C++ UTF-8 substr function

#include <iostream>
 
using namespace std;
 
string utf8_substr(const string& str, unsigned int start=0, unsigned int leng=string::npos);
 
int main(int argc, char *argv[])
{
    string str = string("ni hao");
    string ni_hao = string("\xe4\xbd\xa0\xe5\xa5\xbd\xe4\xb8\x96\xe7\x95\x8c");
 
    cout << utf8_substr(ni_hao) << endl; 
    cout << utf8_substr(ni_hao,0) << endl;
    cout << utf8_substr(ni_hao,1,0) << endl;
    cout << utf8_substr(ni_hao,0,2) << endl;
    cout << utf8_substr(ni_hao,2,2) << endl;
    return 0;
}
 
string utf8_substr(const string& str, unsigned int start, unsigned int leng)
{
    if (leng==0) { return ""; }
    unsigned int c, i, ix, q, min=string::npos, max=string::npos;
    for (q=0, i=0, ix=str.length(); i < ix; i++, q++)
    {
        if (q==start){ min=i; }
        if (q<=start+leng || leng==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==string::npos){ max=i; }
    if (min==string::npos || max==string::npos) { return ""; }
    return str.substr(min,max);
}

string utf8_substr2(const string &str,int start, int length=INT_MAX)
{
    int i,ix,j,realstart,reallength;
    if (length==0) return "";
    if (start<0 || length <0)
    {
        //find j=utf8_strlen(str);
        for(j=0,i=0,ix=str.length(); i<ix; i+=1, j++)
        {
            unsigned char c= str[i];
            if      (c>=0   && c<=127) i+=0;
            else if (c>=192 && c<=223) i+=1;
            else if (c>=224 && c<=239) i+=2;
            else if (c>=240 && c<=247) i+=3;
            else if (c>=248 && c<=255) return "";//invalid utf8
        }
        if (length !=INT_MAX && j+length-start<=0) return "";
        if (start  < 0 ) start+=j;
        if (length < 0 ) length=j+length-start;       
    }
 
    j=0,realstart=0,reallength=0;
    for(i=0,ix=str.length(); i<ix; i+=1, j++)
    {
        if (j==start) { realstart=i; }
        if (j>=start && (length==INT_MAX || j<=start+length)) { reallength=i-realstart; }
        unsigned char c= str[i];
        if      (c>=0   && c<=127) i+=0;
        else if (c>=192 && c<=223) i+=1;
        else if (c>=224 && c<=239) i+=2;
        else if (c>=240 && c<=247) i+=3;
        else if (c>=248 && c<=255) return "";//invalid utf8
    }
    if (j==start) { realstart=i; }
    if (j>=start && (length==INT_MAX || j<=start+length)) { reallength=i-realstart; }
 
    return str.substr(realstart,reallength);
}