Boolean parameters

When I first started developing software, one thing that annoyed me immediately was when people used boolean parameters where their meaning wasn’t immediately obvious.

This is ok:

void setUserIsDeadFlag(bool)

This is not:

User findUserByName(string, bool)

In the documentation, the parameters will have names and possibly further info pertaining to their use, but when I come to write code, I’m forced to add a comment explaining what the parameter is, so that I can read my own code later:

User bob = findUserByName("bob", false /* Don’t include dead users */);

I can’t even trim my comment down, because the parameter name is something like include_dead_users. If I simply used the name in my comment, I’d make it look like I was passing false because I wanted to include dead users, which would be confusing:

User bob = findUserByName("bob", false /* include_dead_users */);

The best solution to this problem (in all the languages I know) is to use enumerations, or at least constants. Here’s C++:

enum DeadUserInclusionPolicy { Include, Exclude };

Before we start rushing off though, what’s wrong with the above? Let’s have a look at a method signature and a call:

void findUserByName(string name, DeadUserInclusionPolicy deadUserInclusionPolicy)
User bob = findUserByName("bob", Exclude);

The enumeration is still no help, because ‘Exclude’ means nothing to the reader of our code. Let’s try again:

namespace DeadUsers { enum InclusionPolicy { Include, Exclude } };

void findUserByName(string name, DeadUsers::InclusionPolicy policy)
User bob = findUserByName("bob", DeadUsers::Exclude);

Finally we have readable ‘user’ code. In VB.NET and C# you don’t need to bother with a namespace and creative naming, because when you pass a value from an enumeration, you have to prefix it with the name of the enumeration, e.g. DeadUsersInclusionPolicy.Exclude

A large proportion of code that I come across has the boolean parameter problem. When I’m using C++ or C#, I add a comment to remind myself what I’m passing - and try to remember to update the comment if I change the value I pass. When I’m using VB.NET, I get to use a feature of the language to make things explicit: named parameters.

IO.Directory.Delete("c:\", Recursive:=True)

That’s better! Not only does this help where I have to call a method with boolean parameters, it also allows me to call methods with multiple parameters with less worry that I’ll get them mixed up:

Dim Bob As New User(Name:="Bob", Surname:="Holness", Whereabouts:="Unknown")