How to convert Byte Array to Hexadecimal String in C++?

#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include <iterator>
#include <sstream>
#include <iomanip>

int main() 
{
  std::vector<unsigned char> v;

  v.push_back( 1 );
  v.push_back( 2 );
  v.push_back( 3 );
  v.push_back( 4 );

  std::ostringstream ss;

  ss << std::hex << std::uppercase << std::setfill( '0' );
  std::for_each( v.cbegin(), v.cend(), [&]( int c ) { ss << std::setw( 2 ) << c; } );

  std::string result = ss.str();

  std::cout << result << std::endl;
  return 0;
}

Or, if you've got a compiler that supports uniform initialization syntax and range based for loops you can save a few lines.

#include <vector>
#include <sstream>
#include <string>
#include <iostream>
#include <iomanip>

int main()
{
  std::vector<unsigned char> v { 1, 2, 3, 4 };
  std::ostringstream ss;

  ss << std::hex << std::uppercase << std::setfill( '0' );
  for( int c : v ) {
    ss << std::setw( 2 ) << c;
  }

  std::string result = ss.str();
  std::cout << result << std::endl;
}

Use boost::alogorithm::hex

std::vector<unsigned char> v{1, 2, 3, 4};
std::string res;
boost::algorithm::hex(v, back_inserter(res));

You can use the C++ Standard Library and or you can use boost::lexical_cast

#include <iostream>
#include <string>
#include <array>
#include <vector>
#include <sstream>
#include <iomanip>
#include <algorithm>

using namespace std;

// use this macro for c++11 feature
#define USE_CPP11

int main(int argc, char* argv[])
{
    array<unsigned char, 3> hexArr = {0x01, 0xff, 0x55};
    const char separator = ' ';             // separator between two numbers
    ostringstream os;
    os << hex << setfill('0');  // set the stream to hex with 0 fill

#ifdef USE_CPP11
    std::for_each(std::begin(hexArr), std::end(hexArr), [&os, &separator] (int i)
    {
        os << setw(2) << i << separator;
    });
#else       // c++03
    typedef array<unsigned char, 3>::const_iterator const_iterator;
    for (const_iterator it = hexArr.begin(); it != hexArr.end(); ++it)
    {
        os << setw(2) << int(*it) << separator;
    }
#endif
    os << dec << setfill(' ');          // reset the stream to "original"

    // print the string
    cout << "the string array is: " << os.str() << endl;

    return EXIT_SUCCESS;
}

One of the fastest way I know in C++ 11:

template <size_t byteCount>
string BytesArrayToHexString( const std::array<byte, byteCount>& src )
{
  static const char table[] = "0123456789ABCDEF";
  std::array<char, 2 * byteCount + 1> dst;
  const byte* srcPtr = &src[0];
  char* dstPtr = &dst[0];

  for (auto count = byteCount; count > 0; --count)
  {
      unsigned char c = *srcPtr++;
      *dstPtr++ = table[c >> 4];
      *dstPtr++ = table[c & 0x0f];
  }
  *dstPtr = 0;
  return &dst[0];
}

A good compiler should not have any problem to apply SSE optimization on this....