Chapter 3 Be Principled

Now it’s much easier to change the innards of AutomaticDoor. The client code does not depend on internal parts of the class anymore. You can remove the State enumeration and replace it with another kind of implementation without users of the class noticing this.

Strong Cohesion

A general piece of advice in software development is that any software entity (i.e., module, component, unit, class, function, etc.) should have a strong (or high) cohesion. In very general terms, cohesion is strong when the module does a well-defined job.

To dive deeper into this principle, let’s look at two examples where cohesion is weak, starting with Figure 3-1.

Figure 3-1.  MyModule has too many responsibilities, and this leads to many dependencies from and to other modules

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In this illustration of the modularization of an arbitrary system, three different aspects of the business domain are placed inside one single module. Aspects A, B, and C have nothing, or nearly nothing, in common, but all three are placed inside MyModule. Looking at the module’s code could reveal that the functions of A, B, and C are operating on different, and completely independent, pieces of data.

Now look at all the dashed arrows in that picture. Each of them is a dependency. The element at the tail of such an arrow requires the element at the head of the arrow for its implementation. In this case, any other module of the system that wants to use services offered by A, or B, or C will make itself dependent from the whole module MyModule. The major drawback of such a design is obvious: it will result in too many dependencies and the maintainability goes down the drain.

To increase cohesion, the aspects of A, B, and C should be separated from each other and moved into their own modules (Figure 3-2).

Figure 3-2.  High cohesion: The previously mixed aspects A, B, and C have been separated into discrete modules

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Now it is easy to see that each of these modules has far fewer dependencies than the old MyModule. It is clear that A, B, and C have nothing to do with each other directly. The only module that depends on all three modules A, B, and C is Module 1.

Another form of weak cohesion is called the shot gun anti-pattern. I think it is generally known that a shot gun is a firearm that shoots a huge amount of small spherical pellets. The weapon typically has a large scatter. In software development, this metaphor is used to express that a certain domain aspect, or single logical idea, is highly fragmented and distributed across many modules. Figure 3-3 depicts such a situation.

Figure 3-3.  Aspect A is scattered over five modules

Even with this form of weak cohesion, many unfavorable dependencies arise. The distributed fragments of Aspect A must work closely together. That means that every module that implements a subset of Aspect A must interact at least with one other module containing another subset of Aspect A. This leads to a large number of

dependencies crosswise through the design. At worst, it can lead to cyclic dependencies, like between Modules 1 and 3, or between Modules 6 and 7. This has, once again, a negative impact on the maintainability and extendibility. Furthermore, the testability is also very poor due to this design.

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This kind of design will lead to something that is called shotgun surgery. A certain type of change regarding Aspect A leads to making lots of small changes to many modules. That’s really bad and should be avoided. We have to fix this by pulling all the parts of the code that are fragments of the same logical aspect together into a single cohesive module.

There are certain other principles—for instance, the single responsibility principle (SRP) of object-oriented design (see Chapter 6)—that foster high cohesion. High cohesion often correlates with loose coupling and vice versa.

Loose Coupling

Consider the small example in Listing 3-8.

Listing 3-8.  A Switch That Powers a Lamp On and Off

class Lamp { public:

void on() { //...

}

void off() { //...

}

};

class Switch { private:

Lamp& lamp;

bool state {false};

public:

Switch(Lamp& lamp) : lamp(lamp) { }

void toggle() { if (state) {

state = false; lamp.off();

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}else {

state = true; lamp.on();

}

}

};

Basically, this piece of code will work. You can first create an instance of the Lamp class. Then this is passed by reference when instantiating the Switch class. Visualized with UML, this small example would look like Figure 3-4.

Figure 3-4.  A class diagram of Switch and Lamp

What’s the problem with this design?

The problem is that the Switch contains a direct reference to the concrete class Lamp. In other word, the switch knows that there is a lamp.

Maybe you would argue, “Well, but that’s the purpose of the switch. It has to power on and off lamps.” That’s true if that is the one and only thing the switch should do.

If that’s the case, this design might be adequate. But go to a DIY store and look at the switches that you can buy there. Do they know that lamps exist?

And what do you think about the testability of this design? Can the switch be tested independently as it is required for unit testing? No, this is not possible. And what will we do when the switch has to power on not only a lamp, but also a fan or an electric roller blind?

In this example, the switch and the lamp are tightly coupled.

In software development, a loose coupling (also known as low or weak coupling) between modules is best. That means that you should build a system in which each of its modules has, or makes use of, little or no knowledge of the definitions of other separate modules.

The key to achieve loose coupling in object-oriented software designs is to use interfaces. An interface declares publicly accessible behavioral features of a class without

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committing to a particular implementation of that class. An interface is like a contract. Classes that implement an interface are committed to fulfill the contract, that is, these classes must provide implementations for the method signatures of the interface.

In C++, interfaces are implemented using abstract classes, as shown in Listing 3-9.

Listing 3-9.  The Switchable Interface

class Switchable { public:

virtual void on() = 0; virtual void off() = 0;

};

The Switch class doesn’t contain a reference to the lamp any more. Instead, it holds a reference to our new interface class called Switchable, as shown in Listing 3-10.

Listing 3-10.  The Modified Switch Class, Whereby Lamp Is Gone

class Switch { private:

Switchable& switchable; bool state {false};

public:

Switch(Switchable& switchable) : switchable(switchable) {}

void toggle() { if (state) {

state = false; switchable.off();

} else {

state = true; switchable.on();

}

}

};

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The Lamp class implements our new interface, as shown in Listing 3-11.

Listing 3-11.  The Lamp Class Implements the Switchable Interface

class Lamp : public Switchable { public:

void on() override { // ...

}

void off() override { // ...

}

};

Expressed in UML, the new design looks like Figure 3-5.

Figure 3-5.  Loosely coupled Switch and Lamp via an interface

The advantages of such a design are obvious. Switch is completely independent from concrete classes that will be controlled by it. Furthermore, Switch can be tested independently by providing a test double implementing the Switchable interface. You want to control a fan instead of a lamp? No problem, as this design is open for extension. Just create a Fan class or other classes representing electrical devices that implement the Switchable interface, as depicted in Figure 3-6.

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