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Enum

An enum defines a type that acts as a wrapper around an integer type (defaults to uint32), with certain named constants, each having a corresponding underlying integer value. It is used much like an enum in C.

enum Flavor
{
    Vanilla = 1;
    Chocolate = 2;
    Mint = 3;
}

Flags

By default, a Bebop enum type is not supposed to represent any underlying values outside of the ones listed.

If you want a more C-like, bitflags-like behavior, add a flags decorator to the enum:

@flags
enum Permissions {
    Read = 0x01;
    Write = 0x02;
    Comment = 0x04;
}

Defined this way, Permissions values like 0 (no permissions) and 3 (Read + Write) are valid too.

Underlying Type

By default, an enum is backed by a uint32. You can change this by specifying a different underlying type:

enum Flavor : uint8
{
    Vanilla = 1;
    Chocolate = 2;
    Mint = 3;
}

With the above definition, Flavor values can only be in the range 0 to 255 and takes up only 1 byte in the serialized form.

Best Practices

  • Unlike in C, all constants must be explicitly given an integer literal value.
  • You should never change or remove a constant from an enum definition. Instead, put deprecated decorator in front of the member.
enum Flavor
{
    Vanilla = 1;
    @deprecated("eat Mint instead")
    Chocolate = 2;
    Mint = 3;
}
  • You’re free to add new constants to an enum at any point in the future.

Why should 0 be a “boring” value in an enum

Bebop makes the unusual choice of being opinionated about what the 0 member of an enum is called. For all but a select set of identifiers, the compiler issues a warning like so:

Bebop recommends that 0 in an enum be reserved for a value named Unknown, Default, or similar.

The full list of acceptable names is: Default, Unknown, Invalid, Null, None, Zero, False.

This warning was inspired by news of the OMIGOD vulnerability, where an attacker could achieve root privileges because of a bug involving an uninitialized authInfo struct in a C program.

Before this fix, the uninitialized uid and gid values were filled with… zero. (The memory here was allocated by calloc.) And uid=0 gid=0 happens to mean “root”!

This sort of bug is easy to write, and OMIGOD is far from an isolated incident. As long as there are popular languages where it’s easy for memory to accidentally be zeroed out, we believe there’s a security benefit in ensuring that 0 always means something boring.

That’s why Bebop encourages you to leave 0 explicitly reserved to mean “invalid” or “default”, and starting the “interesting” values from 1.

DiscouragedEncouraged
enum UserType {
    Admin = 0;  // uh-oh
    Musician = 1;
    Listener = 2;
}
enum UserType {
    Invalid = 0;  // whew!
    Admin = 1;
    Musician = 2;
    Listener = 3;
}