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Overview

Vala is a programming language that aims to bring modern language features to GNOME developers without imposing any additional runtime requirements and without using a different ABI than applications and libraries written in C. It provides a concise way of using GLib and GObject features, but does not attempt to expose all possibilities. In particular, Vala is primarily a statically typed language - this is a design decision, not an oversight.

The only support that Vala applications require at runtime are the standard GLib and GObject libraries. It is possible to use any system library from Vala, provided that a VAPI file is written to describe the interface - Vala is distributed with VAPI descriptions of most libraries commonly used by GNOME applications, and many others as well.

Vala provides easy integration with DBus, by automatically writing boiler plate code where required, for exposing objects, dispatching methods, etc.

Getting started

The classic "Hello, world" example in Vala:

   1 int main (string[] args) {
   2         stdout.printf ("hello, world\n");
   3         return 0;
   4 }

Store the code in a file whose name ends in ".vala", such as hello.vala, and compile it with the command:

valac -o hello hello.vala

This will produce an executable file called hello. "valac" is the Vala compiler; it will also allow you to take more control of the compile and link processes when required, but that is outside the scope of this introductory section.

Documentation conventions

A large amount of this documentation describes the language features precisely using a simple rule notation. The same notation is used to describe language syntax and semantics, with the accompanying text always explaining what is described. The following example shows the form:

  • rule-name:

    • literalstring1
      literalstring2 [ optional-section ]


    optional-section:

    • literalstring3

Here, "rule-name" and "optional-section" describe rules, each of which can be expanded in a particular way. Expanding a rule means substituting one of the options of the rule into the place the rule is used. In the example, "optional-section" can be expanded into "literalstring3" or, in "rule-name", "optional-section" can also be substituted for nothing, as it is declared optional by the square brackets. Wherever "rule-name" is required, it can be substituted for either of the options declared in "rule-name". Anything highlighted, such as all literalstrings here are not rules, and thus cannot be expanded.

Example code is shown as follows. Example code will always be valid Vala code, but will not necessarily be usable out of context.

   1 class MyClass : Object {
   2         int field = 1;
   3 }

Some phrases are used in a specific ways in this documentation and it is often useful to recognise their precise meanings: that is, to create a method, you write a declaration for it. When the program is running and the method exists, it is then defined as per your declaration and can be invoked.

Vala source files

There are two types of Vala input files. Vala source files (with a ".vala" extension) contain compileable Vala code. VAPI files (with a ".vapi" extension) describe an interface to a library, which can be written in either Vala or C. VAPI files are not compileable, and cannot contain any executable code - they are used when compiling Vala source files.

There are no requirements for how Vala source files are named, although there are conventions that can be followed. VAPI files are usually named to matched the pkg-config name of the library they relate to; they are described more fully in the documention about bindings.

All Vala input files should be encoded in UTF-8.

Vala conventions

The logical structure of a Vala project is entirely based on the program text, not the file layout or naming. Vala therefore does not force particular naming schemes or file layouts. There are established conventions derived from how GNOME related applications are normally written, which are strongly encouraged. The choice of directory structure for a project is outside the scope of this documentation.

Vala source files usually contain one main public class, after which the source file is named. A common choice is to convert this main class' name to lowercase, and prefix with its namespace, also in lower case, to form the file name. In a small project the namespace may be redundant and so excluded. None of this is a requirement, it is just a convention.

It is not encouraged to include declarations in more than one namespace in a single Vala source file, simply for reasons of clarity. A namespace may be divided over any number of source files, but will normally not be used outside of one project. Each library or application will normally have one main namespace, with potentially others nested within.

In source code, the following naming conventions are normally followed:

  • Namespaces are named in camel case: NameSpaceName

  • Classes are named in camel case: ClassName

  • Method names are all lowercase and use underscores to separate words: method_name
  • Constants (and values of enumerated types) are all uppercase, with underscores between words: CONSTANT_NAME

Vala supports the notion of a package to conveniently divide program sections. A package is either a combination of an installed system library and its Vala binding, or else is a local directory that can be treated in a similar way. In the latter case it will contain all functionality related to some topic, the scope of which is up to the developer. All source files in package are placed within a directory named for package name. For details on using packages, see the Vala compiler documentation

Vala syntax

Vala's syntax is modelled on C#'s, and is therefore similar to all C-like languages. Curly braces are the basic delimeter, marking the start and end of a declaration or block of code.

There is no whitespace requirement, though this is a standard format that is used in Vala itself, and in many Vala projects. This format is a version of the coding style used for glib and gnome projects, but is not fully described in this document, other than being used for all examples.

There is flexibility in the order of declarations in Vala. It is not required to pre-declare anything in order to use it before its declaration.

Identifiers all follow the same rules, whether for local variables or class names. Legal identifiers must begin with one alphabetic character or underscore, followed by any number (zero or more) of alphanumerics or underscores (/[:alpha:_]([:alphanum:_])*/). It is also possible to use language keywords as identifiers, provided they are prefixed with a "@" when used in this way - the "@" is not considered a part of the identifier, it simply informs the compiler that the token should be considered as an identifier.

GType and GObject

Vala uses the runtime type system called GType. This system allows every type in Vala, including the fundamental types, to be identified at runtime. A Vala developer doesn't need to be aware of GType in most circumstances, as all interaction with the system is automatic.

GType provides Vala with a powerful object model called GObject. To all types descended from GLib.Object class, this model provides for features such as properties and signals.

GType and GObject are entirely runtime type systems, intended to be usable to dynamically typed languages. Vala is primarily a statically typed language, and so is designed not to provide access to all of GType and GObject's features. Instead Vala uses a coherent subset to support particular programming styles.

Vala is designed to use GType and GObject seamlessly. There are occasions, mostly when working with existing libraries, when you might need to circumvent parts of the system. These are all indicated in this documentation.

Memory management

Vala automatically uses the memory management system in GLib, which is a reference counting system. In order for this to work, the types used must support reference counting, as is the case with all GObject derived types and some others.

Memory is allocated and initialised by Vala when needed. The memory management scheme means it is also freed when possible. There is though no garbage collector, and currently reference cycles are not automatically broken. This can lead to memory being leaked. The main way to avoid this problem is to use weak references - these are not counted references and so cannot prevent memory being released, at the cost that they can be left refering to non existent data.

Vala also allows use of pointers in much the same way as C. An instance of a pointer type refers directly to an address in memory. Pointers are not references, and therefore the automatic memory management rules do not apply in the same way. See Types/Pointer types.

There are more details about memory management elsewhere, see Types, see Concepts.

Vala compilation

Vala programs and libraries are translated into C before being compiled into machine code. This stage is intended to be entirely transparent unless you request otherwise, as such it is not often required to know the details.

When performing a more complicated compile or link process than valac's default, valac can be instructed to simply output its intermediate C form of the program and exit. Each Vala source file is transformed into a C header and a C source file, each having the same name as the Vala source file except for the extension. These C files can be compiled without any help from any Vala utility or library.

The only times it is definitely required to be aware of the translation process is when a Vala feature cannot be represented in C, and so the generated C API will not be the same as the Vala one. For example, private struct members are meaningless in C. These issues are indicated in this documentation.

Application entry point

All Vala applications are executed beginning with a method called "main". This must be a non-instance method, but may exist inside a namespace or class. If the method takes a string array parameter, it will be passed the arguments given to the program on execution. If it returns an int type, this value will be passed to the user on the program's normal termination. The entry point method may not accept any other parameters, or return any other types, making the acceptable definitions:

   1 void main () { ... }
   2 int main () { ... }
   3 void main (string[] args) { ... }
   4 int main (string[] args) { ... }

The entry point can be implicit, in the sense that you can write the main code block directly in the file outside the main function.

Projects/Vala/Manual/Overview (last edited 2017-02-06 17:13:37 by AlThomas)