I have to construct a class for strings. C++ already has one, but my assignment is a home-made version. I am very lost as to how to make the constructor work. I have toyed with a few things, but I was given a header file to follow and I have no clue what to do. Any help would be appreciated.

Code:
class String
{
  private:

    unsigned Capacity;  // Number of memory locations reserved
    unsigned Length;    // Number of memory locations in use
    char * Mem;         // Pointer to memory to hold characters

  public:

    // Construct empty string
    //
    String()
    {
      Mem = NULL;
      Capacity = 0;
      Length = 0;
    }
I am clueless as to what to do. The constructor seems to be giving me the most trouble, I am not even sure where to start. All the other parts of the assignment seem possible, but the constructor is confusing me with the Capacity, Length, and * Mem.

Please help with comments, explanations, example code, or anything you think that would be helpful.