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Concepts

This pages describes concepts that are widely applicable in Vala. Specific syntax is not described here, see the linked pages for more details.

Variables, fields and parameters

Any piece of data in a Vala application is considered an instance of a data type. There are various different categories of data types, some being built into Vala, and others being user defined. Details about types are described elsewhere in this documentation, in particular see Types.

Instances of these types are created in various different ways, depending on the type. For example, fundamental types are instantiated with literal expressions, and classed types with the new operator.

In order to access data, the instance must be identifiable in some way, such as by a name. In Vala, there are broadly three ways that this is done, with similar but not identical semantics.

(All these subsections refer to ownership, so it may be useful to read the section on Concepts/References and ownership in conjunction with this section)

Variables

Within executable code in a method, an instance may be assigned to a variable. A variable has a name and is declared to refer to an instance of a particular data type. A typical variable declaration would be:

int a;

This declaration defines that "a" should become an expression that evaluates to an instance of the int type. The actual value of this expression will depend on which int instance is assigned to the variable. "a" can be assigned to more than once, with the most recent assigment being the only one considered when "a" is evaluated. Assignment to the variable is achieved via an assignment expression. Generally, the semantics of an assignment expression depends on the type of the variable.

A variable can take ownership of an instance, the precise meaning of which depends on the data type. In the context of reference types, it is possible to declare that a variable should not ever take ownership of an instance, this is done with the unowned keyword. See Types/Reference types.

If a type is directly instantiated in a variable declaration statement, then the variable will be created owning that new instance, for example:

string s = "stringvalue";

A variable ceases to exist when its scope is destroyed, that is when the code block it is defined in finishes. After this, the name can no longer be used to access the instance, and no new assignment to the variable is allowed. What happens to the instance itself is dependent on the type.

For more details of the concepts in this section, see Statements/Variable declaration and Expressions/Assignment operations.

Fields

A field is similar to a variable, except for the scope that it is defined in. Fields can be defined in namespaces, classes and structs. In the case of classes and structs, they may be either in the scope of the the class or struct, or in the scope of each instance of the class or struct.

A field is valid as long as its scope still exists - for non-instance fields, this is the entire life of the application; for instance fields, this is the lifetime of the instance.

Like variables, fields can take ownership of instances, and it is again possible to avoid this with the unowned keyword.

If a type is directly instantiated in the declaration of the field, then that field will be created owning that new instance.

For more details about fields, see Namespaces, Classes and Structs.

Parameters

Instances passed to methods are accessible within that method with names given in the method's parameter list.

They act like variables, except that they cannot, by default, take ownership of the first instance that is assigned to them, i.e. the instance passed to the method. This behaviour can be changed using explicit ownership transfer. When reassigning to a parameter, the result depends on the parameter direction. Assuming the parameter has no direction modifier, it will subsequently act exactly as a variable.

For more details of methods and parameters, see Methods and Expressions/Ownership transfer expressions.

Scope and naming

A "scope" in Vala refers to any context in which identifiers can be valid. Identifiers in this case refers to anything named, including class definitions, fields, variables, etc. Within a particular scope, identifiers defined in this scope can be used directly:

void main () {
        int a = 5;
        int b = a + 1;
}

Scopes in Vala are introduced in various different ways.

  • Named scopes can be created directly with declarations like namespaces. These are always in existence when the program is running, and can be referred to by their name.
  • Transient scopes are created automatically as the program executes. Every time a new code block is entered, a new scope is created. For example, a new scope is created when a method is invoked. There is no way to refer to this type of scope from outside.
  • Instance scopes are created when a data type is instantiated, for example when a new instance of a classed type is created. These scopes can be accessed via identifiers defined in other scopes, e.g. a variable to which the new instance is assigned.

To refer to an identifier in another scope, you must generally qualify the name. For named scopes, the scope name is used; for instance scopes, any identifier to which the instance is assigned can be used. See Expressions/Member access for the syntax of accessing other scopes.

Scopes have parent scopes. If an identifier is not recognised in the current scope, the parent scope is searched. This continues up to the the global scope. The parent scope of any scope is inferred from its position in the program - the parent scope can easily be identified as it is the scope the current declaration is in.

For example, a namespace method creates a transient scope when it is invoked - the parent of this scope if the namespace which contains the definition of the method. There are slightly different rules applied when instances are involved, as are described at Classes/Class scope.

The ultimate parent of all other scopes is the global scope. The scope contains the fundamental data types, e.g. int, float, string. If a program has a declaration outside of any other, it is placed in this scope.

Qualifying names

The following rules describe when to qualify names:

  • For names in the same scope as the current definition, just the name should be used.
  • For names in scopes of which the current is parent, qualify with just the names of scopes that the current definition is not nested within.
  • For names in other scopes entirely, or that are less deeply nested than the current, use the fully qualified name (starting from the global scope.)

There are some intricacies of scopes described elsewhere in this documentation. See Classes for how scopes are managed for inherited classes.

Vala will lookup names assuming first that they are not fully qualified. If a fully qualified name can be partially matched locally, or in a parent scope that is not the global scope, the compilation will fail. To avoid problems with this, do not reuse names from the global scope in other scopes.

There is one special scope qualifier that can be used to avoid the problem described in the previous paragraph. Prefixing an identifier with global:: will instruct the compiler to only attempt to find the identifier in the global scope, skipping all earlier searching.

Object oriented programming

Vala is primarily an object oriented language. This documentation isn't going to describe object oriented programming in detail, but in order for other sections to make sense, some things need to be explained.

A class in Vala is a definition of a potentially polymorphic type. A polymorphic type is one which can be viewed as more than one type. The basic method for this is inheritance, whereby one type can be defined as a specialized version of another. An instance of a subtype, descended from a particular supertype, has all the properties of the supertype, and can be used wherever an instance of the supertype is expected. This sort of relationship is described as a "subtype instance is-a supertype instance." See Classes.

Vala provides inheritance functionality to any type of class (see Classes/Types of class). Given the following definition, every SubType instance is-a SuperType instance:

class SuperType {
        public int act() {
                return 1;
        }
}
class SubType : SuperType {
}

Whenever a SuperType instance is required, a SubType instance may be used. This is the extent of inheritence allowed to compact classes, but full classes are more featured. All classes that are not of compact type, can have virtual methods, and can implement interfaces.

To explain virtual functions, it makes sense to look at the alternative first. In the above example, it is legal for SubType to also define a method called "act" - this is called overriding. In this case, when a method called "act" is called on a SubType instance, which method is invoked depends on what type the invoker believed it was dealing with. The following example demonstrates this:

SubType sub = new SubType();
SuperType super = sub;

sub.act();
super.act();

Here, when sub.act() is called, the method invoked will be SubType's "act" method. The call super.act() will call SuperType's "act". If the act method were virtual, the SubType.act method would have been called on both occasions. See Classes/Class methods for how to declare virtual methods.

Interfaces are a variety of non-instantiatable type. This means that it is not possible to create an instance of the type. Instead, interfaces are implemented by other types. Instances of these other types may then be used as though they were instances of the interface in question. See Interfaces.

References and ownership

Type instances in Vala are automatically managed to a large degree. This means that memory is allocated to store the data, and then deallocated when the data is no longer required. However, Vala does not have a runtime garbage collector, instead it applies rules at compile time that will predictably deallocate memory at runtime.

A central concept of Vala's memory management system is ownership. An instance is considered still in use as long as there is at least one way of accessing it, i.e. there is some field, variable or parameter that refers to the instance - one such identifier will be considered the instance's owner, and therefore the instance's memory will not be deallocated. When there is no longer any way to access the data instance, it is considered unowned, and its memory will be deallocated.

Value types

When dealing with instances of value types (see Types) knowledge of ownership is rarely important. This is because the instance is copied whenever it is assigned to a new identifier. This will cause each identifier to become owner of a unique instance - that instance will then be deallocated when the identifier ceases to be valid.

There is one exception to this rule: when a struct type instance is passed to a method, Vala will, by default, create the method parameter as a reference to the instance instead of copying the instance. This reference is a weak reference, as described in the following section. If the struct should instead be copied, and the parameter created as a standard value type identifier, the ownership transfer operator should be used (see Expressions/Ownership transfer expressions).

Reference types

With reference types, it is possible for several identifiers to reference the same data instance. Not all identifiers that refer to reference type instance are capable of owning the instance, for reasons that will be explained. It is therefore often required to think about instance ownership when writing Vala code.

Most reference types support reference counting. This means that the instance internally maintains a count of how many references to it currently exist. This count is used to decide whether the instance is still in use, or if its memory can be deallocated. Each reference that is counted in this way is therefore a potential owner of the instance, as it ensures the instance continues to exist. There are situations when this is not desired, and so it is possible to define a field or variable as "weak". In this case the reference is not counted, and so the fact that the reference exists will not stop the instance being possibly deallocated, i.e. this sort of reference cannot take ownership of the instance.

When using reference counted types, the main use for weak references is to prevent reference cycles. These exist when a data instance contains internally a reference to another instance, which in turn contains a reference to the first. In this case it would not be possible to deallocate the instances, as each would be potentially owning the other. By ensuring that one of the references is weak, one of the instances can become unowned and be deallocated, and in the process the other will be dereferenced, and potentially become unowned and be deallocated also.

It is also possible to have reference types which are not reference counted; an example of this is the fundamental string type, others are compact classed types. If Vala were to allow several references to own such instances, it would not be able to keep track of when they all ceased to exist, and therefore would not be able to know when to deallocate the instance. Instead, exactly one or zero identifiers will own the instance - when it is zero, the instance is deallocated. This means that all references to an already owned instance must either be weak references, or ownership must be specifically passed to the new reference, using the ownership transfer operator (see Expressions/Ownership transfer expressions).

Pointer types

Pointer types are of great importance. Pointer types are value types, whose instances are references to some other data instance. They are therefore not actual references, and will never own the instance that they indirectly refer to. See Types/Pointer types.

Projects/Vala/Manual/Concepts (last edited 2017-03-11 19:16:40 by AlThomas)