How do I list an enumeration?

Question

Usually, when I create an enumeration, each of them gets an increment of one, see

enum
{
    A = 0,
    B,
    C,
    D
};

but after I looked through some source codes, I see that people do things like this.

enum
{
    A = 0,
    B = 1 << 0,
    C = 1 << 1,
    D = 1 << 2
};

I understand what this means, but what exactly does this give me? Are there any advantages? I only see that this, in my opinion, looks awkward.

Answer 1

The second form creates flags for use in the bitmask. This is usually done to save space in objects that have several Boolean conditions that control their behavior.

struct foo {
    std::uint32_t bitmask; // up to 32 different flags.
};

foo obj;
obj.bitmask = (B | D); // Sets the bits 0 and 2 

Answer 2

, B | C , D.

, , . = 0 . .