Writing 4 bits to a binary file with ofstream

问题

If I have an unsigned integer between 0 - 16 and I want to write it to a binary file without writing a whole byte to it, how would one shift bits to achieve it?

0-16 means I only need 4 bits, so I should be able to store 2 different numbers in a single byte right?

The following code writes 1 number to 1 byte:

std::ofstream file;
file.open("test.bin", std::ios::binary|std::ios::out);
char oneByteNum = (char)fourByteNum; // Converting from some 4 byte integer to a 1 byte char
file.write(&oneByteNum ,sizeof(char));

Using bitshifts, how can I achieve 2 numbers in 1 byte? I imagine reading the number out of the byte would be a similar, inverse 2 step process too?

回答1:

char oneByteWithTwoNums = (oneByteNum1 << 4) + (oneByteNum2 & 0x0f);

回答2:

Try this:

  compacted = first_number * 0x10 + second-number;

To expand:

  second_number = compacted & 0x0F;
  first_number = compacted >> 4;

回答3:

I wrote up a quick example:

#include <iostream>

typedef unsigned char byte;

byte pack(unsigned int num1, unsigned int num2) {

    // Our byte has the form 0b00000000
    // We want the first four bits to be filled with num1
    byte packed = num1 << 4;

    // We want the last four bits to be filled with num2
    // but, we don't want to overwrite the top four bits, so
    // we mask it with 0x0F (0b0001111)
    packed |= num2 & 0x0F;
    return packed;
}

void unpack(byte b, unsigned int& num1, unsigned int& num2) {
    // Extract the first four bits of our byte
    num1 = b >> 4;

    // Mask off the first four bits of our byte to get only
    // the last four bits
    num2 = b & 0x0F;
}

int main() {

    unsigned int num1 = 5;  // As an example
    unsigned int num2 = 15; // As an example

    byte b = pack(num1, num2);

    unsigned int num3;
    unsigned int num4;
    unpack(b, num3, num4);
    std::cout << num3 << std::endl;
    std::cout << num4 << std::endl;

    return 0;
}

You'll probably want to add checks in your pack/unpack methods to ensure someone doesn't try passing in a value other than [0-15].

来源：https://stackoverflow.com/questions/26516027/writing-4-bits-to-a-binary-file-with-ofstream

标签

c++

binary

bit

bit-shift