Polymorphism means having many forms
Polymorphism occurs when there is a hierarchy of classes and they are related by inheritance
C++ polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function
Polymorphism gives the ability to assign a different meaning, or purpose, to an entity according to its context
Polymorphic variables
Exchangeable objects
Virtual functions
A socket and plug-compatible devices
A function is split into two entities;
Binding - refers to the process to be used for converting functions & variables into machine language addresses
Binding for functions, is associating the function call with the right function definition by the compiler
The binding is done either at compile time or at runtime
In case of one single function definition, there is no confusion. Whenever a function is called, the associated definition is executed
However, in inheritance its likely to have two functions with the same signature (overriding functions)
This means that the function has only one form of call but may have more than one definition
There are 2 cases of binding
Happens at compile time (operations performed by a compiler)
Function definition & function call are linked during compile time
All information needed to call a function is available at the compile time
Its achieved using the normal function calls, function overloading, & operator overloading
Faster execution of a program
Happens at the run time or execution time (program lifecycle phase)
Function calls are not resolved until runtime
All information needed for a function call cannot de determined at compile time
Its achieved using the virtual functions
Slower execution of a program
Flexible - since a single function can handle different type of objects at runtime
In C++ we have two types of polymorphism
Function overloading & Operator overloading are perfect examples
Overloading is a polymorphism where several functions have the same name but with different number of parameters or the type of the parameters
Based on number of parameters passed during function call, compile determines which function to call
The function call is determined during compile time, thus called compile time polymorphism
Overriding is a polymorphism where several functions have the same name, number of parameters & the type of the parameters
Function overriding - When child class declares a method, which is already present in the parent class; - child class overrides the parent class
The function call is determined at runtime, thus called run time polymorphism
Bitbucket: Run Time Polymorphism
| Compile Time Polymorphism | Run Time Polymorphism |
|---|---|
| The function to be invoked is known at the compile time | The function to be invoked is known at the run time |
| Also known as overloading, static & early binding | Also known as overriding, dynamic & late binding |
| Achieved by function & operator overloading | Achieved by virtual functions & pointers |
| Provides fast execution as it is known at the compile time | Provides slow execution as it is known at the run time |
| Less flexible as mainly all the things execute at the compile time | More flexible as all the things execute at the run time |
The classes we have designed so far are called concrete classes
Classes that can be used to instantiate objects
An abstract class is a class with at least one pure virtual function
Abstract class defines an interface
An abstract class cannot be instantiated because there is no definition for the pure virtual member functions
A pure virtual function is specified by placing = 0 in its declaration
The purpose of an abstract class is to provide an appropriate base class from which other classes can inherit
e.g. virtual double getArea (0) = 0;
An abstract class can have both virtual and pure virtual functions
An interface is a special case of an abstract class in which all member functions are pure virtual functions
It is a type of polymorphism in which an operator is overloaded to give user-defined meaning to it
Overloaded operator is used to perform operation on user-defined data type
For example '+' operator can be overloaded to perform addition on various data types, like for Integer, String(concatenation) etc
Objects of user-defined types can play three different roles in a function
Understanding objects in each role helps the process of writing overloading operators
It is the one that is pointed to by the pointer
It must be constant if the function is not supposed to change it; it must be non-constant otherwise
It must be passed to the member function in three ways:
Functions may return an object in three ways:
Must use return-by-value if the returned object is created in the body of the function
Must use return-by-reference or by pointer if the returned object is passed as the parameter to the function
Operator overloading is the definition of two or more operations using the same operator
In the following example, the compiler changes the first operand to 14.0 to do the operation
Categories of Operators
Precedence - No change to the precedence of the operator
Associativity - The operator keeps its associativity
Commutativity - No change to commutativity of the operator
Arity - Unary & binary operators are kept
No new operators - No invention of operators
No combination - No new operators by combining 2 or more operators
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*References*
- C++ Programming - An Object-Oriented Approach, 2019
Behrouz, Richard et.al- Accelerated C++ - Practical Programming by Example, 2000
Andrew, Barbara et.al
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