Re: l-values and r-values

I want to ask another question that related the topic.

One of the important difference between C and C++ is
prefix ++ operator. The operator doesn't yield an l-value
in C but in C++ it yields a l-value.

I tried to overload the prefix ++ oprator for that class;

class MyInt
{
   int m_i;

   friend ostream & operator<< (ostream &, const MyInt &);

public:

   MyInt(int i)
       :
       m_i(i)
   {}

   MyInt(const MyInt & r)
   {
       m_i = r.m_i;
   }

   MyInt & operator+= (const MyInt & anoth)
   {
       m_i += anoth.m_i;
       return *this;
   }

   MyInt operator++ (int);

   MyInt & MyInt::operator ++()
   {
       return *this += 1;
   }
};

MyInt MyInt::operator++ ( int )
{
   MyInt m_r( *this );
   ++*this;

   return m_r;
}

MyInt operator+ (const MyInt & lefts, const MyInt & rights)
{
   MyInt result = lefts;
   result += rights;
   return result;

   // Or
   // return MyInt( lefts ).operator+= ( rights );
}

int main()
{
   MyInt myi( 5 );

   // I am expecting error bu doesn't happen. Why?
   myi++ = 10;

   cout << myi;
}