more about c++ classescs-2303, c-term 20101 more about c++ classes cs-2303 system programming...

More about C++ ClassesCS-2303, C-Term 2010 1

More about C++ Classes

CS-2303System 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)


Reading Assignment

• Deitel & Deitel, 5th edition, Chapter 20


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


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.


Example Class


... /* other methods */TreeNode(const string &newWord);//constructor~TreeNode(); //destructor


};

What does the constructor methodof TreeNode have to do?


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?


Creating Member Objects

class TreeNode {

...

string word;

...

};

TreeNode::TreeNode (string &newWord) {

...

word = newWord;

...

};

A string object is a member ofTreeNode

Therefore, string constructoris automatically called to initialize object word before TreeNodeconstructor method is called

TreeNode constructor can thenbe programmed to set theinitial value of word to benewWord (the parameter)


What About const Members?

class TreeNode {public:... /* other methods */TreeNode(const string &newWord);//constructor~TreeNode(); //destructor


};

A const object may nev

er be

on left side of assignment

Not even in a constructor!


Constructor with const Member

TreeNode::TreeNode(const string &newWord){left = right = NULL;count = 1;word = newWord;

}

class TreeNode {...


};

Error detected by compiler.


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 ','


Example – Initializer List

TreeNode::TreeNode(const string &newWord):word(newWord), //initialize wordcount(1), //initialize countleft(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


Common Programming Error 21.5

• Not providing a member initializer for a const data member is a compilation error.



• 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.


Questions?


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


Example


... /* other methods */TreeNode(const string &newWord);//constructor~TreeNode();

//destructor


};

TreeNode::~TreeNode() {if (left) {

delete left;left = NULL;

}

if (right) {delete right;right = NULL;

}}

Note: destructor for string word andfor count are called automatically(because these are in class scope).

Destructors for left and right have to be called forcibly, because thoseobject were allocated dynamically


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

Deitel & Deitel, §20.8


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


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.



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.


Questions?


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!



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()


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!



• 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?


Questions?


Stream Manipulators

• setfill(), setw()• Sets fill character and field width of stream

• Examplescout << setfill('0') << setw(2) << hour << ":" << setw(2) << minute << ":" << setw(2) << second << endl;

cout.setw(2); // sets all field widths

Deitel & Deitel, Fig 20.2


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()


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• http://www.cplusplus.com/reference/iostream/

• Deitel & Deitel, Chapter 26

http://www.cplusplus.com/reference/iostream/


Questions?

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