How to read a single bit from binary file in C++?

Question

We're doing some experiment on random number. The data given me is stored in a .bin file, and according to the provider, it's stored in independent bits, that is, every bit is the supposed bool random variable.

Now I would like to use this input in program, but as bool take 1 BYTE and int take 4 bytes, I don't know how to read it into a variable in C++. And it seems that the ifstream.read(s,n) also only takes n as number of char.

Is there any way to read bit data from file? Thank you!

You do realize that chars and ints and bytes consist of bits, right? You can't read one bit at a time, but you can read 8 bits at a time. Think of `ifstream.read(s,n)` as reading `8*n` bits, rather than `n` bytes. — Igor Tandetnik, Aug 19 '14 at 21:05

score 8 · Answer 1 · answered Aug 19 '14 at 21:09

8

If every char is 8 bits, then you can discover the state of each bit like this:

// @param bit = 0 - 7
inline bool get_bit(char c, int bit)
{
    return (c >> n) & 1;
}

answered Aug 19 '14 at 21:09

Galik

47,303
4
80
117

score 5 · Answer 2 · answered Aug 19 '14 at 21:07

The smallest unit of information that can be read from a file is a char (8 bits¹). To extract a particular bit from a char, you can do something like:

char c = 0xa5;
int n = 2;
int bit = (c >> n) & 1;
printf("bit = %d\n", bit);

^{¹If I don't include this, then somebody will point out that a char doesn't always have to be 8 bits, and it depends on CHAR_BIT as defined by your compiler, but on all reasonable platforms today, a char is 8 bits.}

How to read a single bit from binary file in C++?

2 Answers2