Lightning overview of C++

Overview

Teaching: 5 min
Exercises: 10 min

Questions

• How do I write and execute C++ code?

Objectives

• To write a hello-world C++ program

Setting up your working area

If you completed the Docker pre-exercises you should already have worked through this episode, under Download the docker images for ROOT and python tools and start container, and you will have

• a working directory cms_open_data_root on your local computer
• a docker container with name my_root created with the working directory cms_open_data_root mounted into the /code directory of the container.

Start your ROOT container with

docker start -i my_root

In the container, you will be in the /code directory and it shares the files with your local cms_open_data_root directory.

Your first C/C++ program

Let’s start with writing a simple hello world program in C. First we’ll edit the source code with an editor of your choice.

Note that you will edit the file in a local terminal on your computer and then run the file in the Docker environment. This is because we mounted the cms_open_data_root directory on your local disk such that it is visible inside the Docker container.

Let’s create a new file called hello_world.cc in the cms_open_data_root directory, using your preferred editor.

The first thing we need to do, is include some standard libraries. These libraries allow us to access the C and C++ commands to print to the screen (stdout and stderr) as well as other basic function.

At the very beginning of your file, add these three lines

#include <cstdlib>
#include <cstdio>
#include <iostream>

The first library, cstdlib, you will see in almost every C++ program as it has many of the very basic functions, including those to allocate and free up memory, or even just exit the program.

The second library, cstdio, contains the basic C functions to print to screen, like printf.

The third library, iostream, contains C++ functions to print to screen or write to files.

Usually people will use one or the other of the C or C++ printing functions, but for pedagogical purposes, we show you both.

Every C++ program must have a main function. So let’s define it here. The scope of this function is defined by curly brackets { }. So let’s add

int main() {


    return 0;
}

The int at the beginning tells us that this function will be returning an integer value. At the end of the main function we have return 0, which usually means the function has run successfully to completion.

Warning!

Note that at the end of return 0, we have a semicolon ;, which is how C/C++ programs terminate lines. If you’re used to programming in python or any other language that does not use a similar terminator, this can be tough to remember. If you get errors when you compile, check the error statements for the lack of ; in any of your lines!

For this function, we are not passing in any arguments so we just have the empty ( ) after the main.

This function would compile, but it doesn’t do anything. Let’s print some text to screen. Before the return 0; line, let’s add these three lines.

    printf("Hello world! This uses the ANSI C 'printf' statement\n");

    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;

    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;

The text itself, should explain what they are doing. If you want to learn more about standard error and standard output, you can read more on Wikipedia.

OK! Your full hello_world.cc should look like this.

Full source code file for hello_world.cc

#include <cstdlib>
#include <cstdio>
#include <iostream>


int main() {

    printf("Hello world! This uses the ANSI C 'printf' statement\n");

    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;

    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;

    return 0;

}

This won’t do anything yet though! We need to compile the code, which means turning this into machine code. To do this, we’ll use the GNU C++ compiler, g++. Once you have saved your file, go to the container shell, make sure (e.g. with ls -l) that you your file is in the current directory and type this in your shell.

g++ hello_world.cc -o hello_world

This compiles your code to an executable called hello_world. You can now run this by typing the following on the shell command line, after which you’ll see the subsequent output.

./hello_world

Hello world! This uses the ANSI C 'printf' statement
Hello world! This uses the C++ 'iostream' library to direct output to standard out.
Hello world! This uses the C++ 'iostream' library to direct output to standard error.

When you are working with the Open Data, you will be looping over events and may find yourself making selections based on certain physics criteria. To that end, you may want to familiarize yourself with the C++ syntax for loops and conditionals.

Key Points

• We must compile our C++ code before we can execute it.