Download - L11: Coupling, Cohesion, Visibility - sfdv4001.wdfiles.comsfdv4001.wdfiles.com/local--files/lectures/L11... · L11: Coupling, Cohesion, Visibility • Dependency • Coupling Internal

L11: Coupling, Cohesion, Visibility• Dependency

• Coupling

Internal data coupling

Global data coupling

Control (or sequence) coupling

Component coupling

Parameter coupling

Subclass coupling

• Cohesion

Coincidental cohesion

Logical cohesion

Temporal cohesion

Communication cohesion

Sequential cohesion

Functional cohesion

Data cohesion

• Visibility: visibility & access and friends

• Chapter 23 of Budd

SFDV4001 / L11 - Coupling, Cohesion & Visibility / 1

Dependency

• If an object cannot meaningfully exist without another object, it is said to be dependent on the second object.

• Eg a child class is almost always dependent on its parent.


Coupling

• Internal data coupling• Global data coupling• Control (or sequence) coupling• Component coupling• Parameter coupling• Subclass coupling


Internal Data Coupling

class Sneaky {public:

void sneak(){ luke->father=“darth”;};

private:Luke *luke;

};

class Luke {public:

Luke() { father = “anakin”;};string father;

};


Global Data Coupling

• Two or more classes are bound together by their reliance on common global data structures

double todaysDow;class One {public: void setDow(){

todaysDow=10534; }};

class Two {public: void printDow(){ cout << todaysDow; }};


Global Data Coupling

• File scope• Namespace scope• Program scope


Control Coupling

• When one class must perform operations in a fixed order, but the order is controlled elsewhere

class MyClass {public:

void doFirst(){...};void doSecond(){...};void doThird(){...};

}


Component Coupling

• One class maintains a data field or value that is an instance of another class.

• Ideally this relationship should be one way.

class Set {...

private:List data;

};


Parameter Coupling

• When one class must invoke routines from another via parameters.

class MyClass {public: void doSomething(Set aSet){

... }};


Subclass Coupling

• Describes the relationship between a class and its parent.

class Parent {...};

class Child: public Parent {...};


Rules to Reduce Coupling

• Always access data members through accessor methods. Eg getFather, setFather.

• Advantages?


Rules to Reduce Coupling

• Within a class method only access:– arguments– instance variables– local variables

• Try to reduce the number of classes each class knows about.

• Use namespaces for global data.


Cohesion

• Coincidental cohesion• Logical cohesion• Temporal cohesion• Communication cohesion• Sequential cohesion• Functional cohesion• Data cohesion


Coincidental Cohesion

• Elements of a class are grouped for no apparent reason.

• A class consists of methods that are not related.

• Usually a sign of poor design.


Logical Cohesion

• Occurs when there is logical connection among the elements of the class but no actual connection in either data or control.

• Eg a library of mathematical functions


Temporal Cohesion

• Elements are bound together because they all must be used at approximately the same time.

• Eg a class that performs program initialisation.


Communications cohesion

• Methods are grouped together because they all access the same input/output data or device.

• The class acts as a device manager.• Eg a Proxy class


Sequential Cohesion

• Elements are linked by the necessity to be activated in a particular order.

• Often used to attempt to avoid sequential coupling.


Function Cohesion

• The elements of a class all relate to the performance of a single function.

• Very desirable form of cohesion


Data Cohesion

• A class defines a set of data values and exports routines that manipulate the data structure.

• The embodiment of an ADT• Eg vector, list etc


Visibility

• An object is visible in a certain context if its name is legal and denotes the object.

• I.e., the object is in scope.


Access and Visibility

class Sneaky {private:

int safe;public:

Sneaky(){safe=10;};int &sorry(){return safe;};print(){cout << safe << endl;}

};

Sneaky x;x.sorry() = 1;x.print();int y = x.sorry(); y = 2;x.print();int &z = x.sorry();z = 3;x.print();


Friends

• C++ has a notion of friend functions and classes.

• A friend function is a function that can access a classes private parts.

• Can be useful but easily abused!


Example

class Matrix {float data[4][4];

public:friend Vector operator*(const Matrix&, const Vector&);

};class Vector {

float data[4];public:

friend Vector operator*(const Matrix&, const Vector&);

}

Vector operator*(const Matrix& m, const Vector& v);


Download - L11: Coupling, Cohesion, Visibility - sfdv4001.wdfiles.comsfdv4001.wdfiles.com/local--files/lectures/L11... · L11: Coupling, Cohesion, Visibility • Dependency • Coupling Internal

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