1. CS-2303 System Programming Concepts
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CS-2303 System Programming Concepts
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
CS-2303, C-Term 2010
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2. Reading Assignment Deitel & Deitel, 5th edition, Chapter 20
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3. Preprocessor Wrappers
Prevent code from being included more than once.
#ifndef – “if not defined”
Skip this code if it has been included already
#define
Define a name so this code will not be included again
#endif
If the header has been included previously
Name is defined already and the header file is not included again.
Prevents multiple-definition errors
Example
#ifndef TIME_H #define TIME_H … // code #endif
Exactly as in C
Figure 20.1 in D&D
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4. Example Class
class TreeNode {
public:
... /* other methods */
TreeNode(const string &newWord); //constructor
~TreeNode(); //destructor
private:
const string word;
int count;
TreeNode *left, *right; };
Note: heavy use of const is typical of good C++ programming style
In this design, once a word is assigned to a TreeNode, that word never changes.
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5. Example Class What does the constructor method of TreeNode have to do?
class TreeNode {
public:
... /* other methods */
TreeNode(const string &newWord); //constructor
~TreeNode(); //destructor
private:
const string word;
int count;
TreeNode *left, *right; };
What does the constructor method of TreeNode have to do?
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6. What Must Constructor Do?
(Allocate memory for object)
Done before constructor method is called
Initialize left, right
To NULL
Initialize count
To 1
Create a string object word and initialize
How?
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7. Creating Member Objects
A string object is a member of TreeNode
Therefore, string constructor is automatically called to initialize object word before TreeNode constructor method is called
class TreeNode {
...
string word; };
TreeNode::TreeNode (string &newWord) {
word = newWord;
TreeNode constructor can then be programmed to set the initial value of word to be newWord (the parameter)
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8. What About const Members?
class TreeNode {
public:
... /* other methods */
TreeNode(const string &newWord); //constructor
~TreeNode(); //destructor
private:
const string word;
int count;
TreeNode *left, *right; };
A const object may never be on left side of assignment
Not even in a constructor!
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9. Constructor with const Member
TreeNode::TreeNode(const string &newWord){ left = right = NULL; count = 1;
word = newWord; }
class TreeNode {
...
private:
const string word;
int count;
TreeNode *left, *right; };
Error detected by compiler.
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10. Solution – Initializer List
A list of member-value pairs
Or member-constructor pairs
Between the () of the constructor header and the {} of the constructor body
Preceded by ':' and separated by ','
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11. Example – Initializer List
TreeNode::TreeNode(const string &newWord)
:word(newWord), //initialize word
count(1), //initialize count
left(NULL),
right(NULL)
{ /* rest of constructor body */
} // TreeNode constructor
Invokes the string constructor to create a new const string object from the argument
Widely used in the design of C++ classes
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12. Common Programming Error 21.5
Not providing a member initializer for a const data member is a compilation error.
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13. Common Programming Error 21.6
A compilation error occurs if
a member object is not initialized with a member initializer
and
the member object’s class does not provide a default constructor
Even if member object’s class defines one or more constructors, but none is a default constructor.
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14. Questions?
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15. Destructors The opposite of constructors
Called to clean up objects before deleting them
Very important if your class contains objects of other classes inside of it.
E.g., string
Dynamically allocates array of characters to hold the string itself
Must be freed before string object can be deleted
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16. Example class TreeNode { public: ... /* other methods */
TreeNode(const string &newWord); //constructor
~TreeNode(); //destructor
private:
const string word;
int count;
TreeNode *left, *right; };
TreeNode::~TreeNode() { if (left) { delete left; left = NULL; } if (right) { delete right; right = NULL; } }
Note: destructor for string word and for count are called automatically (because these are in class scope).
Destructors for left and right have to be called forcibly, because those object were allocated dynamically
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17. When are Constructors Called?
Deitel & Deitel, §20.8
When are Constructors Called?
Global Scope — I.e., objects declared outside of any function
Before main() is called!
Function or Block Scope — I.e., automatic variables and constants
When execution reaches point where object is declared
For static objects, the first time execution reaches point where object is declared
Class Scope — I.e., data members of a class
When class constructor executes initialization list (or enters block scope of constructor function)
Dynamic objects — I.e., objects created by new
Constructor is invoked by new operator
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18. When are Destructors Called?
In opposite order of constructors (mostly)
Dynamic objects
Invoked by delete operator
Class scope — I.e., data members of a class
Invoked by destructor of class object
Function or Block Scope
When just before leaving the scope
Global Scope
After main() has returned
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19. When are Destructors Called – Exceptions
static objects in function or block scope
After main() has returned but before calling destructors of objects in Global Scope
exit() function
Destructors of automatic objects are not called
Usually means abnormal termination of program, file cannot be opened, etc.
abort() function
No destructors are called
Usually means serious error, etc.
Deitel & Deitel, §20.8
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20. Dynamically Allocated Objects
Always use new operator (as in Java)
Returns pointer to object (as with malloc() in C)
Never use malloc()
There is a lot more to creating an object in C++ than simply allocating memory
new calls malloc() to allocate memory, calls constructor, sets up inheritance, finds the right polymorphic functions, etc.
Always use delete operator
Invokes destructor, cleans up, calls free(), etc.
Takes pointer to object
Never call free() directly
For same reasons not to call malloc(); memory leaks, etc.
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21. Questions?
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22. Dynamically Allocated Arrays
Deitel & Deitel, §21.6
Dynamically Allocated Arrays
new can be used to dynamically allocate arrays
Examples:–
int *myArray = new int[10]; double *dArray = new double[2*n+1]; TreeNode *silly = new TreeNode[k];
Calls default constructor for each element
No arguments for initializing individual elements!
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23. Using Dynamically Allocated Arrays
Examples:–
int *myArray = new int[10]; double *dArray = new double[2*n+1]; TreeNode *silly = new TreeNode[k];
Example usage:–
myArray[0], ... myArray[9] dArray[i], dArray[j], ... int c = silly[k-1].getCount()
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24. Deleting Dynamically Allocated Arrays
delete[] myArray; delete[] dArray; delete[] silly;
Calls the destructor for each element before deallocating memory!
Note:–
delete myArray only calls destructor for first element!
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25. Common Programming Error 21.9
Using delete instead of delete[] can lead to runtime logic errors.
To ensure that every element receives a destructor call, be sure to use delete[]
To call the destructor for an individual element, use delete for that element only
Why would anyone want to do this?
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26. Questions?
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27. Stream Manipulators Examples setfill(), setw()
Sets fill character and field width of stream
Examples
cout << setfill('0') << setw(2) << hour << ":" << setw(2) << minute << ":" << setw(2) << second << endl;
cout.setw(2); // sets all field widths
Deitel & Deitel, Fig 20.2
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28. More Stream Manipulators
setprecision()
Sets precisions (in decimal places) of floating point numbers
eof()
Returns a bool indicating whether the end of the stream has been reached
True only after program attempts to read past last character of the stream
Usage:– cin.eof()
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29. Other Useful Stream (Member) Functions
getline()
Reads stream to next newline, returns string
get()
Reads one character from input stream, returns int
put()
Writes one character to output stream
A long list of manipulators at
Deitel & Deitel, Chapter 26
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30. Questions?
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31. Review – Constructors Initialize an object of a class Initializer List
I.e., set all data members to their initial values
Especially members of other classes that need initialization
Initializer List
List of member-value pairs for setting values
Follows header of constructor method, precedes body of constructor method
Strongly recommended for good C++ programming style
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