cschulte@kth.se
Software and Computer Systems
School of Information and Communication Technology
KTH – Royal Institute of Technology
Stockholm, Sweden
ID1218 Lecture 07 2009-11-18

2


C++ Basics
 Basic types and control structures
Functions and Arrays


Pointers
Some pragmatics…
ID1218, Christian Schulte L07, 2009-11-18

3




Familiarize with programming in C/C++
Understand underlying programming model
Understand
 language constructs

 program structure data structures and common algorithms

 memory management development pragmatics
 difference between C and C++
For later courses using C/C++
ID1218, Christian Schulte L07, 2009-11-18

4

Not from scratch, build

Java as programming language

Efficient approach
 focus on essential
 not comprehensive
 apply practically
ID1218, Christian Schulte L07, 2009-11-18

5

C++ for Java Programmers,
Mark Allen Weiss, Prentice Hall, 2004
 good selection of issues and topic
 non-naïve: no repetition what you know
 concise (very important)
 valuable resource after course!!!
ID1218, Christian Schulte L07, 2009-11-18

6




Thick, verbose, but useful
J. Lajoie and S. Lippmann, C++ Primer
3 rd edition, Addison-Wesley, 1998.
Reference from the designer
B. Stroustrup, The C++ Programming Language
3 rd edition, Addison-Wesley, 1997.
Tips for advanced issues
R. B. Murray, C++ Strategies and Tactics
Addison-Wesley, 1993.
C++ Standard library
N. M. Josuttis, The C++ Standard Library
Addison-Wesley, 1999.
ID1218, Christian Schulte L07, 2009-11-18

7

The classic
B. W. Kernighan and D. M. Ritchie, The C Programming
Language
2 nd edition, Prentice Hall, 1997

Great reference, portability, also useful for C++
S. P. Harbinson, G. L. Steele, C – A Reference Manual
5 th edition, Prentice Hall, 2002
ID1218, Christian Schulte L07, 2009-11-18

8

C++ as main vehicle
 basic issues
 pointers and references
 classes, objects, inheritance
 overview: operator overloading, templates, exceptions, input/output, standard library

Contrast with main differences in C
 basic issues (arrays)
 memory management, input/output
 mixing C and C++
ID1218, Christian Schulte L07, 2009-11-18

9

Study radically different model of computation
 not type safe: how to get along
 explicit memory management: no garbage collection, how to manage memory successfully
 close to abstraction level of machines

Ideally, do it in Erlang…
…but can't be done
 take the trouble and use C++/C
 can be used afterwards anyway
ID1218, Christian Schulte L07, 2009-11-18

10

Differential approach
 in contrast to Java: Java is close in many aspects

Practical approach
 try most important aspects in labs/tutorials
ID1218, Christian Schulte L07, 2009-11-18

11

You will not become C++ wizards

You will understand the basic issues
 characteristics of computation model
 characteristics of development pragmatics

You will know where to look for further information
 language
 pragmatics
ID1218, Christian Schulte L07, 2009-11-18

12

How to organize programming projects
 automatize recompilation make

How to install software
 use configure

How to develop
 how to find errors: debugging
 how to improve performance: profiling
ID1218, Christian Schulte L07, 2009-11-18

13

Labs
 warm-up:
 sound processing:

Sudoku solver:
 Hand in: show to TA introduction to pragmatics to be handed in to be handed in
ID1218, Christian Schulte L07, 2009-11-18

14

All of you
 thorough reading of chapters 0 to 3
 take a peek at chapter 11
 do it, the book is a great read!
 go through the exercises!
ID1218, Christian Schulte L07, 2009-11-18

15
ID1218, Christian Schulte L07, 2009-11-18

16
#include <iostream> using namespace std; int main() { cout << "Hello world." << endl; return 0;
}
ID1218, Christian Schulte L07, 2009-11-18

17

Execution starts with main
 integer return value is error code (0 is okay)
 is normal function (a global function)
 functions in classes (methods): member functions
 can take additional arguments (command-line): later

All C++ programs are preprocessed
 #include <…> resolved by preprocessor
 <iostream> refers to C++ system IO
 #include "my.h" includes local file my.h

Other uses: multiply used files (declarations, headers)
ID1218, Christian Schulte L07, 2009-11-18

18


Standard functionality in namespace std

 becomes available by using namespace std; here: cout (standard output), endl (end of line)
Input similarly: int x; cout << "Your age: ";

 cin >> x; cin refers to standard input also common: using C-style input/output
ID1218, Christian Schulte L07, 2009-11-18

19 public class Hello {
} public static void main(String[]) {
System.out.println("Hello world");
}
ID1218, Christian Schulte L07, 2009-11-18

20

Edit file emacs hello.cpp
 other extensions: .cc, .cxx, .C

Invoke compiler to create executable g++ hello.cpp –o hello.exe
 we use the GNU C++ compiler (g++)

Run executable
./hello.exe
ID1218, Christian Schulte L07, 2009-11-18

21
ID1218, Christian Schulte L07, 2009-11-18

22

Integer types

Floating point types

Character types

Boolean type

Non-primitive types
 arrays, objects, pointers, references, …
ID1218, Christian Schulte L07, 2009-11-18

23



Basic integer type int
 few guarantees on range in C++

 today, often 32 bits, anytime soon 64 bits, … depends on machine (think of embedded…)
Can be augmented with short or long

 int never shorter than int never longer than short int (at least 16) long int (at least 32)
Can be modified with signed (default) or unsigned
 short signed int

 short unsigned int just for example!
-32768 … 32767
0 … 65535
ID1218, Christian Schulte L07, 2009-11-18

24

Single precision
 often: 32 bits

Double precision
 often: 64 bits

Again, few guarantees float double
ID1218, Christian Schulte L07, 2009-11-18

25

Character type char
 no guarantee on size, often 8 bits (ASCII, not Unicode!)
 unspecified whether signed or unsigned

Character constants in single quotes


'a', '1' escape sequences: '\n' for newline, etc

Also available: wchar_t for wide characters
ID1218, Christian Schulte L07, 2009-11-18

26

Boolean type
 constants bool true and false

Rather late addition!
 watch out for int with zero as false and any other value true!
ID1218, Christian Schulte L07, 2009-11-18

27

Variables taking an immutable value
 cannot be changed
 abbreviation for commonly used values

Use const modifier const double pi = 3.1415;
 assignment to pi not possible
ID1218, Christian Schulte L07, 2009-11-18

28

Almost the same in C++ as in Java
 in general: C++ carries some historic weight, Java has cleaned up, so watch out for some gotchas

Almost the same

 operators and expressions, conditionals, loops, … assignment: no guarantee of initialization

Additionally

 casts: change/convert/ignore type type definitions
ID1218, Christian Schulte L07, 2009-11-18

29

No left to right order
 readInt() – readInt() with input 2 and 3 can be either 1 or -1
Integer division not guaranteed to round towards zero (book uses down)

8/-5 can be either -1 or -2
Comma operator: a,b evaluates to b
 weird things possible!
ID1218, Christian Schulte L07, 2009-11-18

30

No guarantee on shift a << b
 can be either arithmetic or logic
 arithmetic
 logic
-1 << 1 remains negative
-1 << 1 is positive
ID1218, Christian Schulte L07, 2009-11-18

31

Condition can be either of type int or bool



 if (i) { … } else { … }
If i is different from zero, take then part
If i is zero, take else part not recommended, write if (i != 0) …
Common mistake


 if (x = 0) … actually meant: if (x == 0) … assigns 0 to x , takes the else part write variable to the right, if possible: if (0 == x)
ID1218, Christian Schulte L07, 2009-11-18

32

As in Java, along with break and continue
ID1218, Christian Schulte L07, 2009-11-18

33

Variables not by guarantee initialized
 unless global or static (later)

Always give initial value
 important for objects (later)
ID1218, Christian Schulte L07, 2009-11-18

34

Automatic conversion double x = 6.0; int y; y=x;

Typecasting can be essential int x=6; int y=10; double z = x/y;
 results in 0.0 rather than 0.6
ID1218, Christian Schulte L07, 2009-11-18

35

Cast at least one into double static_cast<double>(x)/y;

Other casts
 dynamic_cast casting objects, later
 const_cast later
 reinterpret_cast just do it as size fits…
 C-style cast ((double) x)
 dangerous, combines everything, don't use!
ID1218, Christian Schulte L07, 2009-11-18

36

Give names to types

Example: define implementation dependent integer type uint32
 machine A:
 typedef unsigned long int uint32; machine B:
 typedef unsigned int uint32; rest of program can use uint32 , does not require modification when being ported to machine C
ID1218, Christian Schulte L07, 2009-11-18

37
ID1218, Christian Schulte L07, 2009-11-18

38

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
ID1218, Christian Schulte L07, 2009-11-18

39 int maxx(int x, int y) { return (x > y) ? x : y;
}
ID1218, Christian Schulte L07, 2009-11-18

40

Print maximum of two numbers cout << maxx(43,27) << endl;

Uses call-by-value

Parameters need not be of type int
 automatic cast long int
 possibly with warning
ID1218, Christian Schulte L07, 2009-11-18

41

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
ID1218, Christian Schulte L07, 2009-11-18

42


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);
ID1218, Christian Schulte L07, 2009-11-18

43


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
ID1218, Christian Schulte L07, 2009-11-18

44


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
ID1218, Christian Schulte L07, 2009-11-18

45

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)
ID1218, Christian Schulte L07, 2009-11-18

46
/*
* Declaration of maximum function
*/ int maxx(int, int);
ID1218, Christian Schulte L07, 2009-11-18

47
#include "maxx.h" int maxx(int x, int y) { return (x > y) ? x : y;
}
ID1218, Christian Schulte L07, 2009-11-18

48
#include <iostream>
#include "maxx.h" int main() { std::cout << maxx(47,23)
<< std::endl; return 0;
}
ID1218, Christian Schulte L07, 2009-11-18

49

Compile each implementation file independently g++ -c main.cpp
g++ -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
ID1218, Christian Schulte L07, 2009-11-18

50



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 atmost-once inclusion
 define a preprocessor variable when included

 test whether preprocessor variable not already defined choose reasonably unique name
ID1218, Christian Schulte L07, 2009-11-18

51
/*
* Declaration of maximum function
*/
#ifndef __MAXX_H__
#define __MAXX_H__ int maxx(int, int);
#endif
ID1218, Christian Schulte L07, 2009-11-18

52

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
ID1218, Christian Schulte L07, 2009-11-18

53
ID1218, Christian Schulte L07, 2009-11-18

54

C-style arrays
 int 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 )
 illegal int a[n] where n is variable!
ID1218, Christian Schulte L07, 2009-11-18

55 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
ID1218, Christian Schulte L07, 2009-11-18

56 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!
ID1218, Christian Schulte L07, 2009-11-18

57

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
ID1218, Christian Schulte L07, 2009-11-18

58


Allocate an array of integers with size n new 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 know size, or used as parameter
 use pointers rather than arrays (a little later)
ID1218, Christian Schulte L07, 2009-11-18

59

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
ID1218, Christian Schulte L07, 2009-11-18

60

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)
ID1218, Christian Schulte L07, 2009-11-18

61

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
ID1218, Christian Schulte L07, 2009-11-18

62
ID1218, Christian Schulte L07, 2009-11-18

63

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
ID1218, Christian Schulte L07, 2009-11-18

64

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
ID1218, Christian Schulte L07, 2009-11-18

65

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 parameters exc(x,5);
ID1218, Christian Schulte L07, 2009-11-18

66
ID1218, Christian Schulte L07, 2009-11-18

67

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
ID1218, Christian Schulte L07, 2009-11-18

68




Declaring a pointer to an integer int* p;
Let p point to address of x int 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;
ID1218, Christian Schulte L07, 2009-11-18

69

…
(&x) 100 x = 10
(&y) 104 y = 7
…
(&p) 200 p = ????
…
After declaration int x = 10;
 int y = 7; int* p; p points somewhere (unitialized)
ID1218, Christian Schulte L07, 2009-11-18

70
…
(&x) 100 x = 10
(&y) 104 y = 7
…
(&p) 200 p = ????
…

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…)
ID1218, Christian Schulte L07, 2009-11-18

71
…
(&x) 100 x = 10
(&y) 104 y = 7
…
(&p) 200 p = &x = 100
…

Let p point to location of x p = &x;
ID1218, Christian Schulte L07, 2009-11-18

72
…
(&x) 100 x = 5
(&y) 104 y = 7
…
(&p) 200 p = &x = 100
…

Assign object pointed to by p a value
*p = 5;
ID1218, Christian Schulte L07, 2009-11-18

73
…
(&x) 100 x = 5
(&y) 104 y = 7
…
(&p) 200 p = &y = 104
…

Let p point to location of y p = &y;
ID1218, Christian Schulte L07, 2009-11-18

74
…
(&x) 100 x = 5
(&y) 104 y = 99
…
(&p) 200 p = &y = 104
…

Assign object pointed to by p a value
*p = 99;
ID1218, Christian Schulte L07, 2009-11-18

75

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 anywhere const int NULL = 0;
 use in initialization, tests, etc
ID1218, Christian Schulte L07, 2009-11-18

76

Get memory from heap int* p = new int;
 allocate memory block big enough for one int

After use, release delete p;
ID1218, Christian Schulte L07, 2009-11-18

77

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
ID1218, Christian Schulte L07, 2009-11-18

78

Storage classes
 automatic and static variables

Arrays are pointers int* a = new int[n];
ID1218, Christian Schulte L07, 2009-11-18

79
ID1218, Christian Schulte L07, 2009-11-18

80

Radically new computation model
 not type safe
 explicit memory management

Few guarantees
 primitive types and their operations

Memory management is very difficult!
ID1218, Christian Schulte L07, 2009-11-18