Object Declarations

{AI05-0262-1} An object_declaration declares a stand-alone object with a given nominal subtype and, optionally, an explicit initial value given by an initialization expression. For an array, access, task, or protected object, the object_declaration may include the definition of the (anonymous) type of the object.

Syntax

Name Resolution Rules

For an object_declaration with an expression following the compound delimiter :=, the type expected for the expression is that of the object. This expression is called the initialization expression.

Legality Rules

{AI95-00287-01} An object_declaration without the reserved word constant declares a variable object. If it has a subtype_indication or an array_type_definition that defines an indefinite subtype, then there shall be an initialization expression.

Static Semantics

{AI05-0264-1} {AI05-0299-1} An object_declaration with the reserved word constant declares a constant object. If it has an initialization expression, then it is called a full constant declaration. Otherwise, it is called a deferred constant declaration. The rules for deferred constant declarations are given in subclause 7.4. The rules for full constant declarations are given in this subclause.

Any declaration that includes a defining_identifier_list with more than one defining_identifier is equivalent to a series of declarations each containing one defining_identifier from the list, with the rest of the text of the declaration copied for each declaration in the series, in the same order as the list. The remainder of this International Standard relies on this equivalence; explanations are given for declarations with a single defining_identifier.

{AI95-00385-01} The subtype_indication, access_definition, or full type definition of an object_declaration defines the nominal subtype of the object. The object_declaration declares an object of the type of the nominal subtype.

Discussion: {AI95-00385-01} The phrase “full type definition” here includes the case of an anonymous array, access, task, or protected type.

{AI95-00373-01} A component of an object is said to require late initialization if it has an access discriminant value constrained by a per-object expression, or if it has an initialization expression that includes a name denoting the current instance of the type or denoting an access discriminant.

Reason: Such components can depend on the values of other components of the object. We want to initialize them as late and as reproducibly as possible.

Dynamic Semantics

{AI95-00363-01} If a composite object declared by an object_declaration has an unconstrained nominal subtype, then if this subtype is indefinite or the object is constant the actual subtype of this object is constrained. The constraint is determined by the bounds or discriminants (if any) of its initial value; the object is said to be constrained by its initial value. When not constrained by its initial value, the actual and nominal subtypes of the object are the same. If its actual subtype is constrained, the object is called a constrained object.

For an object_declaration without an initialization expression, any initial values for the object or its subcomponents are determined by the implicit initial values defined for its nominal subtype, as follows:

The implicit initial value for an access subtype is the null value of the access type.

{AI05-0228-1} The implicit initial value for a scalar subtype that has the Default_Value aspect specified is the value of that aspect converted to the nominal subtype (which might raise Constraint_Error — see 4.6, “Type Conversions”);

Ramification: This is a Dynamic Semantics rule, so the visibility of the aspect_specification is not relevant — if the full type for a private type has the Default_Value aspect specified, partial views of the type also have this implicit initial value.

The implicit initial (and only) value for each discriminant of a constrained discriminated subtype is defined by the subtype.

{AI05-0228-1} For a (definite) composite subtype, the implicit initial value of each component with a default_expression is obtained by evaluation of this expression and conversion to the component's nominal subtype (which might raise Constraint_Error), unless the component is a discriminant of a constrained subtype (the previous case), or is in an excluded variant (see 3.8.1). For each component that does not have a default_expression, if the composite subtype has the Default_Component_Value aspect specified, the implicit initial value is the value of that aspect converted to the component's nominal subtype; otherwise, any implicit initial values are those determined by the component's nominal subtype.

For a protected or task subtype, there is an implicit component (an entry queue) corresponding to each entry, with its implicit initial value being an empty queue.

Implementation Note: The implementation may add implicit components for its own use, which might have implicit initial values. For a task subtype, such components might represent the state of the associated thread of control. For a type with dynamic-sized components, such implicit components might be used to hold the offset to some explicit component.

{AI95-00385-01} The subtype_indication, access_definition, array_type_definition, single_task_declaration, or single_protected_declaration is first elaborated. This creates the nominal subtype (and the anonymous type in the last four cases).

If the object_declaration includes an initialization expression, the (explicit) initial value is obtained by evaluating the expression and converting it to the nominal subtype (which might raise Constraint_Error — see 4.6).

{8652/0002} {AI95-00171-01} {AI95-00373-01} The object is created, and, if there is not an initialization expression, the object is initialized by default. When an object is initialized by default, any per-object constraints (see 3.8) are elaborated and any implicit initial values for the object or for its subcomponents are obtained as determined by the nominal subtype. Any initial values (whether explicit or implicit) are assigned to the object or to the corresponding subcomponents. As described in 5.2 and 7.6, Initialize and Adjust procedures can be called.

Discussion: For a per-object constraint that contains some per-object expressions and some non-per-object expressions, the values used for the constraint consist of the values of the non-per-object expressions evaluated at the point of the type_declaration, and the values of the per-object expressions evaluated at the point of the creation of the object.

The elaboration of per-object constraints was presumably performed as part of the dependent compatibility check in Ada 83. If the object is of a limited type with an access discriminant, the access_definition is elaborated at this time (see 3.7).

Reason: The reason we say that evaluating an explicit initialization expression happens before creating the object is that in some cases it is impossible to know the size of the object being created until its initial value is known, as in “X: String := Func_Call(...);”. The implementation can create the object early in the common case where the size can be known early, since this optimization is semantically neutral.

Ramification: Since the initial values have already been converted to the appropriate nominal subtype, the only Constraint_Errors that might occur as part of these assignments are for values outside their base range that are used to initialize unconstrained numeric subcomponents. See 3.5.

{AI95-00373-01} For the third step above, evaluations and assignments are performed in an arbitrary order subject to the following restrictions:

{AI95-00373-01} Assignment to any part of the object is preceded by the evaluation of the value that is to be assigned.

Reason: Duh. But we ought to say it. Note that, like any rule in the International Standard, it doesn't prevent an “as-if” optimization; as long as the semantics as observed from the program are correct, the compiler can generate any code it wants.

{AI95-00373-01} The evaluation of a default_expression that includes the name of a discriminant is preceded by the assignment to that discriminant.

Reason: Duh again. But we have to say this, too. It's odd that Ada 95 only required the default expressions to be evaluated before the discriminant is used; it says nothing about discriminant values that come from subtype_indications.

{AI95-00373-01} The evaluation of the default_expression for any component that depends on a discriminant is preceded by the assignment to that discriminant.

{AI95-00373-01} {AI05-0092-1} The assignments to any components, including implicit components, not requiring late initialization precede the initial value evaluations for any components requiring late initialization; if two components both require late initialization, then assignments to parts of the component occurring earlier in the order of the component declarations precede the initial value evaluations of the component occurring later.

Reason: Components that require late initialization can refer to the entire object during their initialization. We want them to be initialized as late as possible to reduce the chance that their initialization depends on uninitialized components. For instance:

type T (D : Natural) is
  limited record
    C1 : T1 (T'Access);
    C2 : Natural := F (D);
    C3 : String (1 .. D) := (others => ' ');
  end record;

Component C1 requires late initialization. The initialization could depend on the values of any component of T, including D, C2, or C3. Therefore, we want to it to be initialized last. Note that C2 and C3 do not require late initialization; they only have to be initialized after D.

It is possible for there to be more than one component that requires late initialization. In this case, the language can't prevent problems, because all of the components can't be the last one initialized. In this case, we specify the order of initialization for components requiring late initialization; by doing so, programmers can arrange their code to avoid accessing uninitialized components, and such arrangements are portable. Note that if the program accesses an uninitialized component, 13.9.1 defines the execution to be erroneous.

{AI05-0228-1} [There is no implicit initial value defined for a scalar subtype unless the Default_Value aspect has been specified for the type.] In the absence of an explicit initialization or the specification of the Default_Value aspect, a newly created scalar object might have a value that does not belong to its subtype (see 13.9.1 and H.1).

To be honest: It could even be represented by a bit pattern that doesn't actually represent any value of the type at all, such as an invalid internal code for an enumeration type, or a NaN for a floating point type. It is a generally a bounded error to reference scalar objects with such “invalid representations”, as explained in 13.9.1, “Data Validity”.

Ramification: There is no requirement that two objects of the same scalar subtype have the same implicit initial “value” (or representation). It might even be the case that two elaborations of the same object_declaration produce two different initial values. However, any particular uninitialized object is default-initialized to a single value (or invalid representation). Thus, multiple reads of such an uninitialized object will produce the same value each time (if the implementation chooses not to detect the error).

13 Implicit initial values are not defined for an indefinite subtype, because if an object's nominal subtype is indefinite, an explicit initial value is required.

14 {AI05-0092-1} {AI05-0255-1} As indicated above, a stand-alone object is an object declared by an object_declaration. Similar definitions apply to “stand-alone constant” and “stand-alone variable.” A subcomponent of an object is not a stand-alone object, nor is an object that is created by an allocator. An object declared by a loop_parameter_specification, iterator_specification, parameter_specification, entry_index_specification, choice_parameter_specification, extended_return_statement, or a formal_object_declaration of mode in out is not considered a stand-alone object.

Examples

{AI95-00433-01} Count, Sum  : Integer;
Size        : Integer range 0 .. 10_000 := 0;
Sorted      : Boolean := False;
Color_Table : array(1 .. Max) of Color;
Option      : Bit_Vector(1 .. 10) := (others => True);
Hello       : aliased String := "Hi, world.";
θ, φ        : Float range -π .. +π;

{AI95-00433-01} Limit : constant Integer := 10_000;
Low_Limit : constant Integer := Limit/10;
Tolerance : constant Real := Dispersion(1.15);
Hello_Msg : constant access String := Hello'Access; -- see 3.10.2

Extensions to Ada 83

A variable declared by an object_declaration can be constrained by its initial value; that is, a variable of a nominally unconstrained array subtype, or discriminated type without defaults, can be declared so long as it has an explicit initial value. In Ada 83, this was permitted for constants, and for variables created by allocators, but not for variables declared by object_declarations. This is particularly important for tagged class-wide types, since there is no way to constrain them explicitly, and so an initial value is the only way to provide a constraint. It is also important for generic formal private types with unknown discriminants.

We now allow an unconstrained_array_definition in an object_declaration. This allows an object of an anonymous array type to have its bounds determined by its initial value. This is for uniformity: If one can write “X: constant array(Integer range 1..10) of Integer := ...;” then it makes sense to also allow “X: constant array(Integer range <>) of Integer := ...;”. (Note that if anonymous array types are ever sensible, a common situation is for a table implemented as an array. Tables are often constant, and for constants, there's usually no point in forcing the user to count the number of elements in the value.)

Wording Changes from Ada 83

Deferred constants no longer have a separate syntax rule, but rather are incorporated in object_declaration as constants declared without an initialization expression.

Inconsistencies With Ada 95

{AI95-00363-01} Unconstrained aliased objects of types with discriminants with defaults are no longer constrained by their initial values. This means that a program that raised Constraint_Error from an attempt to change the discriminants will no longer do so. The change only affects programs that depended on the raising of Constraint_Error in this case, so the inconsistency is unlikely to occur outside of the ACATS. This change may however cause compilers to implement these objects differently, possibly taking additional memory or time. This is unlikely to be worse than the differences caused by any major compiler upgrade.

Extensions to Ada 95

Wording Changes from Ada 95

{AI95-00373-01} The rules for evaluating default initialization have been tightened. In particular, components whose default initialization can refer to the rest of the object are required to be initialized last.

Extensions to Ada 2005

Wording Changes from Ada 2005

{AI05-0228-1} Implicit initial values can now be given for scalar types and for scalar array components, using the Default_Value (see 3.5) and Default_Component_Value (see 3.6) aspects; the extension is documented there.

3.3.1 Object Declarations