Finished Reading Clean Code

Today I finished reading Clean Code by Robert Martin aka Uncle Bob. I found out about the book from some coworkers during our weekly programming-discussion meetings. The book claims to leave its readers understanding:

How to tell the difference between good and bad code

How to write good code and how to transform bad code into good code

How to create good names, good functions, good objects, and good classes

How to format code for maximum readability

How to implement complete error handling without obscuring code logic

How to unit test and practice test-driven development

Part 1: Principles, Practices, and Patterns

I really liked part one of the book. The principles, practices, and patterns are well thought out and provide a style of programming that is, unfortunately, rarely seen. Since reading this part, I’ve been writing smaller functions (about 4-5 lines at max) and have seen a payoff in the amount of reusability and readability. Chapters 1-7 read quickly, are informative, and are really great pieces to read.

Part 2: Case Studies

The pace of the book really drops off after chapter 7, and each subsequent chapter I lost more of that urge to pick up the book and continue reading. The final nail in the coffin that actually put me to sleep once was the Case Studies part (chapters 14-16). The example of the Args module was too large and my internal-brain compiler just couldn’t handle JITting three pages of code at once. I pushed through and read this whole part, but I wouldn’t recommend you doing the same.

Part 3: Code Smells

Part three is one chapter long and focuses on Code Smells. Code Smells are patterns in code that when seen give a bad taste in your mouth. They are implementations that shout that they could have been done better. The longer the code smells, the worse it gets, until finally the abomination that once started out as a faint odor now is the muskiness in the room that scares off developers from maintaining the module. This chapter is important to get familiar with. It might not be too useful to memorize the different smells, but knowing which ones exist is good enough, because you can always come back to this chapter to see how the code can be refactored.

Unit Testing

The last item I wanted to cover was unit testing. While the book mentions unit testing a lot, and devotes a chapter towards it, I don’t feel like developers will walk away from this book with an understanding of unit tests and test-driven development. Any developer looking for further understanding on unit tests should pick up a book that is devoted to the practice, since there is just far too much to cover in one chapter. I would recommend reading Test Driven Development: By Example.

Non-member non-friend functions vs private functions

Herb Sutter has said that the most object oriented way to write methods in C++ is using non-member non-friend functions. Should that mean that I should take private methods and turn them into non-member non-friend functions? Any member variables that these methods may need can be passed in as parameters.

Example (before):

class Number {
 public:
  Number( int nNumber ) : m_nNumber( nNumber ) {}
  int CalculateDifference( int nNumber ) { return minus( nNumber ); }
 private:
  int minus( int nNumber ) { return m_nNumber - nNumber; }
  int m_nNumber;
};

Example (after):

int minus( int nLhsNumber, int nRhsNumber ) { return nLhsNumber - nRhsNumber; }
class Number {
 public:
  Number( int nNumber ) : m_nNumber( nNumber ) {}
  int CalculateDifference( int nNumber ) { return minus( nNumber ); }
 private:
  int m_nNumber;
};

Am I on the right track? Should all private methods be moved to non-member non-friend functions? What should be rules that would tell you otherwise?

I’ve asked the question on Stack Overflow. Feel free to leave a response there so I can better track responses and award credit.

Moving Away from Exceptional Code

I once took a training course taught by Ron Jeffries and Chet Hendrickson on Test Driven Development. One of the participants in the course asked both Ron and Chet what are their recommendations on testing exceptional code paths. At first I didn’t think much of the question, since all the unit testing frameworks I’ve used have built-in ways to test exceptional code paths.

• VSTS Unit Testing (.NET) has an attribute that you can put on a test to say that it is expecting an exception:

  [TestMethod, ExpectedException( typeof ( ApplicationException ) ) ]
  public void CreationOfFooWithNullParameterShouldThrowApplicationException()
  {
     var foo = new Foo( null ); //should throw an ApplicationException
  }

• Google Test (C++) places a macro around the method call that is expecting an exception to be thrown:

   ASSERT_THROW(Foo(5), bar_exception);

• unittest (Python) has a special assert to test if an exception was raised, similar to Google Test:

  def testsample(self):
     self.assertRaises(ValueError, random.sample, self.seq, 20)

The answer that Ron gave wasn’t expected, but led me to think about the way that we use exceptions within our codebase.

From what I remember, Ron said that he simply doesn’t test exceptional cases. How? He doesn’t have exceptional cases. He recommended using the Null Object pattern to get away from most of the exceptional cases.

Developers often have to pollute their code with checks to see if a value is null, like so:

XmlNode* userNode = xmlParser.GetNode( "user" );
return ( userNode == NULL ) ? "" : userNode->Text();

In the case above, the call to GetNode would search the XML document for a node with a tagName of “user”. If it finds it, it will return a pointer to the data. If not, then it will return NULL. This makes a lot of sense to developers, but it ruins some of the value of abstraction and good object-oriented programming.

First, any client who makes this call to GetNode has to know how GetNode works (they have to know that GetNode will return NULL if it can’t find the node). Second, there is a type check going on here, basically a runtime check to see if the type returned from GetNode is an actual XmlNode or if it is not.

One way to refactor away from this is to use the Null Object pattern. Martin Fowler talks about the Null Object pattern in his book, Refactoring: Improving the Design of Existing Code. In the section entitled, Introduce Null Object, there is a story told by Ron Jeffries describing how he has used Null Object to treat missing objects as actual objects.

The story goes over how if a record didn’t exist in the database, they would return an object like MissingPersonRecord, which would be derived from PersonRecord. All calls to methods on a MissingPersonRecord would simply return some default behavior and not complain. For example, if the MissingPersonRecord is asked for their name, they would respond with a blank string.

This allows any caller of the methods to not have to worry about missing people in the database. See how the code has changed:

XmlNode* userNode = xmlParser.GetNode( "user" );
return userNode->Text();

There is now no change in behavior if the node doesn’t exist, and there is also no knowledge about how GetNode works (besides the fact that the caller knows that it will never return NULL 😉 ).

One library that I have used that follows this pattern is the TinyXml library in C++. When a caller asks for an XML node, there is no checking to see if that node really exists before making the call. This allows you to write code like so:

TiXmlNode* userNode = xmlParser.GetNode( "user" );
return userNode->FirstChild()->FirstChild()->Text();

A disadvantage to this is that when a Null Object has to be used, the error in the code can take a while to find. You may not know that there is a problem until a user object is displayed on screen without a required field like a “name”.

To solve this, you would have to debug and find the Null Object to determine why it is not a real object. Another approach could be to have the Null Object log a message in the Event Log when specific methods are called on it, but this can create too much noise, especially if you are using the Null Object pattern in the way that TinyXml does.

So, that is a little introduction on the Null Object pattern. To summarize, you can use the Null Object pattern to improve abstraction and polymorphism, and to reduce the lines of code per method.