This Annex contains informal descriptions of some of the terms used in this International Standard. The index provides references to more formal definitions of all of the terms used in this International Standard.

  Abstract type. An abstract type is a tagged type intended for use as an ancestor of other types, but which is not allowed to have objects of its own.

Access type. An access type has values that designate aliased objects. Access types correspond to “pointer types” or “reference types” in some other languages.

Aliased. An aliased view of an object is one that can be designated by an access value. Objects allocated by allocators are aliased. Objects can also be explicitly declared as aliased with the reserved word aliased. The Access attribute can be used to create an access value designating an aliased object.

  Ancestor. An ancestor of a type is the type itself or, in the case of a type derived from other types, its parent type or one of its progenitor types or one of their ancestors. Note that ancestor and descendant are inverse relationships.

Array type. An array type is a composite type whose components are all of the same type. Components are selected by indexing.

  Aspect. An aspect is a specifiable property of an entity. An aspect may be specified by an aspect_specification on the declaration of the entity. Some aspects may be queried via attributes.

  Assertion. An assertion is a boolean expression that appears in any of the following: a pragma Assert, a predicate, a precondition, a postcondition, an invariant, a constraint, or a null exclusion. An assertion is expected to be True at run time at certain specified places.

  Category (of types). A category of types is a set of types with one or more common properties, such as primitive operations. A category of types that is closed under derivation is also known as a class.

Character type. A character type is an enumeration type whose values include characters.

Class (of types). A class is a set of types that is closed under derivation, which means that if a given type is in the class, then all types derived from that type are also in the class. The set of types of a class share common properties, such as their primitive operations.

Compilation unit. The text of a program can be submitted to the compiler in one or more compilations. Each compilation is a succession of compilation_units. A compilation_unit contains either the declaration, the body, or a renaming of a program unit.

Composite type. A composite type may have components.

Construct. A construct is a piece of text (explicit or implicit) that is an instance of a syntactic category defined under “Syntax”.

  Container. A container is an object that contain other objects all of the same type, which could be class-wide. Several predefined container types are provided by the children of package Ada.Containers (see A.18.1).

Controlled type. A controlled type supports user-defined assignment and finalization. Objects are always finalized before being destroyed.

Declaration. A declaration is a language construct that associates a name with (a view of) an entity. A declaration may appear explicitly in the program text (an explicit declaration), or may be supposed to occur at a given place in the text as a consequence of the semantics of another construct (an implicit declaration).

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 Derived type. A derived type is a type defined in terms of one or more other types given in a derived type definition. The first of those types is the parent type of the derived type and any others are progenitor types. Each class containing the parent type or a progenitor type also contains the derived type. The derived type inherits properties such as components and primitive operations from the parent and progenitors. A type together with the types derived from it (directly or indirectly) form a derivation class.

   Descendant. A type is a descendant of itself, its parent and progenitor types, and their ancestors. Note that descendant and ancestor are inverse relationships.

Discrete type. A discrete type is either an integer type or an enumeration type. Discrete types may be used, for example, in case_statements and as array indices.

 Discriminant. A discriminant is a parameter for a composite type. It can control, for example, the bounds of a component of the type if the component is an array. A discriminant for a task type can be used to pass data to a task of the type upon creation.

   Elaboration. The process by which a declaration achieves its run-time effect is called elaboration. Elaboration is one of the forms of execution.

Elementary type. An elementary type does not have components.

Enumeration type. An enumeration type is defined by an enumeration of its values, which may be named by identifiers or character literals.

   Evaluation. The process by which an expression achieves its run-time effect is called evaluation. Evaluation is one of the forms of execution.

Exception. An exception represents a kind of exceptional situation; an occurrence of such a situation (at run time) is called an exception occurrence. To raise an exception is to abandon normal program execution so as to draw attention to the fact that the corresponding situation has arisen. Performing some actions in response to the arising of an exception is called handling the exception. 

Execution. The process by which a construct achieves its run-time effect is called execution. Execution of a declaration is also called elaboration. Execution of an expression is also called evaluation.

   Function. A function is a form of subprogram that returns a result and can be called as part of an expression.

Generic unit. A generic unit is a template for a (nongeneric) program unit; the template can be parameterized by objects, types, subprograms, and packages. An instance of a generic unit is created by a generic_instantiation. The rules of the language are enforced when a generic unit is compiled, using a generic contract model; additional checks are performed upon instantiation to verify the contract is met. That is, the declaration of a generic unit represents a contract between the body of the generic and instances of the generic. Generic units can be used to perform the role that macros sometimes play in other languages.

   Incomplete type. An incomplete type gives a view of a type that reveals only some of its properties. The remaining properties are provided by the full view given elsewhere. Incomplete types can be used for defining recursive data structures.

   Indexable container type. An indexable container type is one that has user-defined behavior for indexing, via the Constant_Indexing or Variable_Indexing aspects.

Integer type. Integer types comprise the signed integer types and the modular types. A signed integer type has a base range that includes both positive and negative numbers, and has operations that may raise an exception when the result is outside the base range. A modular type has a base range whose lower bound is zero, and has operations with “wraparound” semantics. Modular types subsume what are called “unsigned types” in some other languages.

   Interface type. An interface type is a form of abstract tagged type which has no components or concrete operations except possibly null procedures. Interface types are used for composing other interfaces and tagged types and thereby provide multiple inheritance. Only an interface type can be used as a progenitor of another type.

   Invariant. An invariant is an assertion that is expected to be True for all objects of a given private type when viewed from outside the defining package.

   Iterable container type. An iterable container type is one that has user-defined behavior for iteration, via the Default_Iterator and Iterator_Element aspects.

   Iterator. An iterator is a construct that is used to loop over the elements of an array or container. Iterators may be user defined, and may perform arbitrary computations to access elements from a container.

Library unit. A library unit is a separately compiled program unit, and is always a package, subprogram, or generic unit. Library units may have other (logically nested) library units as children, and may have other program units physically nested within them. A root library unit, together with its children and grandchildren and so on, form a subsystem.

 Limited type. A limited type is a type for which copying (such as in an assignment_statement) is not allowed. A nonlimited type is a type for which copying is allowed.

Object. An object is either a constant or a variable. An object contains a value. An object is created by an object_declaration or by an allocator. A formal parameter is (a view of) an object. A subcomponent of an object is an object.

   Overriding operation. An overriding operation is one that replaces an inherited primitive operation. Operations may be marked explicitly as overriding or not overriding.

Package. Packages are program units that allow the specification of groups of logically related entities. Typically, a package contains the declaration of a type (often a private type or private extension) along with the declarations of primitive subprograms of the type, which can be called from outside the package, while their inner workings remain hidden from outside users.

   Parent. The parent of a derived type is the first type given in the definition of the derived type. The parent can be almost any kind of type, including an interface type.

Partition. A partition is a part of a program. Each partition consists of a set of library units. Each partition may run in a separate address space, possibly on a separate computer. A program may contain just one partition. A distributed program typically contains multiple partitions, which can execute concurrently.

   Postcondition. A postcondition is an assertion that is expected to be True when a given subprogram returns normally.

Pragma. A pragma is a compiler directive. There are language-defined pragmas that give instructions for optimization, listing control, etc. An implementation may support additional (implementation-defined) pragmas.

   Precondition. A precondition is an assertion that is expected to be True when a given subprogram is called.

   Predicate. A predicate is an assertion that is expected to be True for all objects of a given subtype.

Primitive operations. The primitive operations of a type are the operations (such as subprograms) declared together with the type declaration. They are inherited by other types in the same class of types. For a tagged type, the primitive subprograms are dispatching subprograms, providing run-time polymorphism. A dispatching subprogram may be called with statically tagged operands, in which case the subprogram body invoked is determined at compile time. Alternatively, a dispatching subprogram may be called using a dispatching call, in which case the subprogram body invoked is determined at run time.

 Private extension. A private extension is a type that extends another type, with the additional properties hidden from its clients.

 Private type. A private type gives a view of a type that reveals only some of its properties. The remaining properties are provided by the full view given elsewhere. Private types can be used for defining abstractions that hide unnecessary details from their clients.

   Procedure. A procedure is a form of subprogram that does not return a result and can only be called by a statement.

   Progenitor. A progenitor of a derived type is one of the types given in the definition of the derived type other than the first. A progenitor is always an interface type. Interfaces, tasks, and protected types may also have progenitors.

Program. A program is a set of partitions, each of which may execute in a separate address space, possibly on a separate computer. A partition consists of a set of library units.

Program unit. A program unit is either a package, a task unit, a protected unit, a protected entry, a generic unit, or an explicitly declared subprogram other than an enumeration literal. Certain kinds of program units can be separately compiled. Alternatively, they can appear physically nested within other program units.

 Protected type. A protected type is a composite type whose components are accessible only through one of its protected operations which synchronize concurrent access by multiple tasks.

Real type. A real type has values that are approximations of the real numbers. Floating point and fixed point types are real types.

Record extension. A record extension is a type that extends another type by adding additional components.

Record type. A record type is a composite type consisting of zero or more named components, possibly of different types.

   Reference type. A reference type is one that has user-defined behavior for “.all”, defined by the Implicit_Dereference aspect.

   Renaming. A renaming_declaration is a declaration that does not define a new entity, but instead defines a view of an existing entity.

Scalar type. A scalar type is either a discrete type or a real type.

   Storage pool. Each access-to-object type has an associated storage pool object. The storage for an object created by an allocator comes from the storage pool of the type of the allocator. Some storage pools may be partitioned into subpools in order to support finer-grained storage management.

   Stream. A stream is a sequence of elements that can be used, along with the stream-oriented attributes, to support marshalling and unmarshalling of values of most types.

   Subprogram. A subprogram is a section of a program that can be executed in various contexts. It is invoked by a subprogram call that may qualify the effect of the subprogram through the passing of parameters. There are two forms of subprograms: functions, which return values, and procedures, which do not.

 Subtype. A subtype is a type together with optional constraints, null exclusions, and predicates, which constrain the values of the subtype to satisfy certain conditions. The values of a subtype are a subset of the values of its type.

   Synchronized. A synchronized entity is one that will work safely with multiple tasks at one time. A synchronized interface can be an ancestor of a task or a protected type. Such a task or protected type is called a synchronized tagged type.

Tagged type. The objects of a tagged type have a run-time type tag, which indicates the specific type with which the object was originally created. An operand of a class-wide tagged type can be used in a dispatching call; the tag indicates which subprogram body to invoke. Nondispatching calls, in which the subprogram body to invoke is determined at compile time, are also allowed. Tagged types may be extended with additional components.

 Task type. A task type is a composite type used to represent active entities which execute concurrently and which can communicate via queued task entries. The top-level task of a partition is called the environment task.

 Type. Each object has a type. A type has an associated set of values, and a set of primitive operations which implement the fundamental aspects of its semantics. Types are grouped into categories. Most language-defined categories of types are also classes of types.

   Type Invariant. See Invariant.

 View. A view of an entity reveals some or all of the properties of the entity. A single entity may have multiple views.