Convert strings between hex format and binary format

Question

Is there any utility or library provides a simple function to convert a string between hex/binary format? I've been searching on SO and currently using look-up table approach. By the way, since it might be a long string, I wouldn't consider to convert the string to integer and process the format conversion, as a long string might be greater than MAX_INT (or other integer data types).

For example:

0xA1 => 10100001
11110001 => 0xF1

PS: My project is using Boost 1.44, a bit out-dated. So if the utility is from Boost, hopefully it's available in 1.44.

Answer 1

You can use a combination of std::stringstream , std::hex and std::bitset to convert between hex and binary in C++03.

Here's an example:

#include <iostream>
#include <sstream>
#include <bitset>
#include <string>

using namespace std;

int main()
{
    string s = "0xA";
    stringstream ss;
    ss << hex << s;
    unsigned n;
    ss >> n;
    bitset<32> b(n);
    // outputs "00000000000000000000000000001010"
    cout << b.to_string() << endl;
}

EDIT:

About the refined question, here's a code example about converting between hex strings and binary strings (you can refactor with a helper function for the hex char<>bits part, and use a map or a switch instead, etc).

const char* hex_char_to_bin(char c)
{
    // TODO handle default / error
    switch(toupper(c))
    {
        case '0': return "0000";
        case '1': return "0001";
        case '2': return "0010";
        case '3': return "0011";
        case '4': return "0100";
        case '5': return "0101";
        case '6': return "0110";
        case '7': return "0111";
        case '8': return "1000";
        case '9': return "1001";
        case 'A': return "1010";
        case 'B': return "1011";
        case 'C': return "1100";
        case 'D': return "1101";
        case 'E': return "1110";
        case 'F': return "1111";
    }
}

std::string hex_str_to_bin_str(const std::string& hex)
{
    // TODO use a loop from <algorithm> or smth
    std::string bin;
    for(unsigned i = 0; i != hex.length(); ++i)
       bin += hex_char_to_bin(hex[i]);
    return bin;
}

Answer 2

The following code includes two functions that will do exactly as you desire. This is based on the answer by Silex but with a few extra string operations to match the example output you gave in your question.

#include <iostream>
#include <sstream>
#include <bitset>
#include <string>
#include <boost/algorithm/string.hpp>

using namespace std;

const unsigned g_unMaxBits = 32;

string Hex2Bin(const string& s)
{
    stringstream ss;
    ss << hex << s;
    unsigned n;
    ss >> n;
    bitset<g_unMaxBits> b(n);

    unsigned x = 0;
    if (boost::starts_with(s, "0x") || boost::starts_with(s, "0X")) x = 2;
    return b.to_string().substr(32 - 4*(s.length()-x));
}

string Bin2Hex(const string& s)
{
    bitset<g_unMaxBits> bs(s);
    unsigned n = bs.to_ulong();
    stringstream ss;
    ss << hex << n;
    return "0x" + boost::to_upper_copy(ss.str());
}

int main()
{
    cout << "0xA1 => " << Hex2Bin("0xA1") << endl;
    cout << "B3 => " << Hex2Bin("B3") << endl;
    cout << "11110001 => " << Bin2Hex("11110001") << endl;

    return 0;
}

Answer 3

The simplest solution without using bitset or any library, only using shift operators and std::hex to convert the whole hex-string .

sample code to convert hex-string to binary-digits :

string s = "FF 0F F0 C3 10";
stringstream ss;
ss << hex << s;

unsigned int n;
while(ss >> n){
   for (int i = 8 -1; i >= 0; i--) 
    cout << ((n >> i) & 1) ? "1" : "0";
 cout << std::endl;
}

sample output :

11111111
00001111
11110000
11000011
00001010

full code snipet

Answer 4

If you want a concise solution in plain c++11. Here you go:

string hextobin(const string &s){
    string out;
    for(auto i: s){
        uint8_t n;
        if(i <= '9' and i >= '0')
            n = i - '0';
        else
            n = 10 + i - 'A';
        for(int8_t j = 3; j >= 0; --j)
            out.push_back((n & (1<<j))? '1':'0');
    }

    return out;
}

string bintohex(const string &s){
    string out;
    for(uint i = 0; i < s.size(); i += 4){
        int8_t n = 0;
        for(uint j = i; j < i + 4; ++j){
            n <<= 1;
            if(s[j] == '1')
                n |= 1;
        }

        if(n<=9)
            out.push_back('0' + n);
        else
            out.push_back('A' + n - 10);
    }

    return out;
}

This would work for any length of string with no padding required. Should be pretty fast since there's no long switch cases and bit operations are fast.

Example usage:

string s = "FF11";
string b = hextobin(s);
cout << b << endl; // prints 1111111100010001
string h = bintohex(b);
cout << h << endl; // prints FF11