Question

I need a library that can URLencode a string/char array.

Now, I can hex encode an ASCII array like here:
http://www.codeguru.com/cpp/cpp/cpp_mfc/article.php/c4029

But I need something that works with Unicode.
Note: On Linux AND on Windows !

CURL has a quite nice:

 char *encodedURL = curl_easy_escape(handle,WEBPAGE_URL, strlen(WEBPAGE_URL));

but first, that needs CURL and it also is not unicode capable, as one sees by strlen

Answer 1

If I read the quest correctly and you want to do this yourself, without using curl I think I have a solution (sssuming UTF-8) and I think this is a conformant and portable way of URL encoding query strings:

#include <boost/function_output_iterator.hpp>
#include <boost/bind.hpp>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <iterator>
#include <iomanip>

namespace {
  std::string encimpl(std::string::value_type v) {
    if (isalnum(v))
      return std::string()+v;

    std::ostringstream enc;
    enc << '%' << std::setw(2) << std::setfill('0') << std::hex << std::uppercase << int(static_cast<unsigned char>(v));
    return enc.str();
  }
}

std::string urlencode(const std::string& url) {
  // Find the start of the query string
  const std::string::const_iterator start = std::find(url.begin(), url.end(), '?');

  // If there isn't one there's nothing to do!
  if (start == url.end())
    return url;

  // store the modified query string
  std::string qstr;

  std::transform(start+1, url.end(),
                 // Append the transform result to qstr
                 boost::make_function_output_iterator(boost::bind(static_cast<std::string& (std::string::*)(const std::string&)>(&std::string::append),&qstr,_1)),
                 encimpl);
  return std::string(url.begin(), start+1) + qstr;
}

It has no non-standard dependencies other than boost and if you don’t like the boost dependency it’s not that hard to remove.

I tested it using:

int main() {
    const char *testurls[] = {"http://foo.com/bar?abc<>de??90   210fg!\"$%",
                              "http://google.com",
                              "http://www.unicode.com/example?großpösna"};
    std::copy(testurls, &testurls[sizeof(testurls)/sizeof(*testurls)],
              std::ostream_iterator<std::string>(std::cout,"\n"));
    std::cout << "encode as: " << std::endl;
    std::transform(testurls, &testurls[sizeof(testurls)/sizeof(*testurls)],
                   std::ostream_iterator<std::string>(std::cout,"\n"),
                   std::ptr_fun(urlencode));
}

Which all seemed to work:

http://foo.com/bar?abc<>de??90   210fg!"$%
http://google.com
http://www.unicode.com/example?großpösna

Becomes:

http://foo.com/bar?abc%3C%3Ede%3F%3F90%20%20%20210fg%21%22%24%25
http://google.com
http://www.unicode.com/example?gro%C3%9Fp%C3%B6sna

Which squares with these examples