1. A Deeper Look at Classes
Professor Hugh C. Lauer CS-2303, System Programming Concepts
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
CS-2303, A-Term 2012
A Deeper Look at Classes

2. const Objects & const Member Functions
Principle of Least Privilege
“Allow access to data only when absolutely needed”
Fundamental principle of good software engineering
const objects
Keyword const
Specifies that an object is not modifiable
Attempts to modify const object  compilation errors
Example
const Time noon (12, 0, 0);
CS-2303, A-Term 2012
A Deeper Look at Classes

3. const Member Functions
Only const member functions can be called for const objects
Member functions declared const may not modify the object in any way
A function is specified as const both prototype and definition.
Constructors and destructors are not const
By definition!
CS-2303, A-Term 2012
A Deeper Look at Classes

4. A Deeper Look at Classes
const Example
A promise that these functions do not modify the object at all
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A Deeper Look at Classes

5. const Example (continued)
Same here!
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A Deeper Look at Classes

6. const Example (continued)
Not const
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A Deeper Look at Classes

7. const Example (continued)
const keyword in function definition, as well as in function prototype
CS-2303, A-Term 2012
A Deeper Look at Classes

8. const Example (continued)
Same here!
And here!
CS-2303, A-Term 2012
A Deeper Look at Classes

9. Purpose of const
To enlist the aid of the compiler in detecting unwanted changes to objects
Widely used in large programming projects
Helps maintain sanity, teamwork, etc.
Get used to using const everywhere! For methods, parameters, etc.
CS-2303, A-Term 2012
A Deeper Look at Classes

10. A Deeper Look at Classes
Questions?
CS-2303, A-Term 2012
A Deeper Look at Classes

11. Default Memberwise Assignment
Absolute C++, §8.3
Default Memberwise Assignment
Assignment operator (=)
Can be used to assign an object to another object of the same type.
Each data member of the right object is assigned to the same data member in the left object.
Not usually want you want to do!
Usually causes serious problems when data members contain pointers to dynamically allocated memory !!
Because pointers are simply copied
CS-2303, A-Term 2012
A Deeper Look at Classes

12. Default Memberwise Assignment – Example
Default initialization of data members
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A Deeper Look at Classes

13. Default Memberwise Assignment (continued)
memberwise assignment assigns data members of date1 to date2
date2 now stores the same date as date1
CS-2303, A-Term 2012
A Deeper Look at Classes

14. A Deeper Look at Classes
But …
What happens if we use the default assignment operator on TreeNode?
class TreeNode {
public:
... private:
const string word; int count; TreeNode *left, *right;
}; // class TreeNode
CS-2303, A-Term 2012
A Deeper Look at Classes

15. Default Memberwise Assignment (concluded)
Many classes must provide their own assignment operator
To intelligently assign one object to another
Need to overload the ‘=‘ operator
CS-2303, A-Term 2012
A Deeper Look at Classes

16. A Deeper Look at Classes
Questions?
CS-2303, A-Term 2012
A Deeper Look at Classes

17. A Deeper Look at Classes
Copy Constructor
Definition
A constructor that has a single parameter of the form
ClassName(ClassName &objectToBeCopied);
Normally
A constructor that has a single constant parameter of the form
ClassName(const ClassName &objectToBeCopied);
CS-2303, A-Term 2012
A Deeper Look at Classes

18. Purpose of Copy Constructor
To make a new object just like the previous object
i.e., to make a copy of the previous object
Intelligently
E.g.,
string S1("Now is the time …"); …
string S2(S1);
S1 contains an internal array of char
S2 now contains its own, separate internal array of char
CS-2303, A-Term 2012
A Deeper Look at Classes

19. A Deeper Look at Classes
Copy Constructors
… are used widely in programs
… are used implicitly by compiler whenever objects are passed by value!
Passing objects to parameters
Returning results from functions
Initializing uninitialized memory from previous values
CS-2303, A-Term 2012
A Deeper Look at Classes

20. A Deeper Look at Classes
Copy Constructor
A constructor that copies another object of the same type — e.g.:–
class TreeNode {
public:
... /* other methods */
TreeNode(const TreeNode &nodeToBeCopied); // Copy Constructor
private:
const string word;
int count;
TreeNode *left, *right; };
CS-2303, A-Term 2012
A Deeper Look at Classes

21. Default Copy Constructor
Compiler provides a default copy constructor
Copies each member of original object into corresponding member of new object
i.e., memberwise assignment
Enables pass-by-value for objects
Used to copy original object’s values into new object to be passed to a function or returned from a function
Not usually want you want to do!
Often causes serious problems when data members contain pointers to dynamically allocated memory !!
Just like default memberwise assignment
CS-2303, A-Term 2012
A Deeper Look at Classes

22. A Deeper Look at Classes
Copy Constructor
Many classes must provide an explicit Copy Constructor
To intelligently copy one object to another
Example:–
string(const string &stringToBeCopied);
Allocates new memory for the new string
Copies characters from one to other
Does not blindly copy members (including internal pointers, etc.)
Why do we need '&'?
CS-2303, A-Term 2012
A Deeper Look at Classes

23. Usage of Copy Constructor
In Initializer list — E.g.:–
TreeNode::TreeNode(const string &newWord)
:word(newWord), //initialize word
count(1), //initialize count
left(NULL),
right(NULL)
{ /* rest of constructor body */
} // TreeNode constructor
The string copy constructor
CS-2303, A-Term 2012
A Deeper Look at Classes

24. A Deeper Look at Classes
Questions?
CS-2303, A-Term 2012
A Deeper Look at Classes

25. A Deeper Look at Classes
The “this” Pointer
Member functions know which object’s data members to manipulate
Every object has access to own address through a pointer called this (a C++ keyword)
Object’s this pointer is not part of object itself
this pointer is passed (by the compiler) as an implicit argument to each of object’s non-static member functions
CS-2303, A-Term 2010
A Deeper Look at Classes

26. A Deeper Look at Classes
Using the this Pointer
Objects may use the this pointer implicitly or explicitly
this is used implicitly when accessing members directly
It is used explicitly when using keyword this
Type of the this pointer depends on
type of the object
whether member function is declared const
CS-2303, A-Term 2010
A Deeper Look at Classes

27. A Deeper Look at Classes
Example using “this”
CS-2303, A-Term 2010
A Deeper Look at Classes

28. Example using “this” (continued)
Directly accessing member x
Explicitly using the this pointer to access member x
Using the dereferenced this pointer and the dot operator
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A Deeper Look at Classes

29. Another Example Using “this”
class ExtendTreeNode { public: ... /* other methods */ ExtendTreeNode(const string &newWord, ExtendTreeNode *parent); //constructor private: const string word; int count; ExtendTreeNode *left, *right ExtendTreeNode *const myParent; };
CS-2303, A-Term 2010
A Deeper Look at Classes

30. Example Using “this” (continued)
ExtendTreeNode::ExtendTreeNode(const string &newWord, ExtendTreeNode *parent) :word(newWord), //initialize word count(1), //initialize count left(NULL), right(NULL), myParent(parent) //point back to myParent { /* rest of constructor body */ } // ExtendTreeNode constructor
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A Deeper Look at Classes

31. Example Using “this”(continued)
ExtendTreeNode *ExtendTreeNode::AddNode(const string &newWord){ if (newWord == word){ count++; return this; } else if (newWord < word) { if (left) return left->AddNode(newWord); else return left = new ExtendTreeNode(newWord, this); } else { /* same for right */ } } // AddNode
Constructor of ExtendTreeNode with pointer back to parent node!
CS-2303, A-Term 2010
A Deeper Look at Classes

32. Cascaded member-function calls
Multiple functions are invoked in the same statement.
Enabled by member functions returning the dereferenced this pointer.
Example
t.setMinute( 30 ).setSecond( 22 );
Calls t.setMinute( 30 ) and returns reference to t
Then calls t.setSecond( 22 );
CS-2303, A-Term 2010
A Deeper Look at Classes

33. Cascaded calls (continued)
Absolute C++, §8.3 (end)
Cascaded calls (continued)
Instead of
void setMinute (const int );
Use
Time& setMinute (const int );
Implementation
Time& Time::setMinute (const int m){ minute = m; return *this; }
Note reference result
CS-2303, A-Term 2010
A Deeper Look at Classes

34. A Deeper Look at Classes
Reference Result
A function may return a reference to an object (instead of a value)
Value  copy of an object
Reference  object itself
Reference result must not be automatic variable
Compiler checks
Reference result may be used wherever an object of same type/class may be used
Subject to const rules
Especially useful for overloaded operators
CS-2303, A-Term 2010
A Deeper Look at Classes

35. A Deeper Look at Classes
Questions?
CS-2303, A-Term 2010
A Deeper Look at Classes