Objects and Named Numbers
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 3.3 Objects and Named Numbers

1
Objects are created at run time and contain a value of a given type. An object can be created and initialized as part of elaborating a declaration, evaluating an allocator, aggregate, or function_call, or passing a parameter by copy. Prior to reclaiming the storage for an object, it is finalized if necessary (see 7.6.1).

#### Static Semantics

2
All of the following are objects:
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the entity declared by an object_declaration;
4
a formal parameter of a subprogram, entry, or generic subprogram;
5
a generic formal object;
6
a loop parameter;
7
a choice parameter of an exception_handler;
8
an entry index of an entry_body;
9
the result of dereferencing an access-to-object value (see 4.1);
10/3
the return object of a function;
11
the result of evaluating an aggregate;
11.1/3
a qualified_expression whose operand denotes an object;
12
a component, slice, or view conversion of another object.
13/3
An object is either a constant object or a variable object. Similarly, a view of an object is either a constant or a variable. All views of a constant elementary object are constant. All views of a constant composite object are constant, except for parts that are of controlled or immutably limited types; variable views of those parts and their subcomponents may exist. In this sense, objects of controlled and immutably limited types are inherently mutable. A constant view of an object cannot be used to modify its value. The terms constant and variable by themselves refer to constant and variable views of objects.
14
The value of an object is read when the value of any part of the object is evaluated, or when the value of an enclosing object is evaluated. The value of a variable is updated when an assignment is performed to any part of the variable, or when an assignment is performed to an enclosing object.
15
Whether a view of an object is constant or variable is determined by the definition of the view. The following (and no others) represent constants:
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an object declared by an object_declaration with the reserved word constant;
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a formal parameter or generic formal object of mode in;
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a discriminant;
18.1/3
a loop parameter unless specified to be a variable for a generalized loop (see 5.5.2);
19/3
a choice parameter or entry index;
20
the dereference of an access-to-constant value;
20.1/3
the return object declared by an extended_return_statement with the reserved word constant;
21/3
the object denoted by a function_call or an aggregate;
21.1/3
the result of evaluating a qualified_expression;
21.2/3
within the body of a protected function (or a function declared immediately within a protected_body), the current instance of the enclosing protected unit;
22
a selected_component, indexed_component, slice, or view conversion of a constant.
23/3
At the place where a view of an object is defined, a nominal subtype is associated with the view. The object's actual subtype (that is, its subtype) can be more restrictive than the nominal subtype of the view; it always is if the nominal subtype is an indefinite subtype. A subtype is an indefinite subtype if it is an unconstrained array subtype, or if it has unknown discriminants or unconstrained discriminants without defaults (see 3.7); otherwise, the subtype is a definite subtype (all elementary subtypes are definite subtypes). A class-wide subtype is defined to have unknown discriminants, and is therefore an indefinite subtype. An indefinite subtype does not by itself provide enough information to create an object; an additional constraint or explicit initialization expression is necessary (see 3.3.1). A component cannot have an indefinite nominal subtype.
23.1/3
A view of a composite object is known to be constrained if:
23.2/3
its nominal subtype is constrained, and is not an untagged partial view; or
23.3/3
its nominal subtype is indefinite; or
23.4/3
its type is immutably limited (see 7.5); or
23.5/3
it is part of a stand-alone constant (including a generic formal object of mode in); or
23.6/3
it is part of a formal parameter of mode in; or
23.7/3
it is part of the object denoted by a function_call or aggregate; or
23.8/3
it is part of a constant return object of an extended_return_statement; or
23.9/3
it is a dereference of a pool-specific access type, and there is no ancestor of its type that has a constrained partial view.
23.10/3
For the purposes of determining within a generic body whether an object is known to be constrained:
23.11/3
if a subtype is a descendant of an untagged generic formal private or derived type, and the subtype is not an unconstrained array subtype, it is not considered indefinite and is considered to have a constrained partial view;
23.12/3
if a subtype is a descendant of a formal access type, it is not considered pool-specific.
24
A named number provides a name for a numeric value known at compile time. It is declared by a number_declaration.
NOTES
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7 A constant cannot be the target of an assignment operation, nor be passed as an in out or out parameter, between its initialization and finalization, if any.
25.1/3
8 The value of a constant object cannot be changed after its initialization, except in some cases where the object has a controlled or immutably limited part (see 7.5, 7.6, and 13.9.1).
26/3
9 The nominal and actual subtypes of an elementary object are always the same. For a discriminated or array object, if the nominal subtype is constrained, then so is the actual subtype.

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