Question

I saw this line of code today and had no idea what it does.

typedef enum {
  SomeOptionKeys = 1 << 0 // ?
} SomeOption;

Some usage or example would be helpful. Thanks!

Answer 1

It looks like it defines an enumerated type that is supposed to contain a set of flags. You’d expect to see more of them defined, like this:

typedef enum {
  FirstOption = 1 << 0,
  SecondOption = 1 << 1,
  ThirdOption = 1 << 2
} SomeOption;

Since they are defined as powers of two, each value corresponds to a single bit in an integer variable. Thus, you can use the bitwise operators to combine them and to test if they are set. This is a common pattern in C code.

You could write code like this that combines them:

SomeOption myOptions = FirstOption | ThirdOption;

And you could check which options are set like this:

if (myOptions & ThirdOption)
{
  ...
}