Time for action – defining member access

Let's take a look at how to specify the access to our class member.

1. Create a new file and save it as book.vala inside the src/ directory. Fill it with this:

   using GLib;
   
   public class Book : Object {
     private string title;
      private string isbn;
   
     public Book(string isbn, string title) {
       this.isbn = isbn;
       this.title = title;
     }
   
     public void printISBN() {
     stdout.printf("%s\n", isbn);
     }
   
     public void printTitle() {
       stdout.printf("%s\n", title);
     }
   }

2. We need to add this to the project. Click on the Project menu and choose Add Source File....
3. In the next dialog, click on the Target option, find hello_vala inside src/, and then, in the file selection box below it, choose book.vala.
4. Modify the main function of hello_vala.vala to look like this:

   using GLib;
   
   public class Main : Object
   {
     public Main ()
     {
       var book = new Book("1234", "A new book");
       book.printISBN ();
     }
   
     static int main (string[] args)
     {
       stdout.printf ("Hello, world\n");  
       var main = new Main();
       return 0;
     }
   }

5. Run it.
6. Note that the program cannot be built.
   

What just happened?

From the error message, we see that it rejects our access to call Book.printISBN (we read this dot notation as the printISBN member from the Book class).

    var book = new Book("1234", "A new book");
    book.printISBN ();

This is what we have in the Main class constructor. There we instantiate Book into the book variable, and call printISBN there.

    void printISBN() {
    stdout.printf(isbn);
    }

However, this is what we have in the Book class. It looks innocent, but it turns out that we missed something crucial that makes this function inaccessible from outside the class.

The access specifiers

Here is a list of access specifiers that are recognized by Vala:

• private: The access is limited within the class or struct.
• public: The access is unlimited
• protected: The access is limited within the class and any class that inherits from the class
• internal: The access is limited within the classes inside the package

When we don't specify anything, the access is set to private by default. That is why our program cannot be built.

Pop quiz – how to fix this?

As mentioned previously, we don't put any specifiers in front of the printISBN function, so it is considered private. We can fix this by putting the correct access specifier in the printISBN function.

Q1. Which specifier from the following options do you think is correct?

1. public, because we want to access it from Main class which is outside the Book class.
2. None; we just need to fix how we call printISBN in the Main constructor.

Basic data types

Let's now move on, learning the basic data types available in Vala, meaning that we will take a look at how to interact with strings, numbers, and Boolean.