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Introduction Summary References
OVERLOADING
Muhammad Adil Raja
Roaming Researchers, Inc.
cbna
April 20, 2015
Introduction Summary References
OUTLINE I
INTRODUCTION
SUMMARY
REFERENCES
Introduction Summary References
INTRODUCTION
• In this chapter we will investigate the idea of overloading.• Overloading based on scopes.• Overloading based on type signatures.• Coercison, Conversion and Casts.• Redefinition.• Polyadicity.
Introduction Summary References
A DEFINITION OF OVERLOADING
• We say a term is overloaded if it has two or moremeanings.
• Most words in natural languages are overloaded, andconfusion is resolved by means of context.
• Same is true of OO languages.• There are two important classes of context that are used to
resolve overloaded names.• Overloading based on Scopes.• Overloading based on Type Signatures.
Introduction Summary References
OVERLOADING BASED ON SCOPES I
• A name scope defines the portion of a program in which aname can be used, or the way it can be used. Scopes areintroduced using lots of different mechanisms:
• Classes or interfaces• Packages or Units• Procedures or Functions in some languages, even Blocks• An advantage of scopes is that the same name can appear
in two or more scopes with no ambiguity.
Introduction Summary References
OVERLOADING BASED ON SCOPES II
AN EXAMPLE
Procedure Fr iend . sendFlowersTo ( anAddress : address ) ;begin
go to f l o r i s t ;g ive f l o r i s t message sendFlowersTo ( anAddress ) ;
end ;
Procedure F l o r i s t . sendFlowersTo ( anAddress : address ) ;begin
i f address i s nearby thenmake up f l ower arrangementt e l l d e l i v e r y person sendFlowersTo ( anAddress ) ;
elselook up f l o r i s t near anAddressphone f l o r i s tg ive f l o r i s t message sendFlowersTo ( anAddress )
end ;
Introduction Summary References
RESOLVING OVERLOADED NAMES
• This type of overloading is resolved by looking at the typeof the receiver.
• Allows the same name to be used in unrelated classes.• Since names need not be distinct, allows short, easy to
remember, meaningful names.
Introduction Summary References
OVERLOADING BASED ON TYPE SIGNATURES
A different type of overloading allows multiple implementationsin the same scope to be resolved using type signatures.
TYPE SIGNATURES
class Example {/ / same name, th ree d i f f e r e n t methods
i n t sum ( i n t a ) { return a ; }i n t sum ( i n t a , i n t b ) { return a + b ; }i n t sum ( i n t a , i n t b , i n t c ) { return a + b + c ; }
}
• A type signature is the combination of argument types andreturn type.
• By looking at the signature of a call, can tell which versionis intended.
Introduction Summary References
RESOLUTION PERFORMED AT COMPPILE TIME
Note that resolution is almost always performed at compiletime, based on static types, and not dynamic values.
RESOLUTION
class Parent { . . . } ;
class Chi ld : public Parent { . . . } ;
void Test ( Parent ∗ p ) { cout << " i n parent " << endl ; }void Test ( Ch i ld ∗ c ) { cout << " i n c h i l d " << endl }
Parent ∗ value = new Chi ld ( ) ;
Test ( value ) ;
Example will, perhaps surprizingly, execute parent function.
Introduction Summary References
STREAM OUTPUT IN C++
Stream output in C++ is a good example of the power ofoverloading. Every primitive type has a different stream outputfunction.
STREAM IOostream & operator << ( ostream & des t i na t i on , i n t source ) ;
ostream & operator << ( ostream & des t i na t i on , short source ) ;ostream & operator << ( ostream & des t i na t i on , long source ) ;ostream & operator << ( ostream & des t i na t i on , char source ) ;ostream & operator << ( ostream & des t i na t i on , char ∗ source ) ;/ / . . . and so on
double d = 3 .14 ;cout << " The answer i s " << d << ’ \ n ’ ;
Introduction Summary References
EASY TO EXTEND
• Since output uses overloading, it is very easy to extend tonew types.
EXTEND
class Frac t i on {public :
F rac t i on ( i n t top , i n t bottom ) { t = top ; b = bottom ; }
i n t numerator ( ) { return t ; }i n t denominator ( ) { return b ; }
private :i n t t , b ;
} ;
ostream & operator << ( ostream & des t i na t i on , F rac t i on & source ){
d e s t i n a t i o n << source . numerator ( ) << " / " << source . denominator ( ) ;return d e s t i n a t i o n ;
}
F rac t i on f (3 , 4 ) ;cout << " The value o f f i s " << f << ’ \ n ’ ;
Introduction Summary References
CONVERSION AND COERCIONS
• When one adds conversions into the mix, resolvingoverloaded function or method calls can get very complex.Many different types of conversions:
• Implicit value changing conversion (such as integer to real).• Implicit conversion that does not change value (pointer to
child class converted into pointer to parent).• Explicit conversions (casts).• Conversion operators (C++ and the like).• See text for illustration of the complex rules.
Introduction Summary References
REDEFINITIONS
• A redefinition occurs when a child class changes the typesignature of a method in the parent class.
• Two different types of rules are used to resolve name:• The merge model.• The scope of the child is merged with the scope of the
parent.• The hierarchical model.• Scopes are separate.• Search is made for first scope containing name, then for
best fit within the scope.
Introduction Summary References
AN EXAMPLE ILLUSTRATING REDEFINITION MODELS
• The following example will illustrate the difference in thesetwo models.
REDEFINITION MODELS
class Parent {public void example ( i n t a )
{ System . out . p r i n t l n ( " i n parent method " ) ; }}
class Chi ld extends Parent {public void example ( i n t a , i n t b )
{ System . out . p r i n t l n ( " i n c h i l d method " ) ; }}
Ch i ld aChi ld = new Chi ld ( ) ;aChi ld . example ( 3 ) ;
Will execute parent method in Java and C# (Merge model) andgive error in C++ (hierarchical model). Delphi allowsprogrammer control over this.
Introduction Summary References
OPTIONAL PARAMETERS
• Some languages allow the programmer to create optionalparameters, usually only at the end of the parameter list:
PARAMETERS
f u n c t i o n Count (A, B : I n tege r ; C : I n tege r = 0 ; D : I n tege r = 0 ) ;begin
(∗ Resul t i s a pseudo−v a r i a b l e used ∗)(∗ to represent r e s u l t o f any f u n c t i o n ∗)
Resul t := A + B + C + D;end
beginWr i t e l n ( Count (2 , 3 , 4 , 5 ) ) ; / / can use fou r argumentsWr i te l n ( Count (2 , 3 , 4 ) ) ; / / or th reeWr i te l n ( Count (2 , 3 ) ) ; / / or two
end
Such a program will have more than one type signature.
Introduction Summary References
ARBITRARY NUMBER OF ARGUMENTS IN C#
• The language C# has an interesting way to includearbitrary number of arguments:
ARGUMENTS
class ParamsExample {public void Wri te ( i n t x ) {
/ / use t h i s w i th one argumentWr i t eS t r i ng ( " Example one " ) ;Wr i t eS t r i ng ( x . ToStr ing ( ) ) ;
}
public void Wri te ( double x , i n t y ) {/ / use t h i s w i th two argumentsWr i t eS t r i ng ( " Example two " ) ;Wr i t eS t r i ng ( x . ToStr ing ( ) ) ;Wr i t eS t r i ng ( y . ToStr ing ( ) ) ;
}
public void Wri te ( params ob jec t [ ] args ) {/ / use t h i s w i th any other combinat ionWr i t eS t r i ng ( " Example three " ) ;for ( i n t i = 0 ; i < args . GetLength ( 0 ) ; i ++)
Wr i t eS t r i ng ( args [ i ] . ToStr ing ( ) ) ;}
}
ParamsExample p ;p . Wr i te ( 4 2 ) ;
Example one 42p . Wr i te (3 .14 ,159 ) ;
Example two 3.14159p . Wr i te ( 1 , 2 , 3 , 4 ) ;
Example three 1234p . Wr i te ( 3 . 1 4 ) ;
Example three 3.14p . Wr i te (3 , " abc " ) ;
Example three 3abc
Introduction Summary References
SUMMARY
• In this chapter we have looked at various aspects ofoverloading.
• Overloading based on Scopes.• Overloading based on Type Signatures.• Redefinition.• Polyadicity (functions with variable number of arguments).
Introduction Summary References
REFERENCES
• Images and content for developing these slides have beentaken from the follwoing book with the permission of theauthor.
• An Introduction to Object Oriented Programming, TimothyBudd.
• This presentation is developed using Beamer:• Singapore, spruce.
