C++: How is boolean represented in memory?

[NeoDreamer]

How are booleans represented in memory? If you had 8 booleans declared one after another, will they be packed tightly in 1 byte?

How are booleans retrieved? No OS will actually fetch 1 bit, right? They must grab an entire word (16 bits in Unix, I believe). If this is so, then wouldn't it be faster to use ints instead of bools? Because the bool would have to be extracted out of the retrieved word (with an XOR???) whereas the int can be used directly.

[jase21]

Theoretically a boolean consumes only 1 bit. But its represented using a byte of memory. Consider if there is only one boolean to be represented then the value will be either 0 or 1.
=>
X X X X X X X b ; b-> 0 or 1.

but if there are two or more boolean variables coming up, then it gets stored in the next location in the same byte of the memory but points to the respective variable.
=>
X X X X X X b1 b0 ; b0, b1 either 0 or 1. until the whole chunk gets filled.
Eventually it initializes the next chunk and so on.

[jase21]

And how its extracted depends on the assembly codes (after compiler or assembler optimization).
AFAIK xor could be used or a compare can be used as well as test instruction etc..

[ajcates]

umm numbers on a computer are really actually bools strung together… this is called binary.

[MTsoul]

cout << sizeof(bool) ?

[last_hopes]

Hey, How can I read out a Car ID from the Memory? I tried it in that way:

[jwilliam]

I can't imagine that it would be more efficient to store booleans in an int... When you fetch data to put in a register I think it has to be 32/64 bits at a time. In the end, the gates that actually perform the logic are working on single bits of data anyway.

[ajcates]

@jwilliam
It can be… look at unix file permissions.

[jwilliam]

I see what you're getting at, but the question was, is it more efficient to store a single boolean in an integer (at least 4 bytes) or in a single bit. If you declare an int in C and then set it to either 0 or 1, it's still going to take up sizeof(int) amount of space in memory.

[Krilnon]

Almost no instructions in any given instruction set deal with memory at the bit level, and you need to be able to address different boolean variables separately (especially if you want to pass separate pointers around). Compilers can choose to pack booleans into individual bits of a word, but it won't always be done.

If you were to have multiple booleans stored in a single word, there would be an added expense to reading and writing them. Some instruction sets can fetch a single bit from a word in a single instruction, but I don't personally know how common that instruction is. The processor would still fetch a whole word from memory, but it would mask off the rest of the bits once it was retrieved.

At any rate, it's great to store as many booleans as you can in a word if you're worried about memory consumption or bandwidth, but it's not really worth the runtime expense to pack and unpack those bits if you're just dealing with the limited amount of declared boolean variables that a programmer will use within almost any program. That's probably why C doesn't even have a boolean type and why almost any file format that stores large files will have some packed form for booleans.

[MTsoul]

On my system, sizeof(bool) == 1. It's the same size as a char. If you want space savings (and not zero overhead, in which case use bool), look up vector<bool>. It implements what Krilnon says, kind of.