More About Classes

Friends and Members

A motivating example

Suppose we wanted to write a function to compare two Fraction objects. An intuitive function call might be the following:

  Fraction f1(1,2);
  Fraction f2(2,4);

  if ( Equals(f1, f2) )		// compare two fraction objects
     cout << "The fractions are equal";

To achieve this call, we would need to build a function with the following prototype:

  bool Equals(Fraction x, Fraction y);

Notice that this would not be a member function of class Fraction. The sample call did not use the dot-operator. Here's a possible definition of the function:

  bool Equals(Fraction x, Fraction y)
  {
    if (x.GetNumerator() * y.GetDenominator() == y.GetNumerator() * x.GetDenominator() )
       return true;
    else
       return false;
  }

Note that this algorithm results from finding a common denominator (multiply the two denominators together), adjusting the numerators, then comparing the numerators (integers). Also notice that this function calls upon the accessor functions from the Fraction class -- GetNumerator() and GetDenominator() -- because it is not a member function. It it outside the Fraction class.

Analyzing the situation

Because this function is performing an operation on two Fraction objects, it is probably being written by the author of the Fraction class. As such, it would be easier (and more efficient in run time) to write it this way:

  bool Equals(Fraction x, Fraction y)
  {
    if (x.numerator * y.denominator == y.numerator * x.denominator )
       return true;
    else
       return false;
  }

However, because this is an outside function, it does not have access to private data, so this would be illegal, as is.

An alternative -- the keyword `friend`

The keyword friend allows a class to grant full access to an outside entity
- By "full access", we mean access to all the class' members, including the private section
- An outside entity can be a function, or even another class (we'll focus on functions for now)
To grant friend status, declaration of the "friend" is made inside the class definition block, with the keyword friend in front of it
- A friend is neither public nor private, because by definition it is not a member of the class. Just a friend. So it does not matter where in the block it is placed.
A friend function to a class will have full access to the private members of the class. So, for example, the second definition of Equals() above would be legal
Here's a version of the Fraction class with some friend functions
- This example contains the Equals() function given above
- This example also defines an Add() function, as a friend, for adding two Fractions together and returning a result
- Includes a sample driver program that makes test calls to Equals() and Add()

Member function instead of `friend`

When a funtion works on two objects, it's often convenient to pass both as parameters and make it a friend
Another option is to use a member function -- but one of the objects must be the calling object
Example: The Equals() function could have been set up as a member function, which would mean this kind of call:
```
  if ( f1.Equals(f2) )		
     cout << "The fractions are equal";
```
This would be a member function. One object is the calling object. The other object is passed as a parameter. This would be the corresponding definition. (Compare it to the friend version).
```
  bool Fraction::Equals(Fraction f)
  {
    if (numerator * f.denominator == f.numerator * denominator )
       return true;
    else
       return false;
  }
```
Here's the Fraction class with Equals() and Add() as member functions
- Includes a sample driver program that makes test calls to Equals() and Add()

Different programmers may have different preferences. Here's a comparison of the calls, side-by-side:

  f3 = f1.Add(f2);		// call to member function version
  f3 = Add(f1, f2);		// call to non-member function version

Also note that the member function version is not necessarily equivalent to the friend function:
```
  Fraction Add(Fraction f);				// member
  friend Fraction Add(Fraction f1, Fraction f2);	// friend
```
- Notice that the parameters are pass-by-value, so copies of f1 and f2 are used in the friend function. Changes will not affect the original objects.
- In the member function, the calling object could be changed by the function.
- We'll see how to adjust for this shortly

Conversion Constructors

Recall how some of the built-in types allow automatic type conversions, like this:

  int x = 5;
  double y = 4.5, z = 1.2;
  y = x;			// legal, via automatic conversion
  z = x + y;			// legal, using automatic conversion

Automatic type conversions can also be set up for classes, via a conversion constructor

A conversion constructor is a constructor with one parameter
- By default, the compiler will use such a constructor to enact automatic type conversions from the parameter type to the class type.
- Example:
```
   Fraction(int n);		// can be used to convert int to Fraction
				// suppose it initializes to n/1
 
```

Constructors can be invoked explicitly to create Fraction objects. Treat the constructor call as if it is returning an object of the constructed type. The conversion constructor will be invoked automatically when type conversion is needed:

  Fraction f1, f2;	// create simple fraction objects
  f1 = Fraction(4);	// explicit call to constructor. Fraction 4/1 is
			// created and assigned to f1
  f2 = 10;		// implicit call to conversion constructor
			//  equivalent to:  f2 = Fraction(10);
  f1 = Add(f2, 5);	// uses conversion constructor to turn 5 into 5/1

In our previous Fraction class examples, the constructor with two parameters has an optional parameter, so it counts as a constructor with one parameter as well. (And is therefore a conversion constructor):
```
  Fraction(int n, int d = 1);
```
In the event that a constructor with a single parameter should not be used for type conversions, this feature can be supressed by using the keyword explicit on the constructor declaration. (This means that only explicit calls can be made)
```
   explicit Fraction(double d); // will NOT be used for automatic conversions
```
This version of class Fraction illustrates usage of the conversion constructor in sample calls in the driver program

Using `const` in classes

Remember that const can be used in a variety of places. And everywhere it is applied in code, it:

Expresses an intent on the part of the programmer, which the compiler enforces (i.e. something is not allowed to change, within some scope)
Expresses the intent more clearly to a user (in this case, another portion of code -- i.e. maybe another programmer)
Affects how certain items can be used.
- Sometimes this is the difference between using L-values vs. R-values in calls to functions
- Sometimes this affects what other items can be used (whether they are also const or not)

Revisiting `const` reference parameters

Pass-by-value vs. Pass-by-reference works the same on objects as it does with built-in types
- If an object is passed by value, a copy is made of the object. Any R-value can be sent on the call
- If an object is passed by reference (without const), no copy is made, and only an L-value can be sent on the call
Objects can be passed by const reference, as well. This way, no copy is made (less overhead), but the object cannot be changed through the reference. Since objects are sometimes large, this is often desirable
This example used pass-by-value parameters:
```
  friend Fraction Add(Fraction f1, Fraction f2);
```
We definitely don't want to change the original fractions that were sent in. But to save overhead, we could use const reference parameters:
```
  friend Fraction Add(const Fraction& f1, const Fraction& f2);
```
Since the parameters are const, R-values can be sent in (just like with pass-by-value).

`const` Member Functions

Another use of const is at the end of a member function prototype, like this:
```
      void Func(int x) const;		// const member function
```
This application of const means that the function may not change the calling object itself.
- This can only be done to member functions of a class. (not stand-alone functions).
- This means that the member function will not change the member data of that object.
- If we have an object y, and then this function is called, the object y will remain in the same state (before and after the function call):
```
    Yadda y;		// object y

    y.Func(5);		// this function will not change the data of y 
 
```
When using this feature, the word const must go on the declaration and on the definition (if the definition is separate)
For good class design, it should be used whenever appropriate. This means on any member functions that should not alter the member data of the calling object:
- Constructors -- their job is to initialize the object (i.e. set the member data), so these would not be const functions
- Mutators -- usually Set functions. Their job is to change member data, so these would also not be const
- Accessors -- a pure accessor function's job is to retrieve data from inside the class. These would typically be const member functions
- Functions for printing -- Show() or Display() functions would be good candidates for const, if their only purpose was to print existing data
This version of class Fraction uses const in all appropriate places.
- Note that it uses const reference parameters on the friend functions
- It also uses const on all member functions that don't change the member data
This version of class Fraction is similar to the one above, but with Add and Equals as member functions
- Notice that const is used on Add and Equals so that the calling object will not be changed

Declaring `const` objects

Declaring primitive type variables as const is easy. Remember that they must be initialized on the same line:
```
  const int SIZE = 10;
  const double PI = 3.1415;
```
Objects can also be declared as const. The constructor will always run to intialize the object, but after that, the object's state (i.e. internal data) cannot be changed
```
  const Fraction ZERO;			// this fraction is fixed at 0/1
  const Fraction FIXED(3,4);		// this fraction is fixed at 3/4
```
To ensure that a const object cannot be changed, the compiler enforced the following rule:
- A const object may only call const member functions
So, using the Fraction class example above with const member functions, the following calls are legal:
```
  FIXED.Show();			// calling const functions
  cout << FIXED.Evaluate();
  int n = ZERO.GetNumerator();
  int d = ZERO.GetDenominator();
```
The following calls would be illegal and would cause compiler errors:
```
  FIXED.SetValue(5,7);
  ZERO.Input();
```
Note that in the original version of Fraction (with no const member functions), ALL of these calls would result in compiler errors, even if the function itself didn't change anything (like Show).
Examples:
- A simple class with no const member functions. Note that there's a const object in the sample main() program, and the function calls won't work
- Sample example, but with appropriate const member functions. This time, the calls in main() will work with the const object

`const` Member Data

Member data of a class can also be declared const. This is a little tricky, because of certain syntax rules.
Remember, when a variable is declared with const in a normal block of code, it must be initialized on the same line:
```
  const int SIZE = 10;
```

However, it is NOT legal to intialize the member data variables on their declaration lines in a class definition block:

 class Thing
 {
 public:
    Thing();		// constructor -- intialize member data in here
    // blah blah blah
 private:
    int x;		// just declare here
    int y = 0;		// this would be ILLEGAL!  cannot initialize here
    const int Z = 10;   // would also be ILLEGAL
 };

But a const declaration cannot be split up into a regular code block. This attempt at a constructor definition would also not work, if Z were const:
```
  Thing::Thing()
  {
     x = 0;
     y = 0;
     Z = 10;
  }
```
Solution: When we have lines like this, which involve declaring and initializing in one step and cannot be split up in normal code, we handle it with a special section of a function called the initialization list. Format:
```
  returnType functionName(parameterList) : initialiation_list
  {
     // function body
  }
```
Simple class example that illustrates the initialization of a const member data variable

Destructors

In addition to the special Constructor function, classes also have a special function called a destructor.

Declaration:
The destructor looks like the default constructor (constructor with no parameters), but with a ~ in front. Destructors cannot have parameters, so there can only be one destructor for a class. Example: The destructor for the Fraction class would be:

  ~Fraction();

Like the constructor, this function is called automatically (not explicitly)

A constructor is called automatically when an object is created.
A destructor is called automatically right before an object is deallocated by the system (usually when it goes out of scope).

The destructor's typical job is to do any clean-up tasks (usually involving memory allocation) that are needed, before an object is deallocated. However, like a constructor, a destructor is a function and can do other things, if desired.

Compile and run this example to see when constructors and destructors are invoked.

More About Classes

Friends and Members

A motivating example

Analyzing the situation

An alternative -- the keyword friend

Member function instead of friend

Conversion Constructors

Using const in classes

Revisiting const reference parameters

const Member Functions

Declaring const objects

const Member Data