C++: Base64 Encode Data

#include <iostream>
#include <fstream>
#include <vector>

typedef unsigned char byte;
using namespace std;

const unsigned m1 = 63 << 18, m2 = 63 << 12, m3 = 63 << 6;

class base64
{
public:
	base64() { char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; }
	string encode( vector<byte> v )
	{
		string res;
		unsigned d, a = 0, l = static_cast<unsigned>( v.size() );
		while( l > 2 )
		{
			d = v[a++] << 16 | v[a++] << 8 | v[a++];	
			res.append( 1, char_set.at( ( d & m1 ) >> 18 ) );	
			res.append( 1, char_set.at( ( d & m2 ) >> 12 ) );	
			res.append( 1, char_set.at( ( d & m3 ) >>  6 ) );
			res.append( 1, char_set.at( d & 63 ) );
			l -= 3;
		}
		if( l == 2 )
		{
			d = v[a++] << 16 | v[a++] << 8;
			res.append( 1, char_set.at( ( d & m1 ) >> 18 ) );	
			res.append( 1, char_set.at( ( d & m2 ) >> 12 ) );	
			res.append( 1, char_set.at( ( d & m3 ) >>  6 ) );
			res.append( 1, '=' );
		}
		else if( l == 1 )
		{
			d = v[a++] << 16;
			res.append( 1, char_set.at( ( d & m1 ) >> 18 ) );	
			res.append( 1, char_set.at( ( d & m2 ) >> 12 ) );	
			res.append( "==", 2 );
		}
		return res;
	}

private:
	string char_set;
};

int main( int argc, char* argv[] )
{
	base64 b;
	basic_ifstream<byte> f( "favicon.ico", ios::binary );
	string r = b.encode( vector<byte>( ( istreambuf_iterator<byte>( f ) ), istreambuf_iterator<byte>() ) );
	copy( r.begin(), r.end(), ostream_iterator<char>( cout ) );
	return 0;
}

Output: