# A Little C++
\index{C++}

This chapter introduces you to some useful insights into C++, to allow
you to use some of the most advanced features in ROOT. It is in no case a
full course in C++.

## Classes, Methods and Constructors


C++ extends C with the notion of a class. If you're used to structures
in C, a class is a `struct` that is a group of related variables,
which is extended with functions and routines specific to this
structure (class). What is the interest? Consider a `struct` that is defined this way:

``` {.cpp}
   struct Line {
      float x1;
      float y1;
      float x2;
      float y2;
   }
```

This structure represents a line to be drawn in a graphical window.
`(x1,y1)` are the coordinates of the first point, `(x2,y2)` the
coordinates of the second point. In the standard C, if you want to
draw effectively such a line, you first have to define a structure and
initialize the points (you can try this):

``` {.cpp}
   Line firstline;
   firstline.x1 = 0.2;
   firstline.y1 = 0.2;
   firstline.x2 = 0.8;
   firstline.y2 = 0.9;
```

This defines a line going from the point `(0.2,0.2)` to the point
`(0.8,0.9)`. To draw this line, you will have to write a function, say
`LineDraw(Line l)` and call it with your object as argument:

``` {.cpp}
   LineDraw(firstline);
```

In C++, we would not do that. We would instead define a class like this:

``` {.cpp}
   class TLine {
      Double_t x1;
      Double_t y1;
      Double_t x2;
      Double_t y2;
      TLine(int x1, int y1, int x2, int y2);
      void Draw();
   }
```

Here we added two functions, that we will call methods or member
functions, to the **`TLine`** class. The first method is used for
initializing the line objects we would build. It is called a
constructor. The second one is the `Draw` method itself. Therefore, to
build and draw a line, we have to do:

``` {.cpp}
   TLine l(0.2,0.2,0.8,0.9);
   l.Draw();
```

The first line builds the object `l `by calling its constructor. The
second line calls the **`TLine`**`::Draw()` method of this object. You
don't need to pass any parameters to this method since it applies to
the object l, which knows the coordinates of the line. These are
internal variables `x1`, `y1`, `x2`, `y2` that were initialized by the
constructor.

## Inheritance and Data Encapsulation


We have defined a **`TLine`** class that contains everything necessary
to draw a line. If we want to draw an arrow, is it so different from
drawing a line? We just have to draw a triangle at one end. It would
be very inefficient to define the class **`TArrow`** from scratch.
Fortunately, inheritance allows a class to be defined from an existing
class. We would write something like:

``` {.cpp}
   class TArrow : public TLine {
      int ArrowHeadSize;
      void Draw();
      void SetArrowSize(int arrowsize);
   }
```

The keyword "`public`" will be explained later. The class **`TArrow`**
now contains everything that the class **`TLine`** does, and a couple
of more things, the size of the arrowhead and a function that can
change it. The Draw method of **`TArrow`** will draw the head and call
the draw method of **`TLine`**. We just have to write the code for
drawing the head!

### Method Overriding


Giving the same name to a method (remember: method = member function
of a class) in the child class (**`TArrow`**) as in the parent
(**`TLine`**) does not give any problem. This is called **overriding**
a method. Draw in **`TArrow`** overrides Draw in **`TLine`**. There is
no possible ambiguity since, when one calls the `Draw()` method; this
applies to an object whose type is known. Suppose we have an object
`l` of type **`TLine`** and an object `a` of type **`TArrow`**. When
you want to draw the line, you do:

``` {.cpp}
   l.Draw();
```

`Draw()` from **`TLine`** is called. If you do:

``` {.cpp}
   a.Draw();
```

`Draw()` from **`TArrow`** is called and the arrow `a` is drawn.

### Data Encapsulation


We have seen previously the keyword "`public`". This keyword means
that every name declared public is seen by the outside world. This is
opposed to "`private`" that means only the class where the name was
declared private could see this name. For example, suppose we declare
in **`TArrow`** the variable `ArrowHeadSize` private.

``` {.cpp}
   private:
      int ArrowHeadSize;
```

Then, only the methods (i.e. member functions) of **`TArrow`** will be
able to access this variable. Nobody else will see it. Even the
classes that we could derive from **`TArrow`** will not see it. On the
other hand, if we declare the method `Draw()` as public, everybody will
be able to see it and use it. You see that the character public or
private does not depend of the type of argument. It can be a data
member, a member function, or even a class. For example, in the case
of **`TArrow`**, the base class **`TLine`** is declared as public:

``` {.cpp}
   class TArrow : public TLine { ...
```

This means that all methods of **`TArrow`** will be able to access all
methods of **`TLine`**, but this will be also true for anybody in the
outside world. Of course, this is true if **`TLine`** accepts the
outside world to see its methods/data members. If something is
declared private in **`TLine`**, nobody will see it, not even
**`TArrow`** members, even if **`TLine`** is declared as a public
base class.

What if **`TLine`** is declared "`private`" instead of "`public`" ?
Well, it will behave as any other name declared private in
**`TArrow`**: only the data members and methods of **`TArrow`** will
be able to access **`TLine`**, its methods and data members, nobody
else. This may seem a little bit confusing and readers should read a
good C++ book if they want more details. Especially since, besides
public and private, a member can be protected. Usually, one puts
private the methods that the class uses internally, like some
utilities classes, and that the programmer does not want to be seen
in the outside world.

With "good" C++ practice (which we have tried to use in ROOT), all
data members of a class are private. This is called data encapsulation
and is one of the strongest advantages of Object Oriented Programming
(OOP). Private data members of a class are not visible, except to the
class itself. So, from the outside world, if one wants to access those
data members, one should use so called "getters" and "setters"
methods, which are special methods used only to get or set the data
members. The advantage is that if the programmers want to modify the
inner workings of their classes, they can do so without changing what
the user sees. The user does not even have to know that something has
changed (for the better, hopefully). For example, in our **`TArrow`**
class, we would have set the data member `ArrowHeadSize` private. The
setter method is `SetArrowSize()`, we do not need a getter method:

``` {.cpp}
   class TArrow : public TLine {
      private:
         int ArrowHeadSize;
      public:
         void Draw();
         void SetArrowSize(int arrowsize);
   }
```

To define an arrow object you call the constructor. This will also
call the constructor of **`TLine`**, which is the parent class of
**`TArrow`**, automatically. Then we can call any of the line or arrow
public methods:

``` {.cpp}
root[] TArrow *myarrow = new TArrow(1,5,89,124);
root[] myarrow->SetArrowSize(10);
root[] myarrow->Draw();
```

## Creating Objects on the Stack and Heap


To explain how objects are created on the stack and on the heap we
will use the `Quad` class. You can find the definition in
`$ROOTSYS/tutorials/quadp/Quad.h` and `Quad.cxx`. The `Quad` class
has four methods. The constructor and destructor, `Evaluate` that
evaluates `ax**2 + bx +c`, and `Solve` which solves the quadratic
equation `ax**2 + bx +c = 0`.

`Quad.h` :

``` {.cpp}
   class Quad {
      public:
         Quad(Float_t a, Float_t b, Float_t c);
         ~Quad();
         Float_t Evaluate(Float_t x) const;
         void Solve() const;
      private:
         Float_t fA;
         Float_t fB;
         Float_t fC;
   };
```

`Quad.cxx`:

``` {.cpp}
   #include <iostream.h>
   #include <math.h>
   #include "Quad.h"

   Quad::Quad(Float_t a, Float_t b, Float_t c) {
      fA = a;
      fB = b;
      fC = c;
   }
   Quad::~Quad() {
      Cout <<"deleting object with coeffts: "<< fA << "," << fB << ","
                                             << fC << endl;
   }
   Float_t Quad::Evaluate(Float_t x) const {
      return fA*x*x + fB*x + fC;
   }
   void Quad::Solve() const {
      Float_t temp = fB*fB - 4.*fA*fC;
      if ( temp > 0. ) {
         temp = sqrt( temp );
         cout << "There are two roots: " << ( -fB - temp ) / (2.*fA)
         << " and " << ( -fB + temp ) / (2.*fA) << endl;
      } else {
         if ( temp == 0. ) {
            cout << "There are two equal roots: " << -fB / (2.*fA)
                                                  << endl;
         } else {
            cout << "There are no roots" << endl;
         }
      }
   }
```

Let us first look how we create an object. When we create an object
by:

``` {.cpp}
root[] Quad my_object(1.,2.,-3.);
```

We are creating an object on the stack. A FORTRAN programmer may be
familiar with the idea; it is not unlike a local variable in a
function or subroutine. Although there are still a few old timers who
do not know it, FORTRAN is under no obligation to save local variables
once the function or subroutine returns unless the SAVE statement is
used. If not then it is likely that FORTRAN will place them on the
stack and they will "pop off" when the RETURN statement is reached. To
give an object more permanence it has to be placed on the heap.

``` {.cpp}
root[] .L Quad.cxx
root[] Quad *my_objptr = new Quad(1.,2.,-3.);
```

The second line declares a pointer to `Quad` called `my_objptr`. From
the syntax point of view, this is just like all the other declarations
we have seen so far, i.e. this is a stack variable. The value of the
pointer is set equal to

``` {.cpp}
new Quad(1.,2.,-3.);
```

`new`, despite its looks, is an operator and creates an object or
variable of the type that comes next, `Quad` in this case, on the
heap. Just as with stack objects it has to be initialized by calling
its constructor. The syntax requires that the argument list follows the
type. This one statement has brought two items into existence, one on
the heap and one on the stack. The heap object will live until the
delete operator is applied to it.

There is no FORTRAN parallel to a heap object; variables either come
or go as control passes in and out of a function or subroutine, or,
like a COMMON block variables, live for the lifetime of the program.
However, most people in HEP who use FORTRAN will have experience of a
memory manager and the act of creating a bank is a good equivalent of
a heap object. For those who know systems like ZEBRA, it will come as
a relief to learn that objects do not move, C++ does not garbage
collect, so there is never a danger that a pointer to an object
becomes invalid for that reason. However, having created an object,
it is the user's responsibility to ensure that it is deleted when no
longer needed, or to pass that responsibility onto to some other
object. Failing to do that will result in a memory leak, one of the
most common and most hard-to-find C++ bugs.

To send a message to an object via a pointer to it, you need to use
the "`->`" operator e.g.:

``` {.cpp}
root[] my_objptr->Solve();
```

Although we chose to call our pointer `my_objptr`, to emphasize that
it is a pointer, heap objects are so common in an object-oriented
program that pointer names rarely reflect the fact - you have to be
careful that you know if you are dealing with an object or its
pointer! Fortunately, the compiler won't tolerate an attempt to do
something like:

``` {.cpp}
root[] my_objptr.Solve();
```

As we have seen, heap objects
have to be accessed via pointers, whereas stack objects can be
accessed directly. They can also be accessed via pointers:

``` {.cpp}
root[] Quad stack_quad(1.,2.,-3.);
root[] Quad *stack_ptr = &stack_quad;
root[] stack_ptr->Solve();
```

Here we have a `Quad` pointer that has been initialized with the
address of a stack object. Be very careful if you take the address of
stack objects. As we shall see soon, they are deleted automatically,
which could leave you with an illegal pointer. Using it will corrupt
and may as well crash the program!

It is time to look at the destruction of objects. A destructor is a
special C++ function that releases resources for (or destroys) an
object of a class. It is the opposite of a constructor that creates the
object of a class when it is called. The compiler will provide a
destructor that does nothing if none is provided. We will add one to
our Quad class so that we can see when it is called. The class names
the destructor but with a prefix \~ which is the C++ one's complement
i.e. bit wise complement, and hence has destruction overtones! We
declare it in the .h file and define it in the `.cxx` file. It does
not do much except print out that it has been called (still a useful
debug technique despite today's powerful debuggers!).

Now run root, load the Quad class and create a heap object:

``` {.cpp}
root[] .L Quad.cxx
root[] Quad *my_objptr = new Quad(1.,2.,-3.);
```

To delete the object:

``` {.cpp}
root[] delete my_objptr;
root[] my_objptr = 0;
```

You should see the print out from its destructor. Setting the pointer
to zero afterwards is not strictly necessary (and Cling does it
automatically), but the object is no more accessible, and any attempt
to use the pointer again will, as has already been stated, cause
grief. So much for heap objects, but how are stack objects deleted? In
C++, a stack object is deleted as soon as control leaves the innermost
compound statement that encloses it. Therefore, it is singularly
futile to do something like:

``` {.cpp}
root[] {  Quad my_object(1.,2.,-3.); }
```

Cling does not follow this rule; if you type in the above line, you
will not see the destructor message. As explained in the Script
lesson, you can load in compound statements, which would be a bit
pointless if everything disappeared as soon as it was loaded! Instead,
to reset the stack you have to type:

``` {.cpp}
root[] gROOT->Reset();
```

This sends the Reset message via the global pointer to the ROOT
object, which, amongst its many roles, acts as a resource manager.
Start ROOT again and type in the following:

``` {.cpp}
root[] .L Quad.cxx
root[] Quad my_object(1.,2.,-3.);
root[] Quad *my_objptr = new Quad(4.,5.,-6.);
root[] gROOT->Reset();
```

You will see that this deletes the first object but not the second. We
have also painted ourselves into a corner, as `my_objptr` was also on
the stack. This command will fail.

``` {.cpp}
root[] my_objptr->Solve();
```

Cling no longer knows what `my_objptr` is. This is a great example of a
memory leak; the heap object exists but we have lost our way to access
it. In general, this is not a problem. If any object will outlive the
compound statement in which it was created then a more permanent
pointer will point to it, which frequently is part of another heap
object. See Resetting the Interpreter Environment in the chapter
"Cling the C++ Interpreter".
