(pointers)=
# Pointers: A too-short explanation (for those who don't know C++ or C) 
**(5 minutes)**

On the previous page we used the pointer symbol "->" (a dash followed
by a greater-than sign) instead of the period "." to issue the
commands to the `TTree`. This is because the variable **`tree1`** isn't
really the `TTree` itself; it's a "pointer" to the `TTree`.

The detailed difference between an object and a pointer in C++ (and
ROOT) is beyond the scope of this tutorial. I strongly suggest that you
[look this up](https://www.cplusplus.com/doc/tutorial/pointers/) in any
introductory text on C++. For now, I hope it's enough to show a couple
of examples:

    [] TH1D hist1("h1","a histogram",100,-3,3)

This creates a new histogram in ROOT, and the name of the histogram
"object" is **`hist1`**. I must use a period to issue commands to the
histogram:

    [] hist1.Draw()

Here's the same thing, but using a pointer instead:

    [] TH1D *hist1 = new TH1D("h1","a histogram",100,-3,3)

Note the use of the asterisk "*" when I define the variable, and the
use of the C++ keyword `new`. In this example, **`hist1`** is not a
'object,' it's a 'pointer' to the location in computer memory where the
object **`hist1`** is stored. I must use the pointer syntax to issue
commands:

    [] hist1->Draw()

Take another look at the file `c1.C` that you created in a previous
example. ROOT uses pointers for almost all the code it creates. {ref}`As I
mentioned <differences>` previously, ROOT
automatically creates variables when it opens files in interactive mode;
these variables are always pointers.

It's a little harder to think in terms of pointers than in terms of
objects. But you have to use pointers if you want to use the C++ code
that ROOT creates for you

You also have to use pointers to take advantage of [object
inheritance](https://www.programiz.com/cpp-programming/inheritance)
and [polymorphism](https://www.w3schools.com/cpp/cpp_polymorphism.asp)
in C++. ROOT relies heavily on object inheritance (some would say too
heavily), and this is often reflected in the code it generates.

:::{figure-md} pointers-fig
:class: align-center

<img src="https://imgs.xkcd.com/comics/pointers.png" alt="xkcd pointers" width="50%">

<https://xkcd.com/138/> by Randall Munroe
:::