3.3 Objects and Named Numbers
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).
All of the following are objects:
- the entity declared by an object_declaration;
- a formal parameter of a subprogram, entry, or generic subprogram;
- a generic formal object;
- a loop parameter;
- a choice parameter of an exception_handler;
- an entry index of an entry_body;
- the result of dereferencing an access-to-object value (see 4.1);
- the result of evaluating a function_call (or the equivalent operator invocation -- see 6.6);
- the result of evaluating an aggregate;
- a component, slice, or view conversion of another object.
An object is either a constant object or a variable object. The value of a constant object cannot be changed between its initialization and its finalization, whereas the value of a variable object can be changed. Similarly, a view of an object is either a constant or a variable. All views of a constant object are constant. A constant view of a variable object cannot be used to modify the value of the variable. The terms constant and variable by themselves refer to constant and variable views of objects.
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.
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:
- an object declared by an object_declaration with the reserved word constant;
- a formal parameter or generic formal object of mode in;
- a discriminant;
- a loop parameter, choice parameter, or entry index;
- the dereference of an access-to-constant value;
- the result of evaluating a function_call or an aggregate;
- a selected_component, indexed_component, slice, or view conversion of a constant.
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.
A named number provides a name for a numeric value known at compile time. It is declared by a number_declaration.
5 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.
6 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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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual