Could a kind soul ELI5 this? Well, maybe ELI8. I did quite a bit of programming in the 90-00s as part of my job, although nowadays I’m more of a script kiddie.
A Boolean is a true/false value. It can only be those two values and there be represented by a single bit (1 or 0).
In most languages a Boolean variable occupies the space of a full byte (8 bit) even though only a single of those bits is needed for representing the Boolean.
That’s mostly because computers can’t load a bit. They can only load bytes. Your memory is a single space where each byte has a numeric address. Starting from 0 and going to whatever amount of memory you have available. This is not really true because on most operating systems each process gets a virtual memory space but its true for many microcontrollers. You can load and address each f these bytes but it will always be a byte. That’s why booleans are stored as bytes because youd have to pack them with other data on the same address other wise and that’s getting complicated.
Talking about getting complicated, in C++ a std::vector<bool> is specialized as a bit field. Each of the values in that vector only occupy a single bit and you can get a vector of size 8 in a single byte. This becomes problematic when you want to store references or pointers to one of the elements or when you’re working with them in a loop because the elements are not of type bool but some bool-reference type.
A boolean value only needs 1 bit (on or off) for true or false. However the smallest bit of addressable memory is a byte (8 bits) hence 7 are technically wasted.
For low memory devices you could instead store 8 different Boolean values in one single byte by using bit masking instead
Wait till you here about every ascii letter. . .
what about them?
Ascii needs seven bits, but is almost always encoded as bytes, so every ascii letter has a throwaway bit.
Let’s store the boolean there then!!
Some old software does use 8-Bit ASCII for special/locale specific characters. Also there is this Unicode hack where the last bit is used to determine if the byte is part of a multi-byte sequence.
This reminds me that I actually once made a class to store bools packed in uint8 array to save bytes.
Had forgotten that. I think i have to update the list of top 10 dumbest things i ever did.
I mean is it really a waste? What’s minimum amount of bits most CPUs read in one cycle.
In terms of memory usage it’s a waste. But in terms of performance you’re absolutely correct. It’s generally far more efficient to check is a word is 0 than to check if a single bit is zero.
I swore I read that mysql dbs will store multiple bools in a row as bit maps in one byte. I can’t prove it though
We need to be able to express 0 and 1 as integers so that functionality is just being overloaded to express another concept.
Wait until the person who made this meme finds out about how many bits are being wasted on modern CPU architectures. 7 is the minimum possible wasted bits but it would be 31 on every modern computer (even 64b machines since they default to 32b ints).
boolean bloat
I first thought you wrote boolean float, not sure if that’s even worse.
boolean root beer float
deleted by creator
In the industrial automation world and most of the IT industry, data is aligned to the nearest word. Depending on architecture, that’s usually either 16, 32, or 64 bits. And that’s the space a single Boolean takes.
That’s why I primarily use booleans in return parameters, beyond that I’ll try to use bitfields. My game engine’s tilemap format uses a 32 bit struct, with 16 bit selecting the tile, 12 bit selecting the palette, and 4 bit used for various bitflags (horizontal and vertical mirroring, X-Y axis invert, and priority bit).
Bit fields are a necessity in low level networking too.
They’re incredibly useful, I wish more people made use of them.
I remember I interned at a startup programming microcontrollers once and created a few bitfields to deal with something. Then the lead engineer went ahead and changed them to masked ints. Because. The most aggravating thing is that an int size isn’t consistent across platforms, so if they were ever to change platforms to a different word length, they’d be fucked as their code was full of platform specific shenanigans like that.
/rant
I always use stdint.h so that my types are compatible across any mcu. And it makes the data type easily known instead of guessing an i t size
Good rant.
Yeah. I once had to do stuff to code that had bit-fields like that and after a while, realised (by means of StackOverflow) that that part is UB and I had to go with bitwise operations instead.
Undefined Behavior…?
Ok, I recalled wrong, it was unspecified
Are you telling me that no compiler optimizes this? Why?
CPUs don’t read one bit a a time.
It would be slower to read the value if you had to also do bitwise operations to get the value.
But you can also define your own bitfield types to store booleans packed together if you really need to. I would much rather that than have the compiler do it automatically for me.
Well there are containers that store booleans in single bits (e.g.
std::vector<bool>- which was famously a big mistake).But in the general case you don’t want that because it would be slower.
Why is this a big mistake? I’m not a c++ person
The mistake was that they created a type that behaves like an array in every case except for
bool, for which they created a special magical version that behaves just subtly different enough that it can break things in confusing ways.Could you provide an example?
Consider what the disassembly would look like. There’s no fast way to do it.
It’s also unnecessary since 8 bytes is a negligible amount in most cases. Serialization is the only real scenario where it matters. (Edit: and embedded)
In embedded, if you are to the point that you need to optimize the bools to reduce the footprint, you fucked up sizing your mcu.
pragma(pack) {
int a:1, b:1, … h:1;
}
IIRC.
Back in the day when it mattered, we did it like
#define BV00 (1 << 0) #define BV01 (1 << 1) #define BV02 (1 << 2) #define BV03 (1 << 3) ...etc #define IS_SET(flag, bit) ((flag) & (bit)) #define SET_BIT(var, bit) ((var) |= (bit)) #define REMOVE_BIT(var, bit) ((var) &= ~(bit)) #define TOGGLE_BIT(var, bit) ((var) ^= (bit)) ....then... #define MY_FIRST_BOOLEAN BV00 SET_BIT(myFlags, MY_FIRST_BOOLEAN)Edit - oops, responded to wrong comment…
Okay. Gen z programmer here. Can you explain this black magic? I see it all the time in kernel code but I have no idea what it means.
The code is a set of preprocessor macros to stuff loads of booleans into one int (or similar), in this case named ‘myFlags’. The preprocessor is a simple (some argue too simple) step at the start of compilation that modifies the source code on its way to the real compiler by substituting #defines, prepending #include’d files, etc.
If myFlags is equal to, e.g. 67, that’s 01000011, meaning that BV00, BV01, and BV07 are all TRUE and the others are FALSE.
The first part is just for convenience and readability. BV00 represents the 0th bit, BV01 is the first etc. (1 << 3) means 00000001, bit shifted left three times so it becomes 00001000 (aka 8).
The middle chunk defines macros to make bit operations more human-readable.
Calling
SET_BIT(myFlags, MY_FIRST_BOOLEAN)gets turned into((myFlags) |= ((1 << 0))), which could be simplified asmyFlags = myFlags | 00000001. (Ignore the flood of parentheses, they’re there for safety due to the loaded shotgun nature of the preprocessor.)It’s called bitshifting and is used to select which bits you want to modify so you can toggle them individually.
1 << 0 is the flag for the first bit
1 << 1 for the second
1 << 2 for the third and so onI think that’s correct. It’s been years since I’ve used this technique tbh 😅
Which part?
With embedded stuff its still done like that. And if you go from the arduino functionss to writing the registers directly its a hell of a lot faster.
if wasting a byte or seven matters to you, then then you need to be working in a lower level language.
It’s 7 bits…
Pay attention. 🤪
7 bytes! Look at Mr. Moneybags here!
Well when it comes to bytes, you could say I’m a bit of a millionaire myself.
True.
Well storing that would only take half a bit.
3GPP has an interesting way of serialising bools on the wire with ASN.1
NULL OPTIONAL
meaning only the type would be stored if true, otherwise it won’t be set at all
That requires some form of self describing format and will probably look like a sparse matrix in the end.
I set all 8 bits to 1 because I want it to be really true.
01111111 = true
11111111 = negative true = false
00001111 = maybe
10101010 = I don’t know
0011 1111 = could you repeat the question
00000001 00000000 00001111 10101010
100001111 = maybe not
Schrödingers Boolean
What if it’s an unsigned boolean?
Cthulhu shows up.
Common misconception… Unsigned booleans (ubool) are always 16-bits.
Could also store our bools as floats.
00111111100000000000000000000000is true and10111111100000000000000000000000is negative true.Has the fun twist that true & false is true and true | false is false .
So all this time true was actually false and false was actually true ?
Depends on if you are on a big endian or little endian architecture.
Come on man, I’m not gonna talk about my endian publicly
Why do alternative facts always gotta show up uninvited to the party? 🥳
You jest, but on some older computers, all ones was the official truth value. Other values may also have been true in certain contexts, but that was the guaranteed one.
I was programming in assembly for ARM (some cortex chip) and I kid you not the C program we were integrating with required 255, with just 1 it read it as false
TIL, 255 is the new 1.
Aka -1 >> 1 : TRUE
