C++ - copy-on-write basic implementation - c++

Question

We are supposed to use copy-on-write on our school project. I've been experimenting with a very simple class, but without any luck. I have this:
#include <iostream>
#include <cstdlib>
#include <cstring>
using namespace std;
class CPerson {
public:
CPerson ();
CPerson (const CPerson&);
~CPerson ();
char* m_name;
char* m_surname;
int m_refs;
void rename (const char*, const char*);
};
CPerson :: CPerson () : m_name(NULL), m_surname(NULL), m_refs(1) {}
CPerson :: CPerson (const CPerson& src) : m_name (src.m_name), m_surname (src.m_surname), m_refs(src.m_refs+1) {} // supposed to be a shallow copy
CPerson :: ~CPerson () {
if (m_refs == 1) {
delete [] m_name;
delete [] m_surname;
}
else --m_refs;
}
void CPerson :: rename (const char* name, const char* surname) {
delete [] m_name;
delete [] m_surname;
m_name = new char [strlen(name)+1];
m_surname = new char [strlen(surname)+1];
strcpy (m_name, name);
strcpy (m_surname, surname);
}
int main () {
CPerson a;
a.rename ("Jack", "Smith");
cout << a.m_name << " " << a.m_surname << endl;
CPerson b(a);
cout << a.m_name << " " << a.m_surname << endl;
cout << b.m_name << " " << b.m_surname << endl;
// good so far...
a.rename ("John", "Anderson"); // should rename both 'a' and 'b'
cout << a.m_name << " " << a.m_surname << endl;
cout << b.m_name << " " << b.m_surname << endl;
// prints random values
return 0;
}
It is strange because when I take out the couts, everything works OK (no leaks, no errors by valgrind).
Any help would be appreciated.

Answer 1

Your design is flawed. You need to implement the reference count on the character data itself, not on the individual CPerson objects, as they are not sharing a single reference count variable with each other.
Try something more like this instead:
#include <iostream>
#include <string>
using namespace std;
struct SPersonData
{
string m_name;
string m_surname;
int m_refcnt;
SPersonData() : m_refcnt(0) {}
void incRef() { ++m_refcnt; }
void decRef() { if (--m_refcnt == 0) delete this; }
};
class CPerson
{
private:
SPersonData *m_data;
public:
CPerson ();
CPerson (const CPerson&);
~CPerson ();
CPerson& operator= (const CPerson&);
string getName() const;
string getSurname() const;
void rename (const string&, const string&);
};
CPerson::CPerson ()
: m_data(NULL) {}
CPerson::CPerson (const CPerson& src)
: m_data (src.m_data)
{
if (m_data) m_data->incRef();
}
CPerson::~CPerson ()
{
if (m_data) m_data->decRef();
}
CPerson& operator= (const CPerson &src)
{
if (this != &src)
{
if (m_data) m_data->decRef();
m_data = src.m_data;
if (m_data) m_data->incRef();
}
return *this;
}
string CPerson::getName() const
{
if (m_data) return m_data->m_name;
return string();
}
string CPerson::getSurname() const
{
if (m_data) return m_data->m_surname;
return string();
}
void CPerson::rename (const string &name, const string &surname)
{
if ((m_data) && (m_data->m_refcnt > 1))
{
m_data->decRef();
m_data = NULL;
}
if (!m_data)
{
m_data = new SPersonData;
m_data->incRef();
}
m_data->m_name = name;
m_data->m_surname = surname;
}
Which can be greatly simplified in C++11 and later by using std::shared_ptr to manage the reference count:
#include <iostream>
#include <string>
#include <memory>
using namespace std;
struct SPersonData
{
string m_name;
string m_surname;
};
class CPerson
{
public:
shared_ptr<SPersonData> m_data;
string getName() const;
string getSurname() const;
void rename (const string&, const string&);
};
string CPerson::getName() const
{
if (m_data) return m_data->m_name;
return string();
}
string CPerson::getSurname() const
{
if (m_data) return m_data->m_surname;
return string();
}
void CPerson::rename (const string &name, const string &surname)
{
if (!((m_data) && m_data.unique()))
m_data = make_shared<SPersonData>();
m_data->m_name = name;
m_data->m_surname = surname;
}
Either way, your test would then look like this:
int main ()
{
CPerson a;
a.rename ("Jack", "Smith");
cout << a.getName() << " " << a.getSurname() << endl;
CPerson b(a);
cout << a.getName() << " " << a.getSurname() << endl;
cout << b.getName() << " " << b.getSurname() << endl;
// good so far...
a.rename ("John", "Anderson"); // should rename only 'a' not 'b'
cout << a.getName() << " " << a.getSurname() << endl;
cout << b.getName() << " " << b.getSurname() << endl;
return 0;
}