Advanced C++ Topics

Chapter 17: Advanced C++ Topics I

Chapter Goals

To learn about operator overloading
To learn how to automatically manage dynamic memory

Operator Overloading

Many classes use operators such as ++, * and ==.
Giving a new meaning to an operator is called operator overloading.

Operator Functions

You can overload an operator by defining a function whose name is operator followed by the operator symbol.
For example, you can define the difference between two Time objects as the number of seconds between them.
```
int operator-(Time a, Time b)
{ return a.seconds_from(b);
}
```
The operator- function is a nonmember function with two parameters.

Now you can use the - operator instead of calling seconds_from.

Time now;
Time morning(9, 0, 0);
long seconds_elapsed = now - morning;

The last statement is equivalent to

long seconds_elapsed = operator-(now, morning);

Syntax 17.1: Overloading Operator Definition

Syntax 17.1 : Overloading Operator Definition

return_type operatoroperator_symbol(parameters)
{
   statements
}

Example:	int operator-(Time a, Time b) { return a.seconds_from(b) }
Purpose:	Supply the implementation of an overloaded operator.

Does it make sense to add two times?

some_return_type operator+(Time a, Time b);

A Time objects represent a point in time, not a duration: what does 3 P.M. + 3 P.M. mean? It does not make any sense to add two points in time!
It does make sense to add a number of seconds to a Time object, resulting in a new Time object.
```
Time operator+(Time a, int sec)
{  Time r = a;
   r.add_seconds(sec);
   return r;
}
```

For example

Time now;
Time later = now + 60; /* 60 seconds later */

Overloading Comparison Operations

A commonly overloaded operator is the == operator, to compare two values.

bool operator==(Time a, Time b)
{  return a.seconds_from(b) == 0;
}

For completeness, it is a good idea to also define a != operator.

bool operator!=(Time a, Time b)
{  return a.seconds_from(b) != 0;
}

You may also find it useful to define a < operator.

bool operator<(Time a, Time b)
{  return a.seconds_from(b) < 0;
}

Input and Output

For many classes, you want to print the object with the familiar << notation.

The output operator takes a parameter of type ostream& (reference, because printing modifies the stream) and the object to be printed.

ostream& operator<<(ostream& out, Time a)
{  out << a.get_hours() << ":";
   if (a.get_minutes() < 10) out << "0";
   out << a.get_minutes() << ":";
   if (a.get_seconds() < 10) out << "0";
   out << a.get_seconds();
   return out;
}

The << operator returns the out stream to enable chaining of the << operator.
```
cout << now << "\n";
```
really means
```
(cout << now) << "\n";
```
that is
```
operator<<(cout, now) << "\n";
```
The call to operator<<(cout, now) returns cout, then cout << "\n" prints a new line.

You can also overload the input operator to read in other types of objects.

istream& operator>>(istream& in, Time& a)
{  int hours;
   int minutes;
   int seconds;
   in >> hours >> minutes >> seconds;
   a = Time(hours, minutes, seconds);
   return in;
}

Note that the >> operator returns the input stream just like the << operator.
Unlike the << operator, the >> operator must have a parameter of Time&.
It is easy to go overboard overloading operators! Using inappropriate operators can make programs more difficult to read.
- Does it make sense to overload *, / or % for Time objects?

Operator Overloading (`overload.cpp`)

Overloading Increment and Decrement Operators

There are actually two forms of the ++ and -- operators.
- A prefix form:
```
++x;
```
- A postfix form:
```
x++;
```

Recall that ++x evaluates to x after the increment, and x++ evaluates to x before the increment.

int i = 0;
int j = 0;
vector<double> s(2);
s[0]=10;
s[1]=20;
double a = s[i++]; /* a is s[0], i is 1 */
double b = s[++j]; /* b is s[1], j is 1 */

In order for the compiler to distinguish between the two versions, the operators must have two different parameter lists.

void operator++(Time& a) /* prefix operator */
{  a.add_seconds(1);
}

void operator++(Time& a, int dummy) /* postfix operator */
{ a.add_seconds(1);
}

The int dummy parameter is not used inside the function, it merely serves to differentiate the two operator++ functions.

Operator Members

Some operators must access the internals of the class and must be member functions.

Member functions use the implicit parameter as the left operand.

A non member operator:

bool operator==(Time a, Time b); /* a == b means operator==(a,b) */

The same operator as a member function.

bool Time::operator==(Time b) const /* a == b means a.operator==(b) */
{  return seconds_from(b) == 0;
}

Also note that the != operator uses the == operator. Each member function has a special parameter variable, called this, which is a pointer to the implicit parameter.

bool Time::operator!=(Time b) const
{ return !(*this == b); /* calls operator== */
}

If the operator is unary, then the member function has no explicit parameter

Time Time::operator++() /* prefix operator */
{ *this = *this + 1; /* calls operator+ */
  return *this;
}

Note the parameter in the postfix version of the ++ operator.

Time Time::operator++(int dummy) /* postfix operator */
{  Time t = *this; 
   *this = *this + 1; /* calls operator+ */
   return t;
}

Operator Overloading (`overload1.cpp`)

Automatic Memory Management

For this discussion, we will use a modification of the Department class that was introduced earlier.

class Department {
   ...
private:
   string name;
   Employee* receptionist;
};

Department::Department(string n, Employee e)
{  name = n;
   receptionist = new Employee(e.get_name(), e.get_salary());
}

/* second constructor */
Department::Department(string n)
{  name = n;
   receptionist = NULL;
}

Destructors

A Department object contains a pointer to an Employee object.

When the Department object is destroyed (goes out of scope, for example) the Employee object is leaked.

In C++, you can define a destructor, a function that is called when an object is about to go out of scope.

The destructor for the Department class should delete the receptionist pointer.

Department::~Department()
{  delete receptionist;
}

Note that calling delete on a NULL pointer is safe, so you don't need a special case for that situation.

Syntax 17.2: Destructor Definition

Syntax 17.2 : Destructor Definition

Class_name::~Class_name()
{
   statements
}

Example:	Department::~Department() { delete receptionist; }
Purpose:	Supply the implementation of a destructor that is invoked whenever and object goes out of scope.

The destructor is automatically invoked (not called directly).

{  Department dept;
   ...
} // dept.~Department() automatically invoked here

...
Department* p = new Department(...);
...
delete p; // p->~Department() automatically invoked here

A class can only have one destructor with no parameters.

You should always supply a destructor when some amount of clean up is required when an object goes out of scope.

Recycling dynamic memory (as shown here).

Close a file.

Relinquish some other resource.

Overloading the Assignment Operator

Consider the following declarations.

Department qc("Qualitiy Control", Employee("Tester, Tina", 50000));
Department dept("Shipping", Employee("Hacker, Harry", 35000));

Suppose we assign one department to the other.

dept = qc;

This assignment causes a memory leak! (See following picture).

When one object goes out of scope (and the other doesn't) the Employee is destroyed, causing a dangling pointer.

The remedy is to overload the operator= to make the assignment safe by deleting the only Employee object, and making a copy of the new Employee object.

Department& Department::operator=(const Department& b)
{  if (this != &b)
   {  name = b.name;
      delete receptionist;
      if (b.receptionist == NULL)
         receptionist == NULL;
      else
         receptionist = new Employee(b.receptionist->get_name(),
            b.receptionist->get_salary());
    }
    return *this;
}

Unlike most other operators, the operator= must be a member function.

Special care must be taken to avoid a destructive "self-assignment."

A return by reference is required to handle chaining of assignments, such as

z = y = x;

You should always overload the = operator if your class has data fields that are pointers, and a simple copy of objects leads to dangerous shared pointers.

Copy Constructor

The purpose of operator= is to set an existing object equal to another object.

Use of = for construction is not always appropriate.

Example: Here, the pointer dept.receptionist is set to a random value. The operator will try to delete the receptionist causing an error.

Department dept = qc; /* is not the assignment operator! */

The copy constructor defines how to construct an object of a class as a copy of another object of the same class.

Department dept(qc);

If you don't define a copy constructor, then the compiler provides a version that simply copies the corresponding data fields of the existing object.

This version of the copy constructor is still inappropriate, leading to the same kind of errors as the default version of the assignment operator.

Here is a valid copy constructor for the Department class.

Department::Department(const Department& b)
{  name = b.name;
   if (b.receptionist == NULL)
      receptionist = NULL;
   else
      receptionist = new Employee(b.receptionist->get_name(),
         b.receptionist->get_salary());
}

The parameter must be const reference because pass by value invokes the copy constructor!

The Big Three

The assignment operator, copy constructor, and destructor are collectively called the "big three."

You must implement all three for any class that manages heap memory.

Each function has the following logic.

Destructor:
- Free all dynamic memory that the object manages.
Copy Constructor:
- Initialize the object as a copy of the explicit parameter object.
Assignment Operator:
- Check whether this == &b. If so do nothing.
- Free the dynamic memory of the object that is no longer needed.
- Set the object as a copy of the explicit parameter object.
- Return *this.