This last week, I spent a couple days converting all of the uses of NULL in Sauce to nullptr. As such, I wanted to take this opportunity to jot down some notes on what the differences are and why I believe that it is worth your time to convert your own code to use nullptr if you haven’t already.

NULL

Similar to many other C++ code bases, Sauce used the following code to define NULL:

#if !defined(NULL)
   #define NULL 0
#endif

While it is true that many commercial software packages and games have shipped with this definition of NULL, it is still problematic.

The reason why is that NULL is simply an integer, not an actual null pointer. The danger stems from the fact that other constructs can be implicitly converted to and from integers. This means that using NULL can hide bugs.

In fact, I’ll be the first to admit that during the transition to nullptr, I found a few of these types of conversion errors in Sauce. Sure, the code still compiled and ran — but it was a bit disheartening to find them nonetheless.

Nullptr

Unlike NULL, nullptr is a keyword (available starting in C++11). It is defined to be implicitly converted to any pointer type, but cannot be implicitly converted to an integer. This allows the compiler to recognize the sort of type mismatches we hope that it would.

Let’s walk through an example to see the difference.

An Example

The problem we will explore in this example is the fact that booleans can be compared with NULL without a compiler warning. This is because the C++ standard declares that there is an implicit conversion from bool to int.

// signature:
Result* DoSomething();
 
// client code:
if (DoSomething() == NULL)
   printf("NULL\n");
else
   printf("NOT NULL\n");

Although this example is a bit contrived, the danger it exemplifies is real.

What happens if we change the return type of DoSomething() from Result* to bool? A change like this is certainly not unheard of — instead of using a full object, maybe we feel like we can reduce the result to a simple boolean.

// signature:
bool DoSomething();
 
// client code:
if (DoSomething() == NULL)
   printf("NULL\n");
else
   printf("NOT NULL\n");

The code still compiles — no additional warnings (even on Warning level 4!). That seems wrong… checking if a boolean is equal to null pointer is nonsensical and the compiler should bark when it comes across code like this, right?

Unfortunately, the compiler can’t detect the issue because we are using a #define as a stand-in for a null pointer. Remember, it’s not really a null pointer, it’s just the same value that a null pointer evaluates to: 0. Therefore, we shouldn’t be surprised when corner cases like this result in unexpected behavior.

So what happens if we replace NULL with the nullptr keyword?

// signature:
bool DoSomething();
 
// client code:
if (DoSomething() == nullptr)
   printf("NULL\n");
else
   printf("NOT NULL\n");

Now the compiler will generate an error stating that there is no conversion from 'nullptr' to 'int'. This is much better. We know that there is a type mismatch in the comparison, and we can repair the issue.

Final Thoughts

Simply put, the nullptr keyword is a true null pointer, while NULL is not.

When I was first deciding whether I was going to undertake the conversion task, I felt a bit overwhelmed at the number of changes that I would have to make (at the time there were over 5000 instances of NULL in Sauce). However, as I mentioned earlier, had I not made the transition to nullptr, those silent implicit conversion bugs would surely still be there. As such, I feel that Sauce is far better off with nullptr.

For the most part, I really like the C++ language. That said, I also have a small list of things that I wish had been done differently. For years, enumerations have been at the top of that list. Enums have a couple of distinct problems that make them troublesome, and while there are techniques to mitigate some of their issues, they still remain fundamentally flawed.

Thankfully, C++11 added scoped enums (or “strongly-typed” enums), which address these problems head-on. In my opinion, the best part about scoped enums is that the new syntax is intuitive and feels natural to the C++ language.

In an effort to build a case for why scoped enums are superior, we will first discuss the aforementioned deficiencies of their unscoped counterparts. Throughout this discussion we will also outline how we addressed some of these concerns in Sauce. Afterward, we will explore scoped enums and the task of transitioning Sauce to use them.

Terminology

Before we begin, let’s briefly establish some terminology. An unscoped enum has the following form:

enum IDENTIFIER
{
   ENUMERATOR,
   ENUMERATOR,
   ENUMERATOR,
};

The identifier is also referred to as the “type” of the enum. The list inside the enum is composed of enumerators. Each enumerator has an integral value.

Problem 1: Enumerators are treated as integers inside the parent scope.

Aliased Values

Consider the case where you have two enums inside the same parent scope. Unfortunately, there is no reinforcement by the compiler to say that a given enumerator is associated with one enum over the other. This can cause a couple issues. Here’s an example:

namespace Example1
{
   enum Shape
   {
      eSphere,
      eBox,
      eCone,
   };
 
   enum Material
   {
      eColor,
      eTexture,
   };
}

Now let’s see what happens when we try to use these enums in some client code:

const Example1::Shape shape = Example1::eSphere;
if (shape == Example1::eSphere)
   printf("SPHERE\n");
if (shape == Example1::eBox)
   printf("BOX\n");
if (shape == Example1::eCone)
   printf("CONE\n");
 
if (shape == Example1::eColor)
   printf("COLOR\n");
if (shape == Example1::eTexture)
   printf("TEXTURE\n");

The code above prints out both “SPHERE” and “COLOR”. This is because unscoped enum enumerators are implicitly converted to integers and the value of shape is 0, which matches both eSphere and eColor.

Sadly, the only workable solution is to manually assign a value to each of the enumerators that is unique within the parent scope. This is far from ideal due to the added maintenance cost.

Enumerator Name Clashes

Additionally, there is a second issue that arises from the fact that enums are swallowed into their parent scope: enumerator name clashes. For instance, consider modifying the previous case to add an “invalid” enumerator to each enum. While this makes sense conceptually, the following code will not compile:

namespace Example2A
{
   enum Shape
   {
      eInvalid,
      eSphere,
      eBox,
      eCone,
   };
 
   enum Material
   {
      eInvalid,
      eColor,
      eTexture,
   };
}

Although enumerator name clashes are not too common, it is generally bad practice to establish coding conventions that depend on the rarity of such situations.

Consequently, this usually forces you to mangle the enumerator names to include the enum type. Modifying the previous example might look something like this:

namespace Example2B
{
   enum Shape
   {
      eShape_Invalid,
      eShape_Sphere,
      eShape_Box,
      eShape_Cone,
   };
 
   enum Material
   {
      eMaterial_Invalid,
      eMaterial_Color,
      eMaterial_Texture,
   };
}

This version of the code will compile, but now the enumerator names look a little weird. Also, it is important to point out that we are now repeating ourselves: the enum identifier and each of the enumerators.

Another way to solve the name clash issue is to wrap the enum with an additional scoping object: namespace, class, or struct. Employing this method will allow us to keep our original enumerator names, which I like. However, it actually introduces a new problem: now we need two names… one for the scope and one for the enum itself.

Admittedly, there are a few different ways to handle this, but for the sake of the example let’s keep things simple:

namespace Example2C
{
   namespace Shape
   {
      enum Enum
      {
         eInvalid,
         eSphere,
         eBox,
         eCone,
      };
   };
 
   namespace Material
   {
      enum Enum
      {
         eInvalid,
         eColor,
         eTexture,
      };
   };
}

While the extra nesting does make the declaration a bit ugly, it solves the enumerator name clash problem. Furthermore, it also forces client code to prefix enumerators with their associated scoping object, which I personally consider a big win.

// in some Example2C function...
 
const Shape::Enum shape = GetShape();
if (shape == Shape::eInvalid)
   printf("Shape::Invalid\n");
if (shape == Shape::eSphere)
   printf("Shape::Sphere\n");
if (shape == Shape::eBox)
   printf("Shape::Box\n");
if (shape == Shape::eCone)
   printf("Shape::Cone\n");
 
const Material::Enum material = GetMaterial();
if (material == Material::eInvalid)
   printf("Material::Invalid\n");
if (material == Material::eColor)
   printf("Material::Color\n");
if (material == Material::eTexture)
   printf("Material::Texture\n");

In fact, before the transition to scoped enums, most of the enums in Sauce were scoped this way. Unfortunately, the availability of choices in situations like this breed inconsistency. Sauce was no exception: namespace, class, and struct were all being employed as scoping objects for enums in different parts of the code base (needless to say, I was pretty disappointed in this discovery).

Problem 2: Unscoped Enums cannot be forward declared.

This bothers me a lot. I’m very meticulous with my forward declarations and header includes, but unscoped enums have, at times, undermined my efforts. I also feel like it subverts the C++ mantra of not paying for what you don’t use.

For instance, if you want to use an enum as a function parameter, the full enum definition must be available, requiring a header include if you don’t already have it.

The following is a stripped-down example of the case in point:

Shape.h

namespace Shape
{
   enum Enum
   {
      eInvalid,
      eSphere,
      eBox,
      eCone,
   };
}

ShapeOps.h

#include "Shape.h"    // <-- BOO!
 
namespace ShapeOps
{
   const char* GetName(const Shape::Enum shape);
}

Unfortunately, there is no way around using a full include with unscoped enums. The situation is even more costly if the enum is inside a class header file that has its own set of includes.

Scoped Enums

Scoped enums were introduced in C++11. I am excited to report that not only do they solve all of the issues discussed above, but they also provide the client code with clean, intuitive syntax.

A scoped enum has the following form:

enum class IDENTIFIER
{
   ENUMERATOR,
   ENUMERATOR,
   ENUMERATOR,
};

That’s right — all you have to do is add the class keyword after enum and you have a scoped enum!

Converting the final example from the last section to use a scoped enum looks like the following:

Shape.h

enum class Shape
{
   eInvalid,
   eSphere,
   eBox,
   eCone,
};

ShapeOps.h

enum class Shape;   // forward declaration -- YAY
 
namespace ShapeOps
{
   const char* GetName(const Shape shape);
}

Here is an example of client code:

const Shape shape = GetShape();
if (shape == Shape::eInvalid)
   printf("Shape::Invalid\n");
if (shape == Shape::eSphere)
   printf("Shape::Sphere\n");
if (shape == Shape::eBox)
   printf("Shape::Box\n");
if (shape == Shape::eCone)
   printf("Shape::Cone\n");

This is exactly what we were looking for all along!

Another advantage of scoped enums is that they cannot be implicitly converted to integers. This solves the enumerator value aliasing we described earlier and is enforced by the compiler.

Transitioning to Scoped Enums

Sauce is a fairly large code base: ~200K lines of code at the time of this writing. It took me a few days to convert 100+ unscoped enums to scoped enums. Due to the fact that I was manually scoping all of the enums, this is not a simple “search and replace” task. Additionally, I spent the extra time replacing includes with forward declarations, when appropriate.

Overall, I strongly believe that the time investment is well worth the time spent. The scoped enum syntax is natural, and the fact that they can be forward declared opens an opportunity to drop your header include count in some places. If you are considering the task of transitioning your legacy code base to scoped enums, I highly recommend it!