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Dynamic Binding C++

Douglas C. Schmidt

Professor Department of EECS

d.schmidt@vanderbilt.edu Vanderbilt University

www.dre.vanderbilt.edu/



schmidt/ (615) 343-8197

Douglas C. Schmidt OO Programming with C++

Motivation



When designing a system it is often the case that developers:

1. Know what class interfaces they want, without precisely knowing

the most suitable representation

2. Know what algorithms they want, without knowing how particular

operations should be implemented



In both cases, it is often desirable to

defer

certain decisions as long

as possible

– Goal: reduce the effort required to change the implementation

once enough information is available to make an informed

decision



1997-2006 Vanderbilt University 1

Douglas C. Schmidt OO Programming with C++

Motivation (cont’d)



Therefore, it is useful to have some form of abstract “place-holder”

– Information hiding & data abstraction provide compile-time &

link-time place-holders



i.e.

, changes to representations require recompiling and/or

relinking...

– Dynamic binding provides a

dynamic

place-holder



i.e.

, defer certain decisions until run-time

without

disrupting

existing code structure



Note, dynamic binding is orthogonal to dynamic linking...



Dynamic binding is less powerful than pointers-to-functions, but

more comprehensible & less error-prone

–

i.e.

, since the compiler performs type checking at compile-time



1997-2006 Vanderbilt University 2

Douglas C. Schmidt OO Programming with C++

Motivation (cont’d)



Dynamic binding allows applications to be written by invoking

general

methods via a base class pointer,

e.g.

class Base { public: virtual int vf (void); };

Base *bp = /* pointer to a subclass */;

bp->vf ();



However, at

run-time

this invocation actually invokes more

specialized

methods implemented in a derived class,

e.g.

class Derived : public Base

{ public: virtual int vf (void); };

Derived d;

bp = &d;

bp->vf (); // invokes Derived::vf()



In C++, this requires that both the general and specialized methods

are virtual methods



1997-2006 Vanderbilt University 3

Douglas C. Schmidt OO Programming with C++

Motivation (cont’d)



Dynamic binding facilitates more ﬂexible and extensible software

architectures,

e.g.

– Not all design decisions need to be known during the initial stages

of system development



i.e.

, they may be postponed until run-time

– Complete source code is not required to extend the system



i.e.

, only headers & object code



This aids both

ﬂexibility

extensibility

– Flexibility = ‘easily recombine existing components into new

conﬁgurations’

– Extensibility = “easily add new components”



1997-2006 Vanderbilt University 4

Douglas C. Schmidt OO Programming with C++

Dynamic vs. Static Binding



Inheritance review

– A pointer to a derived class can always be used as a pointer to a

base class that was inherited

publicly



Caveats:



The inverse is not necessarily valid or safe



Private

base classes have different semantics...

–

e.g.

template <typename T>

class Checked_Vector : public Vector<T> { ... };

Checked_Vector<int> cv (20);

Vector<int> *vp = &cv;

int elem = (*vp)[0]; // calls operator[] (int)

– A question arises here as to which version of operator[] is called?



1997-2006 Vanderbilt University 5

Douglas C. Schmidt OO Programming with C++

Dynamic vs. Static Binding (cont’d)



The answer depends on the type of binding used...

Static Binding

: the compiler uses the type of the pointer to

perform the binding at compile time. Therefore,

Vector::operator[](vp, 0) will be called

Dynamic Binding

: the decision is made at run-time based upon

the type of the actual object. Checked

Vector::operator[]

will be called in this case as (*vp->vptr[1])(vp, 0)



Quick quiz: how must class Vector be changed to switch from static

to dynamic binding?



1997-2006 Vanderbilt University 6

Douglas C. Schmidt OO Programming with C++

Dynamic vs. Static Binding (cont’d)



When to chose use different bindings

–

Static Binding



Use when you are sure that any subsequent derived classes

will not want to override this operation dynamically (just

redeﬁne/hide)



Use mostly for reuse or to form “concrete data types”

–

Dynamic Binding



Use when the derived classes may be able to provide a different

(

e.g.

, more functional, more efﬁcient) implementation that

should be selected at run-time



Used to build dynamic type hierarchies & to form “abstract data

types”



1997-2006 Vanderbilt University 7

Douglas C. Schmidt OO Programming with C++

Dynamic vs. Static Binding (cont’d)



Efﬁciency

vs.

ﬂexibility

are the primary tradeoffs between static &

dynamic binding



Static binding is generally more efﬁcient since

1. It has less time & space overhead

2. It also enables method inlining



Dynamic binding is more ﬂexible since it enables developers to

extend the behavior of a system transparently

– However, dynamically bound objects are difﬁcult to store in

shared memory



1997-2006 Vanderbilt University 8

Douglas C. Schmidt OO Programming with C++

Dynamic Binding in C++



In C++, dynamic binding is signaled by explicitly adding the keyword

virtual in a method declaration,

e.g.

struct Base {

virtual int vf1 (void) { cout << "hello\n"; }

int f1 (void);

};

– Note, virtual methods

must

be class methods,

i.e.

, they cannot be:



Ordinary “stand-alone” functions



class data



Static methods



Other languages (

e.g.

, Eiffel) make dynamic binding the default...

– This is more ﬂexible, but may be less efﬁcient



1997-2006 Vanderbilt University 9

Douglas C. Schmidt OO Programming with C++

C++ Virtual Methods



Virtual methods have a ﬁxed

interface

, but derived

implementations

can change,

e.g.

struct Derived_1 : public Base

{ virtual int vf1 (void) { cout << "world\n"; } };



Supplying virtual keyword is optional when overriding vf1() in

derived classes,

e.g.

struct Derived_2 : public Derived_1 {

int vf1 (void) { cout << "hello world\n"; } // Still virtual

int f1 (void); // not virtual

};



You can declare a virtual method in any derived class,

e.g.

struct Derived_3 : public Derived_2 {

virtual int vf2 (int); // different from vf1!

virtual int vf1 (int); // Be careful!!!!

};



1997-2006 Vanderbilt University 10

Douglas C. Schmidt OO Programming with C++

C++ Virtual Methods (cont’d)



Virtual method dispatching uses object’s “dynamic type” to select

the appropriate method that is invoked at run-time

– The selected method will depend on the class of the

object

being

pointed at &

not

on the pointer type



e.g.

void foo (Base *bp) { bp->vf1 (); /* virtual */ }

Base b;

Base *bp = &b;

bp->vf1 (); // prints "hello"

Derived_1 d;

bp = &d;

bp->vf1 (); // prints "world"

foo (&b); // prints "hello"

foo (&d); // prints "world"



1997-2006 Vanderbilt University 11

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