Private inheritance got "an inaccessible base" error

Question

Here is MY_STRING class implementation. Since I wish only some of specified interfaces visible I use private inheritance instead of public.

class MY_STRING : private std::string
{
public:
   MY_STRING() : std::string() {}
   MY_STRING(const char* s) : std::string(s) {}
   using std::string::operator +=;
};
int main()
{
    MY_STRING s = "1", x = "2";
    s += x;
}

But I got a compilation error: 'std::basic_string' is an inaccessible base of 'MY_STRING'. Even though I had a ugly solution as follows

const MY_STRING& operator += (const MY_STRING& s) { static_cast<std::string*>
    (this)->operator+=(*static_cast<const std::string*>(&s)); return *this; }

I still wonder why the error arises and if there is more elegant solution.

Answer 1

You can only use std::string::operator+= from within member functions of MY_STRING. Outsiders cannot use it. You can't "promote" it with using or anything.

That's what private inheritance means. Private inheritance is, in effect, the same as having a private member variable, just that syntax is different within the member functions of MY_STRING.

To fix this you will either have to not use private inheritance, or write a forwarding function for those that you want to publish.

Also, your forwarding function seems unnecessarily complicated, try:

MY_STRING &operator+= (MY_STRING const &s)
    { std::string::operator+=(s); return *this; }

You don't want to return a const reference when called on a non-const object (and += doesn't make sense for a const object).