Substring of a std::string in utf-8? C++11

Question

I need to get a substring of the first N characters in a std::string assumed to be utf8. I learned the hard way that .substr does not work... as... expected.

Reference: My strings probably look like this: mission:\n\n1億2千万匹

Answer 1

I found this code and am just about to try it out.

std::string utf8_substr(const std::string& str, unsigned int start, unsigned int leng)
{
    if (leng==0) { return ""; }
    unsigned int c, i, ix, q, min=std::string::npos, max=std::string::npos;
    for (q=0, i=0, ix=str.length(); i < ix; i++, q++)
    {
        if (q==start){ min=i; }
        if (q<=start+leng || leng==std::string::npos){ max=i; }

        c = (unsigned char) str[i];
        if      (
                 //c>=0   &&
                 c<=127) i+=0;
        else if ((c & 0xE0) == 0xC0) i+=1;
        else if ((c & 0xF0) == 0xE0) i+=2;
        else if ((c & 0xF8) == 0xF0) i+=3;
        //else if (($c & 0xFC) == 0xF8) i+=4; // 111110bb //byte 5, unnecessary in 4 byte UTF-8
        //else if (($c & 0xFE) == 0xFC) i+=5; // 1111110b //byte 6, unnecessary in 4 byte UTF-8
        else return "";//invalid utf8
    }
    if (q<=start+leng || leng==std::string::npos){ max=i; }
    if (min==std::string::npos || max==std::string::npos) { return ""; }
    return str.substr(min,max);
}

Update: This worked well for my current issue. I had to mix it with a get-length-of-utf8encoded-stdsstring function.

This solution had some warnings spat at it by my compiler:

Answer 2

You could use the boost/locale library to convert the utf8 string into a wstring. And then use the normal .substr() approach:

#include <iostream>
#include <boost/locale.hpp>

std::string ucs4_to_utf8(std::u32string const& in)
{
    return boost::locale::conv::utf_to_utf<char>(in);
}

std::u32string utf8_to_ucs4(std::string const& in)
{
    return boost::locale::conv::utf_to_utf<char32_t>(in);
}

int main(){

  std::string utf8 = u8"1億2千万匹";

  std::u32string part = utf8_to_ucs4(utf8).substr(0,3);

  std::cout<<ucs4_to_utf8(part)<<std::endl;
  // prints : 1億2
  return 0;
}