Question

I’ve been given this as a .H file for homework, and I’m tasked with creating the .C to go with it. It’s really simple stuff, I’m sure I’m missing something small.

#ifndef String_H
#define String_H

#include <iostream>

class String
{
public:

  // constructor: initializes String with copy of 0-terminated C-string  
  String(const char *p);

  // destructor (when can shared data be released?)
  ~String();

  // copy constructor (how does this change reference counts?)
  String(const String &x);

  // assignment operator  (how does this change reference counts?)
  String &operator=(const String &x); 

  // return number of non-0 characters in string
  int size() const;

  // return reference count
  int ref_count() const;

  // returns pointer to character array
  const char *cstr() const;

private:

  // data containing character strings
  // shared by Strings when copying/assigning
  struct SharedCString
  {
    char *data; // 0-terminated char array
    int n;      // number of non-0 characters in string
    int count;  // reference count, how many Strings share this object?
  };

  SharedCString *shared;
};

#endif

In my constructor, when I try to set the value of the SharedCString’s count to 1, I get a segmentation fault.

I was trying to pass it using:

   shared->count = 1;

I’m not sure why this doesn’t work.

Answer 1

If you get a segfault on the line

shared->count = 1;

then shared is NULL or is garbage, pointing to memory that your process does not own.

You should do

shared = new SharedCString;

before accessing it.