Question

This is a naive example I have coded to study C++ operator overloading. When I execute it, code hangs at the statement c = a + b; and control never reaches c.display();

As part of debugging if I put a cout << ptr << '\n'; in the assignment operator overloaded function it does print out HelloWorld, so string doesn’t seem to be malformed.

Why does it hang then? What am I missing ??

class mystring
{
    char *ptr;

    public:

   mystring(char *str = "")
   {
      ptr = new char[strlen(str) + 1];
      strcpy(ptr,str);

   } 

   mystring operator +(mystring s)
   {
      char *str = new char[strlen(ptr) + strlen(s.ptr) + 1];//where should this memory be freed
      strcpy(str,ptr);
      strcat(str,s.ptr);
      return mystring(str);
   }

   void operator =(mystring s)
   {
       strcpy(ptr,s.ptr);

      //cout << ptr << '\n'; \\Debug - this prints out HelloWorld but still hangs 
   }

   void display()
   {
       cout << ptr << '\n';
   }

   ~mystring()
   {
      delete [] ptr; 
   }
};

int main()
{
   mystring a="Hello",b="World",c;

  c = a + b;

  c.display();

  getchar();

}

EDIT: Compiler: MS-Visual C++ 2010 Express / Windows.

Answer 1

I think that what you are getting is a memory error. This line:

c = a + b;

does the following:

c.constructor()
c.operator=(a.operator+(b));

and your operator= fails to allocate memory

void operator =(mystring s)
{
  // ptr is allocated enough memory for "", i.e. one byte
  strcpy(ptr,s.ptr); // copying more than one byte into one byte array
  //cout << ptr << '\n'; // this works, but you've trashed memory with the strcpy
} // stack might be corrupted here, depends where this is, so execution can end up anywhere

What you need is:

void operator = (mystring &s) // reference!
{
  delete [] ptr;
  ptr = new char [strlen (s.ptr + 1)];
  strcpy (ptr, s.ptr);
}