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The "hello world" program

Hello world program written in hello.cpp file just prints Hello World! to the terminal:

#include <stdio.h>

int main() {
    // We are trying to be friendly here!
    puts("Hello World!");
    return 0;
}

If we compile and run it we get the following:

λ › c++ -std=c++17 -o hello hello.cpp
λ › ./hello
Hello World!

Here is what we can learn from it

Let's dig into all of these!

📺 Watch the related YouTube video!

Some words and symbols are more special🦄 than other

  • In the code above, some words and symbols are highlighted
  • This is done by parsing the language into meaningful parts
  • Whitespaces and symbols separate these parts
  • Some highlighted words are built-in and always the same:
    • return always marks an exit point from a function
    • int always represents an "integer number" type
    • See all C++ keywords at cppreference.com
  • Things like "Hello World!" and 0 are literal constants
  • Other change their meaning depending on context, e.g. puts can appear elsewhere in the code and mean a different thing
  • We will see many more of all of these later

Whitespaces mostly play no role

If not inside of string constants like "Hello World!" And not splitting key words or names of entities 🙌

All of the following programs are equivalent:

#include <stdio.h>

int      main() {
puts(   "Hello 🌍!"   )  ;

    return 0;
}
#include <stdio.h>




int main(){puts("Hello 🌍!");
return 0;}
#include <stdio.h>
int main()
{puts(
             "Hello 🌍!"
    );
    return 0;
}
#include <stdio.h>
int main() {             puts
(
"Hello 🌍!");
return
0
;}

Use clang-format tool to deal with this madness!

  • 🤬 People tend to argue (a lot!) about how to style their code
  • 💡 Avoid long discussions by using a tool clang-format!

To style the hello.cpp do the following:

λ › clang-format -i hello.cpp

This rewrites the original hello.cpp file with the styled version of itself

You can also tweak the style!

Just put a .clang-format file into a directory up the tree from your source files, here are the possible settings

A "good" clang-format config

  • 🤔 Different people will have different opinions here
  • ✅ This is the .clang-format file I use for my projects:
---
Language: Cpp
BasedOnStyle: Google
AllowShortBlocksOnASingleLine: true
AllowShortCaseLabelsOnASingleLine: true
AllowShortFunctionsOnASingleLine: All
AllowShortLoopsOnASingleLine: true
AllowShortIfStatementsOnASingleLine: true
BinPackArguments: false
BinPackParameters: false
...
  • 🤝 The most important thing is to have a format file in the project, regardless of which style it enforces

What are all those brackets about?

  • {} are mostly used to:
    • Initialize variables
    • Define "scopes" [stay tuned!]
  • () are mostly used for:
    • Arithmetic operations
    • For writing and calling functions
    • Sometimes for initializing variables [older style]
  • <> are mostly used in:
    • Comparison operations
    • Includes [stay tuned!]
    • Templates [later in the course]

The # is not just for social networks!

  • In the C++ world, a line that starts with a # marks a preprocessor directive, e.g. #include
  • Preprocessor is just a text replacement tool
  • It is the first step of the full compilation process:
    %%{init: {'theme':'neutral', 'themeVariables': { 'fontSize': '30pt'}}}%% graph LR Preprocessor --> Compilation --> Assembly --> Linker
  • We'll talk about the other steps later

An example is worth a 1000 words

  • An #include is just a text substitution
  • Let's compile hello.cpp with a flag --save-temps: 
    λ › c++ -std=c++17 -o hello --save-temps hello.cpp
  • This keeps a bunch of files, we care about hello.ii: 
    λ › tail -n 4 hello.ii
int main() {
    puts("Hello 🌍!");
    return 0;
}
  • This file ends with our program
  • Above our code there is the contents of the stdio.h file

Let's get #inclusive

  • There are two ways to use #include (just a convention):
    • #include "file" - searches for the file first in the folder where the current source file lives
    • #include <file> - searches for the file first in the include path [stay tuned]
  • #include <file> and #include "file" simply replace themselves with the content of the file
  • file can be any file with any extension, but usually has extension .h or .hpp

The ; symbol

  • There are multiple types of statements in C++
  • We clearly can't rely on whitespaces to separate them
  • So most (but not all!) statements require a ; at the end
  • Helps separate one statements from another
  • We will learn which statements need a ; in the future
  • For now just prepare to type ; a lot! 😉

No comments 🙊

  • Comments are statements that are relevant only to us
  • Compiler completely ignores them
  • Symbol // comments out the whole line
  • Symbols /* and */ comment out everything between them
  • Let's add comments to our example from before: 
    int main() {
// This is some important comment!
return /* another comment here */ 0;
}
  • Compiling with --save-temps yields hello.ii: 
    int main() {

  return 0;
}

Functions elevator pitch

  • Functions help encapsulate and decouple functionality
  • In the example above, puts is a function that prints a string Hello World! to screen
  • A function like puts can easily be 100s of lines long
  • main is a function too! And a special one at that 🌈🦄
  • Exists in every C++ program - it's where the program starts!
  • We can (and will!) write lots of our own functions
  • Let's write one right now!

The PrintSmth function

  • This is not the most useful function in the world!
  • We also won't talk about a strict definition of what is a function here, we will cover it later
  • We aim at building intuition here!
#include <stdio.h>

void PrintSmth() {
    puts("Smth");
}

int main() {
    PrintSmth();  // This is how to call it!
    return 0;
}

What does it do?
(It's not very useful 🤷)

  • The function just calls puts with the text "Smth"
  • But it is our own function!
  • And we can call it from the main function
  • We have a new keyword here: void
  • void stands for "nothing"
  • In our case it means that the function returns nothing

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