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13.9.1 Data Validity

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.
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.
Abnormal objects and invalid representations are explained in this subclause. 

Dynamic Semantics

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: 
An assignment to the object is disrupted due to an abort (see 9.8) or due to the failure of a language-defined check (see 11.6).
The object is not scalar, and is passed to an in out or out parameter of an imported procedure, the Read procedure of an instance of Sequential_IO, Direct_IO, or Storage_IO, or the stream attribute T'Read, if after return from the procedure the representation of the parameter does not represent a value of the parameter's subtype.
The object is the return object of a function call of a nonscalar type, and the function is an imported function, an instance of Unchecked_Conversion, or the stream attribute T'Input, if after return from the function the representation of the return object does not represent a value of the function's subtype.
  For an imported object, it is the programmer's responsibility to ensure that the object remains in a normal state. 
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

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

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:
If the representation of the object represents a value of the object's type, the value of the type is used.
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

 A call to an imported function or an instance of Unchecked_Conversion is erroneous if the result is scalar, the result object has an invalid representation, and the result is used other than as the expression of an assignment_statement or an object_declaration, as the object_name of an object_renaming_declaration, or as the prefix of a Valid attribute. If such a result object is used as the source of an assignment, and the assigned value is an invalid representation for the target of the assignment, then any use of the target object prior to a further assignment to the target object, other than as the prefix of a Valid attribute reference, is erroneous. 
 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 and it did not originate from an attribute_reference applied to an aliased variable view of a controlled or immutably limited object. An access value whose dereference is erroneous can exist, for example, because of Unchecked_Deallocation, Unchecked_Access, or Unchecked_Conversion.
21  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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