definitions of same

function also: why read same header more than once?

Use preprocessor (also macro processor) to guarantee at-most-once inclusion

define a preprocessor variable when included test whether preprocessor variable not already

defined choose reasonably unique name


18

Fixed Header File maxx.h

/* * Declaration of maximum function */

#ifndef __MAXX_H__#define __MAXX_H__

int maxx(int, int);

#endif


19

Organizing Compilation

How to organize compilation recompilation needed if included header file

changes compilation can be time-consuming: > 1000 files? only recompile what is needed

Use make: express dependencies among files files only recompiled, if dependencies change rules for how to perform compilation

.cpp .o .o .exe

later (first lab session) more

Arrays

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C-style Arrays

C-style arraysint a[42];

creates an array of 42 integers access cout << a[1]; assignment a[1] = a[2]+a[3]; ranges from a[0] to a[41]

Dimension of array must be constant can be evaluated at compile time to constant

(eg 2*4) illegalint a[n] where n is variable!


22

Using Arrays as Parameters

int find_max(int a[], int n) { int m = a[0]; for (int i = 1; i<n; i++) if (a[i] > m) m=a[i]; return m;} Array of arbitrary size int a[]

requires to pass size as extra parameter int n


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Using Arrays as Parameters

int find_max(int a[42]) { int m = a[0]; for (int i = 1; i<42; i++) if (a[i] > m) m=a[i]; return m;} Supports only arrays statically known to

have size 42!


24

Allocating Arrays

What if size is not known statically memory for array must be allocated from heap after use, memory must be explicitly freed

C++ style memory management use new [] and delete [] special versions for arrays normal versions to be used later for objects


25

Allocating Arrays

Allocate an array of integers with size nnew int[n];

Free memory array (no matter its size or type)

delete [] a; The following does not work

int a[] = new int[n]; a must have known size, or used as parameter use pointers rather than arrays


26

Primitive Arrays of Constants Initialize arrays in declaration

declare array to be not assignable

const int DIM[] = {31,28,31,30,31,30, 31,31,30,31,30,31};

declares array of size 12


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C-Style Strings

Use arrays of chars! Often

const char s[] = "A C-string.";

contains all letters given plus 0 at the end (end-of-string marker) has size 12 (additional 0)


28

Vectors and C++ Strings

Vectors are C++ arrays #include <vector> automatic resize automatic memory management copied when passed as parameters

Strings #include <string> same advantages as above support for comparison, copying on assignment, …

Please read book about both vectors and strings

Parameter Passing

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Call By-value

Default mechanism already seen works by copying: straightforward for primitive

types but copying also for objects: to be discussed

later!

What if one return value is not enough? use call by-reference


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Call By-reference

Function to exchange to values, first attempt (wrong):

void exc(int a, int b) { int t = a; a=b; b=t;}

just works on copies passed to exc need to pass references instead


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Call By-reference

Function to exchange to values (correct):void exc(int &a, int &b) { int t = a; a=b; b=t;}

a and b are passed by reference effect is on actual parameters

int x = 4; int y = 5;exc(x,y);

constants are not allowed as actual parametersexc(x,5);

Pointers

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Pointers

Are a consequence that memory management is not abstracted away

Erlang and Java: all variables hold references to values, operations are performed implicitly on value

C and C++ distinguish objects (also primitive values) pointers: values which point to memory

address of objects


35

Pointers

Declaring a pointer to an integerint* p;

Let p point to address of xint x = 5;p = &x;

& is called unary address-of operator Read value at pointer: prints 5

cout << *p; * is called unary dereference operator

Store value at memory referenced at p: prints 7

*p = 7; cout << x;


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Pointers Illustrated…

After declarationint x = 10;int y = 7;int* p;

p points somewhere (unitialized)

(&x) 100 x = 10

(&y) 104 y = 7

…

…

(&p) 200 p = ????

…


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Segmentation Fault or Bus Error…

Assign object pointed to by p a value*p = 124;

but: not necessarily, p can also point to some other location (overrides other variables contents…)

(&x) 100 x = 10

(&y) 104 y = 7

…

…

(&p) 200 p = ????

…


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Redirecting…

Let p point to location of xp = &x;

(&x) 100 x = 10

(&y) 104 y = 7

…

…

(&p) 200 p = &x = 100

…


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Assigning…


(&x) 100 x = 5

(&y) 104 y = 7

…

…

(&p) 200 p = &x = 100

…


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Redirecting…

Let p point to location of yp = &y;

(&x) 100 x = 5

(&y) 104 y = 7

…

…

(&p) 200 p = &y = 104

…


41

Assigning…


(&x) 100 x = 5

(&y) 104 y = 99

…

…

(&p) 200 p = &y = 104

…


42

Common Idioms

Testing that pointers refer to same location

p1 == p2 Testing that objects pointed to have

same value*p1 == *p2

Use NULL for ptr not pointing anywhereconst int NULL = 0;

use in initialization, tests, etc


43

Heap Memory Management

Get memory from heapint* p = new int;

allocate memory block big enough for one int

After use, releasedelete p;


44

Getting It Wrong

Forget to delete program crashes when running out of memory

Delete to early lucky case: program crashes due to OS knowing

that memory has been freed unlucky case: already reused, arbitrary things can

happen!

Delete twice runtime error


45

Call-by-reference in C

C does not provide references: use pointers instead

void exc(int *a, int *b) { int t = *a; *a=*b; *b=t;}

effect is on values pointed toint x = 4; int y = 5;exc(&x,&y);

Arrays are Pointers

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Arrays are Pointers

C-Style arrays are basically pointers point to beginning of array

Array access a[i] translates to *(a+i) Common idiom: pointer arithmetic

pointer +/- integer: offset to pointer pointer – pointer: distance between pointers


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Pointer Arithmetic

Static arrayint a[3] = {3,2,1};

(a+0) 100

3

(a+1) 104

2

…

1(a+2) 108 …

…


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Pointer Arithmetic

Static arrayint a[3] = {3,2,1};

a[2] = *(a+1);

(a+0) 100

3

(a+1) 104

2

…

2(a+2) 108 …

…


50

Creating Dynamic Arrays

An array of size nint* a = new int[n];

Release memory laterdelete [] a;

never forget the []: important for arrays of objects (calling destructor)


51

Computing String Length

unsigned int sl(char* s) { unsigned int n = 0; while (s[n] != 0) n++; return n;} Use pointer s as array

sl("test test");


52


unsigned int sl(char* s) { char* b = s; while (*s != 0) s++; return s-b;} Use pointer s as pointer

sl("test test");


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unsigned int sl(const char* s) { const char* b = s; while (*s != 0) s++; return s-b;} Do not allow that string passed as

argument is modified the content is const, not the pointer!

Objects and Classes

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An Integer Cell

class IntCell {private: int x;public: IntCell(int y=0) { x=y; } int get() { return x; } void set(int y) { x=y; }};


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Reminder: Access Control

public: visible to everybody protected: visible to subclasses only

inheritance explained later private: visible only to defining class

default is private!


57

Creating an Integer Cell

To be used locallyIntCell c;cout << c.get() << endl;c.set(4);cout << c.get() << endl;


58

Creating an Integer Cell

Use globally: allocate memory on heapIntCell* c = new IntCell(5);cout << c->get() << endl;c->set(42);

refer to by pointer! do not forget to delete: delete c;


59

Accessors vs Mutators

Fundamental difference set: changes object state (mutator) get: does not change state (accessor)

In C++ accessors need to be declared const

int get() const { return x;}

compiler enforces that state is not changed well, can be controlled with const-cast…


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Accessors

int square(const IntCell* ic) { return ic->get()*ic->get();}

Requires that get is declared const! due to const IntCell*


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Implicit Type Conversion

The following code worksIntCell ic;ic = 4;

due to constructor used for type conversion If not desired, declare as explicit:

explicit IntCell(int y=0) { x=y;

} only for one parameter constructors


62

Initializer Lists

Rewrite constructor toIntCell(int y=0) : x(y) {}

after colon: comma separated list in order of declaration of members

Old version first initialize with default constructor for

member then assign value

New version only initialized

Matters for non primitive data members!


63

Passing Objects as Parameters By default, objects are copied

void test(IntCell m) { m.set(7);}IntCell n;test(n);cout << n.get() << endl;

still prints zero! copying can be expensive if not wanted: pass by reference or const reference!


64

Copying and Assignment

Copying is controlled by copy constructorIntCell(const IntCell& c) : x(c.x) {}

Assignment is controlled by assignment operatorIntCell& operator=(const IntCell& c) { if (this != &c) x=c.x; return *this;}

These are synthesized by compiler if missing required for resource management


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A Different IntCell

Maintain integer via pointerclass IntCell {private: int* x;

public: IntCell(int y=0) : x(new int) { *x = y; } …

} how to manage the allocated memory?


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Copy Constructor

IntCell(const IntCell& c) : x(new int) { *x = *c.x;}

Used for parameter passing Used for initialization (assume c is IntCell)

IntCell d(c);


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Assignment Operator

IntCell& operator=(const IntCell& c) {

if (this != &c) {

delete x; x = c.x;

}

return *this;

} Returns an IntCell to allow assignment

sequences

a = b = c;


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Destructor

~IntCell() {

delete x;

}

When object is deleted by delete (for heap allocated) by going out of scope (for automatically allocated)

destructor is invoked for resource management


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Default Constructor

A constructor with no arguments (or all arguments with default values)

automatically generated, if no constructors provided

Important for initializationIntCell c;

invokes the default constructor

Summary

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Summary

Functions parameter passing: by value (copy), by

reference, by constant reference Arrays

are pointers pointer arithmetic

Objects constructors: default, copy assignment operator destructor

getting started with c++ id1218 lecture 082009-11-23 christian schulte [email protected] software and...

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