How to write a tester for a program that writes one bit at a time?

How to write a tester for a program that writes one bit at a time?

I am trying to write a tester for the following program to see if it
functions correctly, however, I'm not sure if I implemented flush()
correctly and for some reason I don't get any output. Can someone suggest
code that will test this class to see if I implemented flush and writeBit
correctly?
#ifndef BITOUTPUTSTREAM_HPP
#define BITOUTPUTSTREAM_HPP
#include <iostream>
class BitOutputStream {
private:
char buf; // one byte buffer of bits
int nbits; // how many bits have been written to buf
std::ostream& out; // reference to the output stream to use
public:
/* Initialize a BitOutputStream that will
* use the given ostream for output.
* */
BitOutputStream(std::ostream& os) : out(os) {
buf = nbits = 0; // clear buffer and bit counter
}
/* Send the buffer to the output, and clear it */
void flush() {
out.put(buf);
flush(); // not sure if this is how I should flush the buffer?
buf = nbits = 0;
}
/* Write the least sig bit of arg into buffer */
int writeBit(int i) {
// If bit buffer is full, flush it.
if (nbits == 8)
flush();
// Write the least significant bit of i into
// the buffer at the current index.
// buf = buf << 1; this is another option I was considering
// buf |= 1 & i; but decided to go with the one below
int lb = i & 1; // extract the lowest bit
buf |= lb << nbits; // shift it nbits and put in in buf
// increment index
nbits++;
return nbits;
}
};