Clean Code - Part 2

In Part 1 I talked about writing less code and writing simple code. In this post I will cover writing stateless code and writing readable code.

Write Stateless Code

pure function is a function where the return value is only determined by its input values, without observable side effects. This is how functions in math work: Math.cos(x) will, for the same value of x, always return the same result.

Source: SitePoint

Things break when a program gets into a “bad state”. There are a couple of ways to make this less likely to happen: making data immutable and writing functions that don’t have side-effects (pure functions).

Immutable data

If something shouldn’t change, mark it as immutable and let the compiler enforce that. A good rule of thumb is to mark things as const (const&, const* etc.) and/or readonly by default, and make them mutable only when truly needed [1].

A simple example:

struct StringUtil
    static std::string Concat1(std::string& str1, std::string& str2)
        return str1 + str2;

    static std::string Concat2(std::string& str1, std::string& str2)
        return str1.append(str2);

Both StringUtil::Concat1 and StringUtil::Concat2 return the same thing for the same input, the difference being that Concat2, as opposed to Concat1, modifies its first argument. In a bigger function, such a change might be introduced accidentally and have unexpected consequences down the line.

A simple way to address this is by explicitly marking the arguments as const:

struct StringUtil
    static std::string Concat1(const std::string& str1, std::string& str2)
         return str1 + str2;

    static std::string Concat2(const std::string& str1, std::string& str2)
         return str1.append(str2); // Won't compile - can't call append on str1

In this case, Concat2 won’t compile, so we can rely on the compiler to eliminate this type of unintended behavior.

Another example: a simple UpCase function which calls toupper on each character of the given string, upcasing it in place:

void UpCase(char *string)
    while (*string)
        *string = toupper(*string);

Calling it with

char* myString = "Foo";

will lead to a crash at runtime - the function will try to call toupper on the characters of myString. The problem is that myString is a char* to a string literal which gets compiled into the read-only data segment of the binary. This cannot be modified.

To catch this type of errors at compile-time, we again only need to mark the immutable data as such:

const char* myString = "Foo";
UpCase(myString); // Won't compile - can't call UpCase on myString

In contrast with the previous example, the argument to UpCase is mutable by design (the API is modifying the string in-place), but marking myString as const tells the complier this is non-mutable data, so it can’t be used with this API.

Pure functions

Another way to reduce states is to use pure functions. Unfortunately there isn’t a lot of syntax-level support for this in C++ and C# (C++ supports const member functions, which guarantee at compile time that calling the member function on an instance of the type won’t change the attributes of that instance) [2]

This goes back to the recommendation from Part 1 of using generic algorithms and predicates rather than implementing raw loops. In many cases, traversal state is encapsulated in the library algorithm or in an iterator, and predicates ideally don’t have side-effects.

var squares = numbers.
                Where(number => number % 2 != 0).
                Select(number => number * number);

Above code (also from Part 1) doesn’t hold any state: traversal is handled by the Linq methods, the predicates are pure.

In general, try to encapsulate state in parts of the code built to manage state, and keep the rest stateless. Note that immutable data and pure functions are also an advantage in concurrent applications, since they can’t generate race conditions.

Key takeaways:

  • Prefer pure functions to stateful functions and, if state is needed, keep it contained
  • By default mark everything as const (or readonly), and only remove the constraint when mutability is explicitly needed

Write Readable Code

In computer science, the expressive power (also called expressiveness or expressivity) of a language is the breadth of ideas that can be represented and communicated in that language […]

  • regardless of ease (theoretical expressivity)
  • concisely and readily (practical expressivity)

Source: Wikipedia

Code is read many more times than it is written/modified, so it should be optimized for readability. What I mean by this is making the intent of the code clear at a glance - this includes giving good descriptive names to variables, functions, and types, adding useful comments where appropriate (a comment should describe what the code does if it is non-obvious; a comment like foo(); // calls foo() is not a useful comment), and in general structure the code for easy reading.

For a counterexample, think back on a piece of code you read that elicited a WTF. That’s the kind of code you don’t want to write.

I won’t insist much here, since there are countless books and industry best practices for improving code readability.

Another way to make the code more readable is to have a good knowledge of the language you are using. The strength of a language lies in its particularities, so use them whenever appropriate. This means writing idiomatic code, which implies knowledge of the language idioms. Don’t write C++ code like C code, write it like C++ code. Don’t write C# code as C++, write it as C# etc.

Also, keep up to date on the language. Language syntax evolves to address needs, so in general modern syntax introduces simpler, better ways to implement things than old syntax. Take object allocation and initialization in C++ as an example:

Foo* foo = (Foo*)malloc(sizeof(Foo));

This is the C way of allocating and initializing a structure on the heap, then deinitializing and freeing it. Allocation and initialization are separate steps, with opportunity to leak both memory (by omitting the free call) and managed resources (by omitting the deinit call). Not to mention opportunity to end up with an initialized struct (by omitting the init call), or accidental double-initialization, double-deinitialization, double-free etc.

C++ introduced classes, and the following syntax:

Foo* foo = new Foo();
delete foo;

new both allocates memory and calls the constructor, while delete calls the destructor then releases the memory. Many of the problems in the C example go away, but there is still the problem of leaking the resource by omitting the delete call, and the issue of calling delete twice on the same memory address.

To address these issues, smart pointers were introduced in the language:

std::shared_ptr<Foo> foo(new Foo());

Smart pointers encapsulate reference counting (how many shared_ptr objects point to the same memory address), and automatically release the resource when the last reference goes away. This gets rid of most problems, but there is an even better way of allocating heap objects:

auto foo = std::make_shared<Foo>();

make_shared has the advantage of improved performance, by allocating memory in a single operation for both the object and the shared pointer’s own control block [3]. It also prevents leaks due to interleaving [4]. So as the C++ language evolved, new constructs appeared to address potential problems. Keeping up to date with these updates, and incorporating them into your code will reduce the opportunity for bugs, make the code more concise, and thus more readable.

Beautiful Code

I encourage you to not stop at writing working code, rather strive to write beautiful code. I have the following quote from Apprenticeship Patterns on the wall behind my monitors, so I can see it while I work:

There’s always a better/faster/smarter way to do what you’re currently doing

So don’t stop as soon as something works, ask yourself is this the best way to implement this?

Key takeaways:

  • Come up with good names
  • Write meaningful comments
  • Keep up to date with your language
  • Don’t just write working code, write beautiful code.


As I was working on putting together the talk that inspired this post, I realized there are a few more rules of thumb which I could cover. The current working draft is:

  • Write safe code
  • Write leakproof code
  • Write responsive code
  • Write testable code

Sometime in the future I hope to continue the series with the above, in the meantime, I’ll leave you with this one sentence summary:

Always code as if the person who ends up maintaining your code is a violent psychopath who knows where you live

Source: Coding Horror

[1]At the time of this writing, there is an active proposals to extend the C# language with an immutable keyword.
[2]C# has a PureAttribue in the System.Diagnostics.Contracts namespace (purity not compiler-enforced) and there is an active proposal to add a keyword for it too.
[3]This is a non-binding requirement in the standard, meaning a standard library implementation doesn’t have to do this, but most implementations will. You can read more about it here.
[4]Interleaving occurs since call order is not guaranteed. For example, in bar(std::share_ptr<Foo>(new foo()), baz()), there is no guarantee that call order will be new foo(), then the shared pointer’s constructor, then baz(). Calls might get interleaved and executed as new foo(), then baz(), then the shared pointer constructor, in which case an exception thrown by baz() would leak the newly allocated Foo object, since the shared pointer didn’t get ownership of it yet.