Can or should I make bools bit fields?

Question

Is this legal or recommended? I read that you should only use integer types as bitfields, but does this apply to boolean types? Is this OK, or is this bad practice or undefined behavior somehow?

struct MyStruct {
    // ...
    bool SomeBooleanProperty:1;
    // ...
};

it does not matter as bool can one have two integer values which perfectly fits 1 bit bitfield — 0___________, Jun 23 '21 at 22:29
It is perfectly fine to have a bitfield of type `bool` and this is explicitly stated in the C17 standard at 6.7.2.1 (5) (note that `_Bool` is the same as `bool`). — Nate Eldredge, Jun 23 '21 at 22:32
Though I have heard that `bool` and `unsigned char` have subtle differences — user16217248, Jun 23 '21 at 22:33
The differences have to do with assigning values other than `true` and `false`, but you're not going to do that anyway, right? — Nate Eldredge, Jun 23 '21 at 22:34
As far as the advisability, it's subject to all the same pros and cons of using bitfields in general, which are discussed ad nauseum elsewhere, e.g. https://stackoverflow.com/questions/24933242/when-to-use-bit-fields-in-c/24934836#24934836 — Nate Eldredge, Jun 23 '21 at 22:34

chux - Reinstate Monica · Accepted Answer · 2021-06-24T01:58:56.163

5

Can ... I make bools bit fields?

Yes. It is one of 3 well defined choices.

A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type. It is implementation-defined whether atomic types are permitted. C17dr § 6.7.2.1 5

.... should I make bools bit fields?

Yes, if it makes code more clear.

Note: this is one place to not use int x:1 as it is implementation defined if x has values [0,1] or [-1,0]. Use signed int x:1 or unsigned x:1 or _Bool x:1 for [-1,0], [0,1], [0,1] respectively.

For x:1, bool does have a clearer functionally specification than signed int when assigning an out-of-range value. See comment. For unsigned, just the LSbit is copied.

edited Jun 24 '21 at 01:58

answered Jun 23 '21 at 23:12

chux - Reinstate Monica

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Note that `_Bool` and the int versions behave differently, e.g. `x = 2;` will produce a true `_Bool` but zero for the other types – M.M Jun 24 '21 at 00:44
@M.M Agree zero for `bool` and `unsinged`. Likely zero for `signed int` - yet that is an implementation defined conversions. That point does give `bool` an edge. – chux - Reinstate Monica Jun 24 '21 at 02:02

Can or should I make bools bit fields?

1 Answers1