This document discusses generics in C++. It introduces template functions and template classes as ways to reuse code by leaving key types unspecified. Template functions allow functions to operate on different data types, while template classes allow classes to work with different types. The document also discusses bounded genericity, which allows placing restrictions on template arguments, and issues with inheritance and generics.

1. Introduction Template Functions Template Classes Bounded Genericity Summary References
GENERICS
Muhammad Adil Raja
Roaming Researchers, Inc.
cbna
April 21, 2015
Generics Roaming Researchers, Inc. cbna

2. Introduction Template Functions Template Classes Bounded Genericity Summary References
OUTLINE I
INTRODUCTION
TEMPLATE FUNCTIONS
TEMPLATE CLASSES
BOUNDED GENERICITY
SUMMARY
REFERENCES
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3. Introduction Template Functions Template Classes Bounded Genericity Summary References
INTRODUCTION
The idea of a generic (or template) is yet another approach
to software reuse.
The basic idea is to develop code by leave certain key
types unspecified, to be filled in later.
In many ways this is like a parameter that is filled with may
different values.
Here, however, the parameters are types, and not values.
Generics are used both with functions and with classes.
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4. Introduction Template Functions Template Classes Bounded Genericity Summary References
TEMPLATE FUNCTIONS
The following illustrates a simple template function in C++,
and its use.
TEMPLATE FUNCTION
template <class T> T max(T l e f t , T r i g h t ) {
i f ( l e f t < r i g h t )
return r i g h t ;
return l e f t ;
}
int a = max(3 , 27);
double d = max(3.14 , 2.75); / / see how types d i f f e r
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5. Introduction Template Functions Template Classes Bounded Genericity Summary References
CAN EVEN BE USED WITH USER-DEFINED TYPES
EXAMPLE
class Fraction {
public :
Fraction ( int top , int bottom ) { t = top ; b = bottom ; }
int numerator ( ) { return t ; }
int denominator ( ) { return b ; }
bool operator < ( Fraction & r i g h t )
{ return t ∗ r i g h t . b < r i g h t . t ∗ b ; }
private :
int t , b ;
} ;
Fraction x (3 , 4 ) ;
Fraction y (7 , 8 ) ;
Fraction z = max( x , y ) ;
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6. Introduction Template Functions Template Classes Bounded Genericity Summary References
TEMPLATE CLASSES
While template functions are useful, it is more common to
use templates with classes.
TEMPLATE CLASSES
template <class T> class Box {
public :
Box (T i n i t i a l ) : value ( i n i t i a l ) { }
T getValue ( ) { return value ; }
setValue (T newValue ) { value = newValue ; }
private :
T value ;
} ;
Box<int > iBox ( 7 ) ;
cout << iBox . getValue ( ) ;
7
iBox . setValue ( 1 2 ) ;
cout << iBox . getValue ( ) ;
12
Notice how the programmer filled in the template argument
when creating a new variable.Generics Roaming Researchers, Inc. cbna

7. Introduction Template Functions Template Classes Bounded Genericity Summary References
CAN BE FILLED WITH DIFFERENT ARGUMENTS
EXAMPLE
iBox . setValue (3.1415); / / ERROR − i n v a l i d type
Box <double> dBox ( 2 . 7 ) ;
cout << dBox . getValue ( ) ;
2.7
dBox . setValue (3.1415);
cout << dBox . getValue ( ) ;
3.1415
iBox = dBox ; / / ERROR − mismatched types
In the next chapter we will see how generics are used to create
collection classes.
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8. Introduction Template Functions Template Classes Bounded Genericity Summary References
BOUNDED GENERICITY
Some languages (Eiffel, others) allow the programmer to place
a type on the template argument:
GENERICITY
class
Hash_Table [ H −> Hashable ]
. . .
The restriction says that the argument (here named H) can
only be a subclass of Hashable.
This feature allows the compiler to do stronger type
checking, and produce more meaningful error messages.
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9. Introduction Template Functions Template Classes Bounded Genericity Summary References
INHERITANCE AND GENERICS
Remember the class Box.
Suppose a class Person has subclasses BoyChild and
GirlChild.
What is the relationship between Box[Person] and
Box[BoyChild]?
Unfortunately, runs into problems with the principle of
substitution.
Assume Box[BoyChild] is a subclass of Box[Person], would
make the following legal:
EXAMPLE
Box [ Person ] aBox = new Box [ BoyChild ] ;
Person aGirl = new G i r l C h i l d ;
aBox . set ( aGirl ) ;
A similar argument can be made for the reverse.Generics Roaming Researchers, Inc. cbna

10. Introduction Template Functions Template Classes Bounded Genericity Summary References
INHERITANCE AND ARRAYS
Can make a similar argument for arrays:
EXAMPLE
BoyChild [ ] boys = new BoyChild [ 1 0 ] ;
Person [ ] people = boys ; / / copy or pointer semantics?
G i r l C h i l d s a l l y = new G i r l C h i l d ;
people [ 1 ] = s a l l y ;
If pointer semantics are used for the array assignment then
this can produce type errors.
Java allows the assignment, but uses a run-time check to
catch the last assignment error!
Other languages make the array assignment illegal.
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11. Introduction Template Functions Template Classes Bounded Genericity Summary References
SUMMARY
A polymorphic Variable is a variable that can reference
more than one type of object.
Polymorphic variables derive their power from interaction
with inheritance, overriding and substituion.
A common polymorphic variable is the implicit variable that
maintains the reciever during the execution of a method.
Downcasting is the undoing of a polymorphic assignment.
Pure polymorphism occurs when a polymorphic variable is
used as an argument.
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12. Introduction Template Functions Template Classes Bounded Genericity Summary References
REFERENCES
Images and content for developing these slides have been
taken from the follwoing book with the permission of the
author.
An Introduction to Object Oriented Programming, Timothy
Budd.
This presentation is developed using Beamer:
Szeged, monarca.
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