New implementation

Be concise.

Be useful.

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Please try to avoid dependencies to third-party libraries and frameworks.

Other implementations
char s[32];
snprintf(s, sizeof(s), "%x", i);
template <typename I>
std::string n2hexstr(I w, size_t hex_len = sizeof(I)<<1) {
    static const char* digits = "0123456789ABCDEF";
	std::string str(hex_len, '-');
    for (size_t i=0, j=(hex_len-1)*4 ; i<hex_len; ++i,j-=4)
       str[i] = digits[(w>>j) & 0x0f];
	return str;
}
#include <sstream>
std::ostringstream stream;
stream << std::hex << x;
s = stream.str();
String s = x.ToString("x")
import std.conv;
s = to!string(x, 16);
import std.format;
s = format("%x", x);
s = Integer.to_string(x, 16)
S = io_lib:fwrite("~.16B",[X]).
  write (*,'(Z8.8)') 999
import "strconv"
s := strconv.FormatInt(x, 16)
import "fmt"
import "math/big"
s := fmt.Sprintf("%x", x)
import Text.Printf (printf)
s :: String
s = printf "%x" 999
const s = x.toString(16)
String s = Integer.toHexString(x);
s = string.format("%x",x)
$s = dechex(999);
uses sysutils;
s := IntToHex(x);
my $s = sprintf("%x", $i);
s = hex(x)
s = x.to_s(16)
let s = format!("{:X}", x);