Inheritance in Matlab Classes

E177 February 14, 2008http://jagger.me.berkeley.edu/~pack/e177

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Classes

Recall, classes give data an identity, and associate functions to the identity.

A class can also inherit some parts of its identity and associated functions from another class.

This is called inheritance, and allows for hierarchical layout of classes, and promotes reuse of methods.

referred to as the child class

referred to as the parent class

@vehicle class (front wheel steering)

Simple constructor vehicle classfunction M = vehicle(xyloc,v,theta,omega,wh)

M.Location = xyloc; % 2-by-1

M.Velocity = v; % 2-by-1

M.Orientation = theta; % 1-by-1

M.AngularRate = omega; % 1-by-1

M.RectDim = wh; % 2-by-1

M = class(M,’vehicle’);

An ismoving methodfunction B = ismoving(A)

B = norm(A.Velocity)>0;

An isrearwheelskidding method…some relation between Orientation and Velocity

@car class

Simple constructor car class, inheriting vehiclefunction M = car(BP,SA,DT,R,xy,v,th,om,wh)

Veh = vehicle(xy,v,th,om,wh);

M.BreakPressure = BP;

M.SteeringAngle = SA;

M.DriveTorque = DT;

M.WheelRadius = R;

M = class(M,’car’,Veh);

A = car(2.4,…);

class(A)

isa(A,’car’), isa(A,’vehicle’), ismoving(A)

Make M an object of class car Inherit data from Veh, and all methods from class(Veh)

SomeCarMethod.m

A method for objects of class car can reference and modify the inherited vehicle object

function M = SomeCarMethod(A)

% This function can access fields…

A.BreakPressure % class=double

A.SteeringAngle % class=double

A.DriveTorque % class=double

A.WheelRadius % class=double

A.vehicle % class=vehicle

Example: FrontWheelSlipAngle, relation betweenSteeringAngle, Orientation, Velocity

Scope for inherited classes

At this point, we have

ConsiderV1 = vehicle([2;3],[10;3],1.2,0.1,[1;4]);

C1 = car(0,0.2,100,190,[1;2],[5;6],1,0,[2;4]);

ismoving(V1)

ismoving(C1)

SomeCarMethod(V1)

SomeCarMethod(C1)

car.m

SomeCarMethod.m

@car

vehicle.m

ismoving.m

isrealwheelskidding.m

@vehicle

Inside ismoving, only the fields of vehicle class can be accessed directly, even though input arg is a car object.

@bike class

Simple constructor bike class, inheriting vehicle

function M = bike(FCR,RCR,F,R,SA,RA,…

xy,v,theta,wh)

Veh = vehicle(xy,v,theta,wh);

M.FrontChainRing = FCR;

M.RearChainRing = RCR;

M.FrontGear = F;

M.RearGear = R;

M.SteeringAngle = SA;

M.RollAngle = RA;

M = class(M,’bike’,Veh);

@wheelchair class

Simple constructor wheelchair class, inheriting vehicle

function M = wheelchair(RW,Lw,Rw,…

xy,v,theta,wh)

Veh = vehicle(xy,v,theta,wh);

M.RearWheelRadius = RWR;

M.LeftWheelRate = Lw;

M.RightWheelRate = Rw;

M = class(M,’wheelchair’,Veh);

Front wheels are very small and light, so perhaps…function A = FrontWheelSlipAngle(W)

A = 0;

Folder layout

Each class has a constructor and methods.

car.m

FrontWheelSlipAngle.m

...

@car

vehicle.m

ismoving.m

isrealwheelskidding

@vehicle

wheelchair.m

FrontWheelSlipAngle.m

...

@wheelchair

bike.m

incrementFrontChainRing.m

decrementFrontChainRing.m

@bike

@ReferAssignGetSetMany ideas of

get, set, subsref, subsasgn

will to carry over to all classes we write. For example,–Reference of M.Value is the same as get(M,’Value’)–Multi-depth subsref has specific recursive meaning–Assignment of M.Value=B is the same as set(M,’Value’,B)–Multi-depth subsasgn has specific recursive meaning–In some cases, () reference and assignment should be handled by the

built-in methods for Matlab arrays–set should work with or without output arguments–Special Matlab-like conventions for get and set should be followed

• Calls with 1-input argument

• Property names can be partially specified

• Multiple args in set(A,P1,V1,P2,V2)

This is a good motivation to write a class that handles all of this functionality, and all other classes will simply inherit this.

@ReferAssignGetSetQuestions/Options for a given class

– Is property reference allowed, as in Object.Property– Is property assignment allowed, as in Object.Property = Value– Is () reference allowed, as in Object(Idx1,Idx2)

• if so, are builtin methods used, or does the class programmer supply the reference method?

– Is () assignment allowed, as in Object(Idx1,Idx2) = Value• if so, are builtin methods used, or does the class programmer supply the

assignment method?

– Is {} reference allowed, as in Object{Idx1,Idx2}• if so, are builtin methods used, or does the class programmer supply the reference

method?

– Is {} assignment allowed, as in Object{Idx1,Idx2} = Value• if so, are builtin methods used, or does the class programmer supply the

assignment method?

– Does set use assignin when called with no output arguments?

set(A,Prop,Value) %uses inputname, overwrites A A = set(A,Prop,Value) % overwrites A, directly B = set(A,Prop,Value) % A unchanged