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29 Cards in this Set
- Front
- Back
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-Associating many meanings to one function
-Virtual functions provide this capability -Fundamental principle of object-oriented programming! |
Polymorphism
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Can be "used" before it’s "defined"
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Virtual Function
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Want inherited function center() to use function Triangle::draw() NOT function Figure::draw()
But class Triangle wasn’t even WRITTEN when Figure::center() was! Doesn’t know "triangles"! |
Example use of virtual function
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implement late binding
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Virtual functions
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"Don’t know how function is implemented"
"Wait until used in program" "Then get implementation from object instance" |
Virtual functions
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Program must:
Compute daily gross sales Calculate largest/smallest sales of day Perhaps average sale for day All come from individual bills But many functions for computing bills will be added "later"! When different types of sales added! |
So function for "computing a bill" will be virtual!
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Impact: Later, derived classes of Sale can define THEIR versions of function bill
Other member functions of Sale will use version based on object of derived class! They won’t automatically use Sale’s version! |
Notice reserved word "virtual" in declaration of member function bill
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Virtual functions changed
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overridden
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Non-virtual functions changed
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redefined
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One major disadvantage of virtual functions
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overhead
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virtual void draw() = 0;
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pure virtual function
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require no definition
Forces all derived classes to define "their own" version |
Pure virtual functions
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Class with one or more pure virtual functions
Can only be used as base class No objects can ever be created from it Since it doesn’t have complete "definitions" of all it’s members! |
abstract base class
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Given: Derived is derived class of Base
Derived objects can be assigned to objects of type Base But NOT the other way! Consider previous example: A DiscountSale "is a" Sale, but reverse not true |
Extended Type Compatibility
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a nuisance of virtual functions that can be solved with pointers to dynamic variables
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slicing problem
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needed to de-allocate dynamically allocated data
Good policy for all _____ to be virtual |
destructors
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Base *pBase = new Derived; … delete pBase;
Would call base class destructor even though pointing to Derived class object! |
Making destructor virtual fixes this
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From descendant type to ancestor type
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Upcasting
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Casting from ancestor type to descended type
dangerous Assumes information is "added" |
Downcasting
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delays decision of which member function is called until runtime
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Late binding
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functions have no definition
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Pure virtual functions
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Classes with at least one are abstract
No objects can be created from abstract class Used strictly as base for others to derive |
Pure virtual functions
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_____ class objects can be assigned to _____ class objects
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Derived, base
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Allow very "general" definitions for functions and classes
Type names are "parameters" instead of actual types Precise definition determined at run-time |
templates
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template<class T>
Tells compiler what’s coming is "template" And that T is a type parameter |
template prefix
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C++ allows keyword _____ in place of keyword "class" in templates
But most use "class" anyway |
typename
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Typically we have them separate
For templates separate not supported on most compilers! |
Function declarations and definitions
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template<class T1, class T2>
Not typical Usually only need one "replaceable" type Cannot have "unused" template parameters Each must be "used" in definition Error otherwise! |
Multiple Type Parameters
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Refers to implementing templates
Express algorithms in "general" way: Algorithm applies to variables of any type Ignore incidental detail Concentrate on substantive parts of algorithm Function templates are one way C++ supports _____ |
Algorithm Abstraction
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