Hello World

Choosing a Compiler

There are many C++ compilers.

1. GNU compiler collection (GCC): The standard C/C++ compiler provided on most Linux systems. Linux-only.
2. Clang: Clang provides various extensions that can be used to build custom compiler add-ons. Cross-platform. Standard on Mac.
3. Microsoft Visual C++ (MSVC): The standard C/C++ compiler for Windows.
4. Intel Compiler: Sometimes used. Optimizations for intel products.
5. MinGW: Like gcc, except works on Windows.

Generally, we prioritize compiling for Linux systems. This means we typically focus on gcc and occasionally clang. Clang is a more modern compiler with many advanced features and better warnings and error messages. However, we generally focus on gcc.

Introduction

In this example, we will demonstrate how print text to a terminal. One of the main use of printing is for debugging. Tools such as Google Logger, are used to make print statements to help determine where a program crashes, without having to use an expensive debugger all the time.

Setup

Docker

You can get the iowarp container to reduce dependency install times:

git clone https://github.com/iowarp/iowarp-install.git
docker pull iowarp/iowarp-user:latest
cd iowarp-install/docker/user
docker compose up -d  # Only for recent dockers
docker-compose up -d  # Only for older dockers

To connect to the container:

docker exec -it iowarp bash

note

Docker does not save state. So when you build tutorials or create your own code, the data gets lost when the container shuts down.

Linux

git clone https://github.com/grc-iit/grc-tutorial.git

Go to the tutorial:

cd grc-tutorial
export GRC_TUTORIAL=${PWD}
cd ${GRC_TUTORIAL}/cpp/01-cpp-hello-world

C++ Source File

Below is the code in hello_world.cc.

#include <stdio.h>
#include <iostream>

int main() {
  printf("Hello world 0\n");
  std::cout << "hello world 1" << std::endl;
  return 0;
}

We will describe this step-by-step.

C++ Header Files

#include <stdio.h>
#include <iostream>

#include is used to instruct the C++ compiler where to find header files. In our case, the header files being included are "stdio.h" and "iostream". Header files typically contain the definitions of functions. They have the same syntax as a source file, but they don't typically have as much code logic.

To find header files, compilers will search a number of places. For example, "/usr/include" will typically contain "stdio.h" and "/usr/include/c++" may contain iostream. These paths are subject to change depending on the gcc version and OS.

In our case, "stdio.h" is where printf is defined, and "iostream" is where std::cout is defined.

Program start

int main() {
  // code here...
}

The main function is required in order to execute a C++ program. When you launch a program, the main function is where the code execution begins.

Console output

printf("Hello world 0\n");
std::cout << "hello world 1" << std::endl;

1. Text is always in double quotes ""
2. "\n" in "Hello world 0" means "new line". Analogous to hitting the Enter key on your keyboard in a text editor.
3. std::cout uses left shift operator (<<) to print to console
4. std::endl is equivalent to '\n'
5. printf takes the text as a function parameter

Both printf and cout are valid ways of printing. Typically cout is preferred in C++ programs, but you'll likely see both in example programs.

Program return

int main() {
  // code here...
  return 0;
}

Main returns an integer "int". This indicates the success or failure of a program. Generally, a return value of "0" indicates the program succeeded. Any other value indicates a failure and the reason for failure.

Building

We will build this code manually using gcc. In general, building things manually is a bad idea. Build tools (CMake nowadays for C/C++) automate much of the process. However, the knowledge of how the compiler is is helpful.

Here we will use gcc to compile the program "hello_world.cc".

g++ hello_world.cc -o hello_world

This will produce an executable file called "hello_world". To run the code:

./hello_world

The output will be:

Hello world 0
hello world 1

To view the return code of the program in Linux:

echo $?

The output will be:

0

which is what we returned using the return statement.