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A "type", loosely described, is just an abstract set of 0 or more data fields. A type may be instantiated by creating an entity that contains values that map to the fields of the type. In Vala, a type generally consists of:
- A type name, which is used in various contexts in Vala code to signify an instance of the type.
- A data structure that defines how to represent an instance of the type in memory.
- A set of methods that can be called on an instance of the type.
These elements are combined as the definition of the type. The definition is given to Vala in the form of a declaration, for example a class declaration.
Vala supports three kinds of data types: value types, reference types, and meta types. Value types include simple types (e.g. char, int, and float), enum types, and struct types. Reference types include object types, array types, delegate types, and error types. Type parameters are parameters used in generic types.
Value types differ from reference types in that there is only ever one variable or field that refers to each instance, whereas variables or fields of the reference types store references to data which can also be referred to by other variable or fields. When two variables or fields of a reference type reference the same data, changes made using one identifier are visible when using the other. This is not possible with value types.
Meta types are created automatically from other types, and so may have either reference or value type semantics.
Instances of value types are stored directly in variables or fields that represent them. Whenever a value type instance is assigned to another variable or field, the default action is to duplicate the value, such that each identifier refers to a unique copy of the data, over which it has ownership. When a value type is instantiated in a method, the instance is created on the stack.
Where a literal is indicated, this means the actual type name of a built in struct type is given. The definition of these types is included in Vala, so these types are always available.
A struct type is one that provides just a data structure and some methods that act upon it. Structs are not polymorphic, and cannot have advanced features such as signals or properties. See Structs for documentation on how to define structs and more details about them. See Expressions/Struct instantiation for how to instantiate structs.
Each variable or field to which a struct stype instance is assigned gains a copy of the data, over which it has ownership. However, when a struct type instance is passed to a method, a copy is not made. Instead a reference to the instance is passed. This behaviour can be changed by declaring the struct to be a simple type.
In Vala, the fundamental types are defined as struct types whose data structure is known internally to Vala. They have one anonymous field, which is automatically accessed when required. All fundamental value types are defined as simple types, and so whenever the instance is assigned to a variable or field or passed as a function parameter, a copy of the data is made.
The fundamental value types fall into one of three categories: the boolean type, integral types, and floating point types.
Integral types can contain only integers. They are either signed or unsigned, each of which is considered a different type, though it is possible to cast between them when needed.
Some types define exactly how many bits of storage are used to represent the integer, others depend of the environment. long, int short map to C data types and therefore depend on the machine architecture. char is 1 byte. unichar is 4 bytes, i.e. large enough to store any UTF-8 character.
All these types can be instantiated using a literal expression, see Expressions/Literal expressions.
Floating point types
Floating point types contain real floating point numbers in a fixed number of bits (see IEEE 754).
All these types can be instantiated using a literal expression, see Expressions/Literal expressions.
The bool type
Can have values of true of false. Although there are only two values that a bool instance can take, this is not an enumerated type. Each instance is unique and will be copied when required, the same as for the other fundamental value types.
This type can be instantiated using literal expressions, see Expressions/Literal expressions.
An enumerated type is one in which all possible values that instances of the type can hold are declared with the type. In Vala enumerated types are real types, and will not be implicitly converted. It is possible to explicitly cast between enumerated types, but this is not generally advisable. When writing new code in Vala, don't rely on being able to cast in this way.
A variation on an enumerated type is a flag type. This represents a set of flags, any number of which can be combined in one instance of the flag type, in the same fashion as a bitfield in C.
See Enumerated types (Enums) for documentation on defining and using enumerated types.
Instances of reference types are always stored on the heap. Variables of reference types contain references to the instances, rather than the instances themselves. Assinging an instance of a reference type to a variable or field will not make a copy of the data, instead only the reference to the data is copied. This means that both variables will refer to the same data, and so changes made to that data using one of the references will be visible when using the other.
Instances of any reference type can be assigned a variable that is declared "weak". This implies that the variable must not be known to the type instance. A reference counted type does not increase its reference count after being assigned to a weak variable: a weak variable cannot take ownership of an instance.
non-array-type [ dimension-separators ]
A class definition introduces a new reference type - this is the most common way of creating a new type in Vala. Classes are a very powerful mechanism, as they have features such as polymorphism and inheritance. Full discussion of classes is found at Classes.
Most classed types in Vala are reference counted. This means that every time a classed type instance is assigned to a variable or field, not only is the reference copied, but the instance also records that another reference to it has been created. When a field or variable goes out of scope, the fact that a reference to the instance has been removed is also recorded. This means that a classed type instance can be automatically removed from memory when it is no longer needed. The only classed types that are not reference counted are compact classes.. Memory management is discussed at Overview/Memory management. If the instance is not of a reference counted type, then the ownership must be explicitly transferred using the # operator - this will cause the original variable to become invalid. When a classed-type instance is passed to a method, the same rules apply. The types of classes available are discussed at Classes/Types of class.
TODO: Check correctness.
An array is a data structure that can contains zero or more elements of the same type, up to a limit defined by the type. An array may have multiple dimensions; for each possible set of dimensions a new type is implied, but there is a meta type available that describes an array of any size with the same number of dimensions, i.e. int is not the same type as int, while int is the same type as either.
See Expressions/Array instantiation for how to instantiate an array type.
A delegate is a data structure that refers to a method. A method executes in a given scope which is also stored, meaning that for instance methods a delegate will contain also a reference to the instance.
Delegates are technically a referenced type, but since methods are immutable, this distinction is less important than for other types. Assigning a delegate to a variable or field cannot copy the method indicated, and no delegate is able to change the method in any way.
See Delegates for full documentation.
Instances of error types represent recoverable runtime errors. All errors are described using error domains, a type of enumerated value, but errors themselves are not enumerated types. Errors are discussed in detail in several sections of this documentation, see: Errors, Enumerated types (Enums)/Error domains and Methods.
Vala has built in support for Unicode strings, via the fundamental string type. This is the only fundamental type that is a reference type. Like other fundamental types, it can be instantiated with a literal expression (Expressions/Literal expressions.) Strings are UTF-8 encoded, the same as Vala source files, which means that they cannot be accessed like character arrays in C - each Unicode character is not guaranteed to be stored in just one byte. Instead the string fundamental struct type (which all strings are instances of) provides access methods along with other tools.
While strings are technically a reference type, they have the same default copy semantics as structs - the data is copied whenever a string value is assigned to a variable or field, but only a reference is passed as a parameter to a method. This is required because strings are not reference counted, and so the only way for a variable or field to be able to take ownership of a string is by being assigned a copy of the string. To avoid this behaviour, string values can be assigned to weak references (in such a case no copy is made).
The concept of ownership is very important in understanding string semantics. For more details see Concepts/References and ownership.
Vala allows definitions of types that can be customised at runtime with type parameters. For example, a list can be defined so that it can be instantiated as a list of ints, a list of Objects, etc. This is achieved using generic declarations. See Generics.
The name of a type can be used to implicitly create a nullable type related to that type. An instance of a nullable type T? can either be a value of type T or null.
A nullable type will have either value or reference type semantics, depending on the type it is based on.
The name of a type can be used to implicitly create a pointer type related to that type. The value of a variable declared as being of type T* represents the memory address of an instance of type T. The instance is never made aware that its address has been recorded, and so cannot record the fact that it is referred to in this way.
Instances of any type can be assigned to a variable that is declared to be a pointer to an instance of that type. For referenced types, direct assignment is allowed in either direction. For value types the pointer-to operator "&" is required to assign to a pointer, and the pointer-indirection operator "*" is used to access the instance pointed to. See Expressions/Pointer expressions.
The void* type represents a pointer to an unknown type. As the referent type is unknown, the indirection operator cannot be applied to a pointer of type void*, nor can any arithmetic be performed on such a pointer. However, a pointer of type void* can be cast to any other pointer type (and vice-versa) and compared to values of other pointer types. See Expressions/Type operations.
A pointer type itself has value type semantics.
There are two types if type conversions possible in Vala, implicit conversions and explicit casts. In expressions, Vala will often convert fundamental types in order to make calculations possible. When the default conversion is not what you require, you can cast explicitly so that all operands are of compatible types. See Expressions for details of automatic conversions.
Vala will also automatically perform conversions related to polymorphism where the required cast is unambiguous and can be inferred from the context. This allows you to use a classed-type instance when an instance of any of its superclasses or implemented interfaces is required. Vala will never automatically cast to a subtype, as this must be done explicitly. See Concepts/Object oriented programming, see Classes.
For explicit casting expressions, see Expressions/Type operations.