C++ "darkstring", own implemented string manipulation

#include "../str.h"
#include <iostream>
class darkstring
{
  friend darkstring & operator +=( darkstring & lhs, char const* rhs );
  friend darkstring  operator +( darkstring const & lhs, char const* rhs );
  friend darkstring  operator +( darkstring const & lhs, darkstring const & rhs );
  friend darkstring  operator +( char const * lhs, darkstring const & rhs );
  friend std::ostream & operator <<( std::ostream &  lhs,  darkstring const & rhs );
  private:
    char *dstr;
    unsigned int dstrlength;
  public:
    darkstring(const char* str)
    {
      int i=0,length=mystrlen(str);
      dstrlength=length;
      dstr=new char[length+1];
      while(i < length)
      {
        dstr[i]=str[i];
        i++;
      }
      dstr[i]='\0';
    }
    unsigned int size() const
    {
      return dstrlength;
    }
};
darkstring & operator+=(darkstring &  lhs,  char const* rhs )
{
  int newLength=lhs.size() + mystrlen(rhs), lhs_size=lhs.size();
  int i=0,y=0;
  char *newSTR=new char[newLength];
  while(i<lhs_size)
  {
    newSTR[i]=lhs.dstr[i];
    i++;
  }
  while(i<newLength)
  {
    newSTR[i]=rhs[y];
    y++;
    i++;
  }
  //Nullbyte
  newSTR[i]='\0';
  delete[](lhs.dstr);
  lhs.dstr=newSTR;
  return lhs;
}
std::ostream & operator<<(std::ostream &  lhs,  darkstring const & rhs )
{
  return lhs<<rhs.dstr;
}
darkstring  operator +( darkstring const & lhs, char const* rhs )
{
  darkstring newSTR=lhs.dstr;
  newSTR+=rhs;
  return newSTR;
}
darkstring  operator +( darkstring const & lhs, darkstring const & rhs )
{
  darkstring newSTR=lhs.dstr;
  newSTR+=rhs.dstr;
  return newSTR;
}
darkstring  operator +( char const * lhs, darkstring const & rhs )
{
  darkstring newSTR=lhs;
  newSTR+=rhs.dstr;
  return newSTR;
}
int main(int argc, char** argv)
{
  darkstring str="Hallo ";
  str +=" Welt";
  std::cout << "----Referenz links------"<<std::endl;
  std::cout << str+"--!!--!!" << std::endl;
  std::cout << "---Beides Referenzen----"<<std::endl;
  std::cout << str+str << std::endl;
  std::cout << "---Referenz rechts------"<<std::endl;
  std::cout << "!!--!!--"+str<<std::endl;
}