How to Write a binary file in c++

Question

I'm trying to implement the Huffman's encoding algorithm in c++.

my question is : after i got the equivalent binary string for each character , how can i write those zeros and ones as binary on a file not as string 0 or string 1 ?

thanks in advance ...

Answer 1

Obtaining individually the encoding of each character in a different data structure is a broken solution, because you need to juxtapose the encoding of each character in the resulting binary file: storing them individually makes that as hard as directly storing them contiguously in a vector of bits.

This consideration suggests using a std::vector<bool> to perform your task, but it is a broken solution because it can't be treated as a c-style array, and you really need that at output time.

This question asks precisely which are the valid alternatives to std::vector<bool>, so I think answers to that question fits perfectly your question.

BTW, what I would do is to just wrap a std::vector<uint8_t> under a class which suits yout needs, like the code attached:

#include <iostream>
#include <vector>
#include <cstdint>
#include <algorithm>
class bitstream {
private:
    std::vector<std::uint8_t> storage;
    unsigned int bits_used:3;
    void alloc_space();
public:
    bitstream() : bits_used(0) { }

    void push_bit(bool bit);

    template <typename T>
    void push(T t);

    std::uint8_t *get_array();

    size_t size() const;

    // beware: no reference!
    bool operator[](size_t pos) const;
};

void bitstream::alloc_space()
{
    if (bits_used == 0) {
        std::uint8_t push = 0;
        storage.push_back(push);
    }
}

void bitstream::push_bit(bool bit)
{
    alloc_space();
    storage.back() |= bit << 7 - bits_used++;
}

template <typename T>
void bitstream::push(T t)
{
    std::uint8_t *t_byte = reinterpret_cast<std::uint8_t*>(&t);
    for (size_t i = 0; i < sizeof(t); i++) {
        uint8_t byte = t_byte[i];
        if (bits_used > 0) {
            storage.back() |= byte >> bits_used;
            std::uint8_t to_push = (byte & ((1 << (8 - bits_used)) - 1)) << bits_used;
            storage.push_back(to_push);
        } else {
            storage.push_back(byte);
        }
    }
}

std::uint8_t *bitstream::get_array()
{
    return &storage.front();
}

size_t bitstream::size() const
{
    const unsigned int m = 0;
    return std::max(m, (storage.size() - 1) * 8 + bits_used);
}

bool bitstream::operator[](size_t size) const
{
    // No range checking
    return static_cast<bool>((storage[size / 8] >> 7 - (size % 8)) & 0x1);
}

int main(int argc, char **argv)
{
    bitstream bs;
    bs.push_bit(true);
    std::cout << bs[0] << std::endl;
    bs.push_bit(false);
    std::cout << bs[0] << "," << bs[1] << std::endl;
    bs.push_bit(true);
    bs.push_bit(true);
    std::uint8_t to_push = 0xF0;
    bs.push_byte(to_push);
    for (size_t i = 0; i < bs.size(); i++)
        std::cout << bs[i] << ",";
    std::cout << std::endl;
}

Answer 2

I hope this code can help you.

• You start from a sequence of bytes (1s and 0s) representing the continuous encoding of every character of the input file.
• You take every byte of the sequence and add a bit into a temporary byte (char byte)
• Every time you fill a byte, you write it to file (you could also wait, for efficiency, to have a bigger data)
• At the end, you write the remaining bits to file, filled with trailing zeros, for example
• As akappa correctly pointed out, the else branch can be removed if byte is set to 0 after each file writing operation (or, more generically, every time it has been totally filled and flushed somewhere else), so only 1s must be written.

---

void writeBinary(char *huffmanEncoding, int sequenceLength)
{
    char byte = 0;
    // For each bit of the sequence
    for (int i = 0; i  < sequenceLength; i++) {
        char bit = huffmanEncoding[i];

        // Add a single bit to byte
        if (bit == 1) {
            // MSB of the sequence to msb of the file
            byte |= (1 << (7 - (i % 8)));
            // equivalent form: byte |= (1 << (-(i + 1) % 8);
        }
        else {
            // MSB of the sequence to msb of the file
            byte &= ~(1 << (7 - (i % 8)));
            // equivalent form: byte &= ~(1 << (-(i + 1) % 8);
        }

        if ((i % 8) == 0 && i > 0) {
            //writeByteToFile(byte);
        }
    }

    // Fill the last incomplete byte, if any, and write to file
}

Answer 3

You cant write to a binary file with only bits; the smallest size of data written is one byte (thus 8 bits).

So what you should do is create a buffer (any size).

char BitBuffer;

Writing to a buffer:

int Location;
bool Value;

if (Value)
    BitBuffer |= (1 << Location);
else
    BitBuffer &= ~(1 << Location)

The code (1 << Location) generates a number with all 0's except the position specified by Location. Then, if Value is set to true, it sets corresponding bit in Buffer to 1, and to 0 in other case. The binary operations used are fairly simple, if you don't understand them, it should be in any good C++ book/tutorial.

Location should be number in range <0, sizeof(Buffer)-1>, so <0,7> in this case.

Writing buffer to a file is relatively simple when using fstream. Just remember to open it as binary.

ofstream File;
File.open("file.txt", ios::out | ios::binary);
File.write(BitBuffer, sizeof(char))

EDIT: Noticed a bug and fixed it.

EDIT2: You can't use << operators in binary mode, i forgot about it.

Alternative solution : Use std::vector<bool> or std::bitset as a buffer.

This should be even simpler, but I thought I could help you a little bit more.

void WriteData (std::vector<bool> const& data, std::ofstream& str)
{
    char Buffer;
    for (unsigned int i = 0; i < data.size(); ++i)
    {
       if (i % 8 == 0 && i != 0)
           str.write(Buffer, 1);
       else
           // Paste buffer setting code here
           // Location = i/8;
           // Value = data[i];
    }
    // It might happen that data.size() % 8 != 0. You should fill the buffer
    // with trailing zeros and write it individually.
}