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Certain actions that can potentially lead to erroneous execution are not
directly erroneous, but instead can cause objects to become abnormal.
Subsequent uses of abnormal objects can be erroneous.
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A scalar object can have an invalid representation, which means that the
object's representation does not represent any value of the object's
subtype. The primary cause of invalid representations is uninitialized
variables.
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Abnormal objects and invalid representations are explained in this
subclause.
Dynamic Semantics
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When an object is first created, and any explicit or default
initializations have been performed, the object and all of its parts are
in the normal state. Subsequent operations generally leave them normal.
However, an object or part of an object can become abnormal in the
following ways:
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An assignment to the object is disrupted due to an abort, See section 9.8 Abort of a Task - Abort of a Sequence of Statements, or
due to the failure of a language-defined check, See section 11.6 Exceptions and Optimization.
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The object is not scalar, and is passed to an in out or out parameter of
an imported procedure or language-defined input procedure, if after
return from the procedure the representation of the parameter does not
represent a value of the parameter's subtype.
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Whether or not an object actually becomes abnormal in these cases is not
specified. An abnormal object becomes normal again upon successful
completion of an assignment to the object as a whole.
Erroneous Execution
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It is erroneous to evaluate a primary that is a name denoting an
abnormal object, or to evaluate a prefix that denotes an abnormal
object.
Bounded (Run-Time) Errors
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If the representation of a scalar object does not represent a value of
the object's subtype (perhaps because the object was not initialized),
the object is said to have an invalid representation. It is a bounded
error to evaluate the value of such an object. If the error is detected,
either Constraint_Error or Program_Error is raised. Otherwise, execution
continues using the invalid representation. The rules of the language
outside this subclause assume that all objects have valid
representations. The semantics of operations on invalid representations
are as follows:
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If the representation of the object represents a value of the object's
type, the value of the type is used.
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If the representation of the object does not represent a value of the
object's type, the semantics of operations on such representations is
implementation-defined, but does not by itself lead to erroneous or
unpredictable execution, or to other objects becoming abnormal.
Erroneous Execution
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A call to an imported function or an instance of Unchecked_Conversion is
erroneous if the result is scalar, and the result object has an invalid
representation.
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The dereference of an access value is erroneous if it does not designate
an object of an appropriate type or a subprogram with an appropriate
profile, if it designates a nonexistent object, or if it is an
access-to-variable value that designates a constant object. Such an
access value can exist, for example, because of Unchecked_Deallocation,
Unchecked_Access, or Unchecked_Conversion.
NOTES
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(18) Objects can become abnormal due to other kinds of actions that
directly update the object's representation; such actions are generally
considered directly erroneous, however.
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