How to implement safe copy constructor for class that has internal placement new (with std::string)

Question

Here's my code:

struct RS_Token
{
    char id;
    char cleanup;
    unsigned char array[sizeof (std::string) > sizeof (double) ? sizeof (std::string) : sizeof (double)];

    RS_Token(int a) :
        id(a),
        cleanup(0)
    {
    }
    RS_Token(int a, const char* pstr) : // identifier or text
        id(a),
        cleanup(1)
    {
        new (array) std::basic_string<unsigned char>((unsigned char*)pstr);
    }
    RS_Token(int a, int b) : // integer
        id(a),
        cleanup(0)
    {
        new (array) int(b);
    }
    RS_Token(int a, double b) : // float (double)
        id(a),
        cleanup(0)
    {
        new (array) double(b);
    }

    ~RS_Token()
    {
        if (cleanup)
        {
            std::basic_string<unsigned char>* p = reinterpret_cast<std::basic_string<unsigned char>*>(array);

            p->~basic_string();
        }
    }
};

Any suggestions on how to add a copy constructor that properly handles the case where a std::string has been allocated internally, would be appreciated.

Answer 1

I'm not sure that what you're doing is at all a good design, but to answer your question about placement-new: You provide the constructor arguments just like in any other new expression:

Construct new string:

typedef std::basic_string<unsigned char> ustring;

RS_Token(const char* pstr)
{
  void * p = static_cast<void*>(array);
  new (p) ustring(pstr, pstr + std::strlen(pstr));
}

Copy-construct:

RS_Token(const RS_Token & other)
{
  void * p = static_cast<void*>(array);
  new (p) ustring(*reinterpret_cast<const ustring *>(other.array));
}

Assign:

RS_Token & operator=(const RS_Token & other)
{
  ustring & s = *reinterpret_cast<ustring *>(array);
  s = *reinterpret_cast<const ustring *>(other.array);
  return this;
}