Question

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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.