A question about class definition in C++ " : 1" [duplicate]

Question

Possible Duplicate:
What does 'unsigned temp:3' means

I encountered a problem when I'm reading the code of Clang.

class LangOptions {
public:
    unsigned Trigraphs         : 1;  // Trigraphs in source files.
    unsigned BCPLComment       : 1;  // BCPL-style '//' comments.
    ...
};

This is the first time I saw the syntax " : 1", what's the " : 1" stands for? Thanks!

Answer 1

It's a bitfield, which means that the value will only use one bit, instead of 32 (or whatever sizeof(unsigned) * <bits-per-byte> is on your platform).

Bitfields are useful for writing compact binary data structures, although they come with some performance cost as the compiler/CPU cannot update a single bit, but needs to perform AND/OR operations when reading/writing a full byte.

Answer 2

Trigraphs and BCPLComment use 1 bit only for saving values.

For example,

struct S
{
   signed char type : 2;
   signed char num  : 4;
   signed char temp : 2;
};

uses only 8 bit of memory. struct S may use a single byte or memory. sizeof(S) is 1 for the case of some implementations. But type and temp is equal to 0,1,2 or 3. And num is equal 0,1,2,..., 15 only.