How to convert u32string to int in C++11?

Question

How can we convert u32string to int in C++11?

Additional, what method should I use to convert part of such string to int - lets say having begin and end iterator available?

I've tried:

u32string test=U"14";
cout << atoi(test.c_str());

but it throws:

candidate function not viable: no known conversion from 'const char32_t *' to 'const char *' for 1st argument
extern int atoi (__const char *__nptr)

Answer 1

#include <locale>   // wstring_convert
#include <codecvt>  // codecvt_utf8
#include <iostream> // cout
#include <string>   // stoi and u32string

int main() {
  std::wstring_convert<std::codecvt_utf8<char32_t>, char32_t> convert;

  std::u32string str = U"14";
  std::cout << std::stoi(convert.to_bytes(str));
}

This depends on UTF-8 and the "C" locale using the same representation for digits.

---

GCC's standard library implementation libstdc++ does not include the codecvt header or std::wstring_convert yet. libc++ does include both of these, as does Visual Studio's standard library implementation. If you have to use libstdc++ you may find it easiest to just implement a simple conversion function yourself.

#include <algorithm> // transform
#include <iterator>  // begin, end, and back_inserter

std::string u32_to_ascii(std::u32string const &s) {
  std::string out;
  std::transform(begin(s), end(s), back_inserter(out), [](char32_t c) {
    return c < 128 ? static_cast<char>(c) : '?';
  });
  return out;
}

int u32toi(std::u32string const &s) { return stoi(u32_to_ascii(s)); }

Answer 2

Edited because my first answer was stupid.

Here is what i managed to do, however its probably not very efficient, and it assumes your string is valid.

#include <string>
#include <iostream>

int     main()
{
  std::u32string str = U"14";

  std::string res;
  for (auto c = str.begin(); c != str.end(); ++c)
    {
      char t = *c;
      res.push_back(t);
    }
  std::cout << "\nVal = " << atoi(res.c_str()) << std::endl;
  return (0);
}