C++ UTF-8 codepoint conversion

#include <iostream>
#include <sstream>
 
using namespace std;
 
int codepoint(const string &u);
string codepointhex(const string &u);
string utf8chr(int cp);
 
int main(int argc, char *argv[])
{
 
    for(int i=32; i<127; i++) //printable ascii range
    {
        cout << "i" << i << ":" <<utf8chr(i) << endl;
    }
    for(int i=192; i<382; i++) // À to ž
    {
        cout << "i" << i << ":" <<utf8chr(i) << endl;
    }
    for(int i=0x4f60; i<0x4f80; i++) // 你 to 使
    {
        cout << "i" << i << ":" <<utf8chr(i) << endl;
    }
 
    string input0 = "A"; //A is ascii 65
    string input1 = "\xc3\xa8"; // è
    string input2 = "\xe4\xbd\xa0"; //你
    cout << input0 << codepoint(input0) << "," << codepointhex(input0) << endl; //65,0x41
    cout << input1 << codepoint(input1) << "," << codepointhex(input1) << endl; //232,0xe8
    cout << input2 << codepoint(input2) << "," << codepointhex(input2) << endl; //20320,0x4f60
 
    return 0;
}
 
int codepoint(const string &u)
{
    int l = u.length();
    if (l<1) return -1; unsigned char u0 = u[0]; if (u0>=0   && u0<=127) return u0;
    if (l<2) return -1; unsigned char u1 = u[1]; if (u0>=192 && u0<=223) return (u0-192)*64 + (u1-128);
    if (u[0]==0xed && (u[1] & 0xa0) == 0xa0) return -1; //code points, 0xd800 to 0xdfff
    if (l<3) return -1; unsigned char u2 = u[2]; if (u0>=224 && u0<=239) return (u0-224)*4096 + (u1-128)*64 + (u2-128);
    if (l<4) return -1; unsigned char u3 = u[3]; if (u0>=240 && u0<=247) return (u0-240)*262144 + (u1-128)*4096 + (u2-128)*64 + (u3-128);
    return -1;
}
 
string codepointhex(const string &u)
{
    stringstream ss;
    string s;
    ss << showbase << hex << codepoint(u);
    ss >> s;
    return s;
}
 
 
string utf8chr(int cp)
{
    char c[5]={ 0x00,0x00,0x00,0x00,0x00 };
    if     (cp<=0x7F) { c[0] = cp;  }
    else if(cp<=0x7FF) { c[0] = (cp>>6)+192; c[1] = (cp&63)+128; }
    else if(0xd800<=cp && cp<=0xdfff) {} //invalid block of utf8
    else if(cp<=0xFFFF) { c[0] = (cp>>12)+224; c[1]= ((cp>>6)&63)+128; c[2]=(cp&63)+128; }
    else if(cp<=0x10FFFF) { c[0] = (cp>>18)+240; c[1] = ((cp>>12)&63)+128; c[2] = ((cp>>6)&63)+128; c[3]=(cp&63)+128; }
    return string(c);
}