A “Hello World!” Program in C++

A “Hello World!” Program in C++#

Introduction#

Now that you hopefully have access to a working compiler, and a place to write, we’ll move on to giving you some practical code to compile. A common first program for people to code in a new programming language is to write just enough code to print the phrase “Hello World!” to the screen (or other output device) and then exit.

The popularity of this tradition actually dates back to the beginnings of the C programming language in 1978 and the book “The C Programming Language” by Brian Kernighan and Dennis Ritchie.

Let’s compare some simple implementations in Python, C and C++.

Hello World in Python#

By now, you should all know enough about Python scripts that you can write a quick implementation of the “Hello World!” program. This will probably look something like the following:

hello.py:

    print("Hello world!")

Here we use the builtin Python print function to print to screen, and can either write "Hello World!" or 'Hello World!' to create our string to print.

Hello World in C#

A C based hello world function might look something like the following:

hello.c:

    #include <stdio.h>

    int main(void) {
        printf("Hello World!\n");
        return 0;
    }

Unlike Python, we need to use a function from outside the core language, in this case the function printf from the standard library stdio, in order to print our message to screen. At the compilation stage we do this with command #include, a C preprocessor directive, which in practice works very similarly to the import statement in Python by pulling other code into our work.

Where Python runs through commands beginning at the top of a script file, C programs (at least, those printing to screen in the terminal) start at the beginning of the special main function. This function usually returns an integer, where 0 is taken to mean that things worked successfully, while other values are generally taken as a sign that something went wrong.

Finally, in C/C++ we must use the double quotes " to indicate a string (the ' character won’t work, and is only used with single characters), and since the printf function doesn’t end lines automatically, we must do it ourselves using the special end-of-line \n “escape” character.

Hello World in C++#

A canonical C++ based hello world function might look something like the following:

hello.cpp:

    #include <iostream>

    int main() {
        std::cout << "Hello World!" << std::endl; 
        return 0;
    }

You’ll see that in this case, the code looks very similar to the C example. In fact, most C code is valid C++ code (possibly with a few small changes). The only major thing that looks different is that rather than using a function like printf we are using the special << operator (i.e. a token which works a bit like + or -) to stream our message to std::cout, an “output stream” which in this case represents printing to the screen. We could have used "\n" again to end the line, but in C++ we can also use std::endl to do the same thing, as well as flushing the output buffer (which can be useful in some cases).

Some key differences from Python#

Whitespace in your code doesn’t matter (although it still makes things easier to read when code is nicely formatted). Instead {} are used to define blocks of code.
Individual statements must end with a ;. It’s easy to forget this at first, but the compiler will remind you (often in a way that’s hard to understand until you are used to it).
Rather than starting at the top of the file, the code starts running at the top of the main function. There can only be one main function in a given programme.
Functions (and variables) must have a fixed data type associated with them the first time they are declared.

Exercise: Run the Hello World programs#

Run the three different programs in Python, C and C++. You already know how to run Python scripts. You can look back to the previous section to see how to run C & C++ codes.

Once you can run the programs, try using the variable based version to try changing the message:

What happens if you leave out the \n instruction in the C example?
Can you add a second line of text, first in the same call to printf/std::cout, then by adding a second one?

Summary#

In this section we’ve seen an example of one of the shortest possible C++ programs, and run it to produce output. In the next section we’ll start to apply C++ to some numerical problems.