Question

Say, i have binary protocol, where first 4 bits represent a numeric value which can be less than or equal to 10 (ten in decimal).

In C++, the smallest data type available to me is char, which is 8 bits long. So, within my application, i can hold the value represented by 4 bits in a char variable. My question is, if i have to pack the char value back into 4 bits for network transmission, how do i pack my char’s value back into 4 bits?

Answer 1

Sure, just use one char for your value:

std::ofstream outfile("thefile.bin", std::ios::binary);

unsigned int n;  // at most 10!
char c = n << 4; // fits
outfile.write(&c, 1);  // we wrote the value "10"

The lower 4 bits will be left at zero. If they’re also used for something, you’ll have to populate c fully before writing it. To read:

infile.read(&c, 1);
unsigned int n = c >> 4;