#include <iostream>
#include <codecvt>
#include <string>
#include <locale>

std::string cutString(const std::string& in, size_t len)
{
    std::wstring_convert<std::codecvt_utf8<wchar_t>> cvt;
    auto wstring = cvt.from_bytes(in);
    if(len < wstring.length())
    {
        wstring = wstring.substr(0,len);
        return cvt.to_bytes(wstring);
    }    
    return in;
}
int main(){
    std::string test = "\xe4\xbd\xa0\xe5\xa5\xbd\xe4\xb8\x96\xe7\x95\x8c"; //&#20320;&#22909;&#19990;&#30028; length 4
    std::cout << test << '\n' << cutString(test,2) << '\n';
    return 0;
}

std::wstring_convert<std::codecvt_utf8<wchar_t>> cvt;
auto res = cvt.from_bytes(in);
wstring = wstring.substr(0,len);
return cvt.to_bytes(res);

 if(len < wstring.length())