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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).
{AI95-00426-01} 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.
{AI95-00426-01} 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.
Discussion: We explicitly list the routines involved in order to avoid future arguments. All possibilities are listed.
We did not include Stream_IO.Read in the list above. A Stream_Element should include all possible bit patterns, and thus it cannot be invalid. Therefore, the parameter will always represent a value of its subtype. By omitting this routine, we make it possible to write arbitrary I/O operations without any possibility of abnormal objects. 
  {AI95-00426-01} [For an imported object, it is the programmer's responsibility to ensure that the object remains in a normal state.] 
Proof: This follows (and echos) the standard rule of interfacing; the programmer must ensure that Ada semantics are followed (see B.1). 
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. 
This paragraph was deleted.{AI95-00114-01}
Ramification: The in out or out parameter case does not apply to scalars; bad scalars are merely invalid representations, rather than abnormal, in this case. 
Reason: {AI95-00114-01} The reason we allow access objects, and objects containing subcomponents of an access type, to become abnormal is because the correctness of an access value cannot necessarily be determined merely by looking at the bits of the object. The reason we allow scalar objects to become abnormal is that we wish to allow the compiler to optimize assuming that the value of a scalar object belongs to the object's subtype, if the compiler can prove that the object is initialized with a value that belongs to the subtype. The reason we allow composite objects to become abnormal is that such object might be represented with implicit levels of indirection; if those are corrupted, then even assigning into a component of the object, or simply asking for its Address, might have an unpredictable effect. The same is true if the discriminants have been destroyed. 

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:
Discussion: The AARM is more explicit about what happens when the value of the case expression is an invalid representation.
Ramification: {AI95-00426-01} This includes the result object of functions, including the result of Unchecked_Conversion, T'Input, and imported functions. 
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. 
Implementation Note: {AI95-00426-01} This means that the implementation must take care not to use an invalid representation in a way that might cause erroneous execution. For instance, the exception mandated for case_statements must be raised. Array indexing must not cause memory outside of the array to be written (and usually, not read either). These cases and similar cases may require explicit checks by the implementation. 

Erroneous Execution

 {AI95-00167-01} {AI05-0279-1} 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. 
Ramification: {AI95-00167-01} In a typical implementation, every bit pattern that fits in an object of a signed integer subtype will represent a value of the type, if not of the subtype. However, for an enumeration or floating point type, as well as some modular types, there are typically bit patterns that do not represent any value of the type. In such cases, the implementation ought to define the semantics of operations on the invalid representations in the obvious manner (assuming the bounded error is not detected): a given representation should be equal to itself, a representation that is in between the internal codes of two enumeration literals should behave accordingly when passed to comparison operators and membership tests, etc. We considered requiring such sensible behavior, but it resulted in too much arcane verbiage, and since implementations have little incentive to behave irrationally, such verbiage is not important to have.
{AI95-00167-01} If a stand-alone scalar object is initialized to a an in-range value, then the implementation can take advantage of the fact that the use of any out-of-range value has to be erroneous. Such an out-of-range value can be produced only by things like unchecked conversion, imported functions, and abnormal values caused by disruption of an assignment due to abort or to failure of a language-defined check. This depends on out-of-range values being checked before assignment (that is, checks are not optimized away unless they are proven redundant).
Consider the following example: 
{AI95-00167-01} type My_Int is range 0..99;
function Safe_Convert is new Unchecked_Conversion(My_Int, Integer);
function Unsafe_Convert is new Unchecked_Conversion(My_Int, Positive);
X : Positive := Safe_Convert(0); -- Raises Constraint_Error.
Y : Positive := Unsafe_Convert(0); -- Bounded Error, may be invalid.
B : Boolean  := Y'Valid; -- OK, B = False.
Z : Positive := Y+1; -- Erroneous to use Y.
{AI95-00167-01} {AI95-00426-01} The call to Unsafe_Convert is a bounded error, which might raise Constraint_Error, Program_Error, or return an invalid value. Moreover, if an exception is not raised, most uses of that invalid value (including the use of Y) cause erroneous execution. The call to Safe_Convert is not erroneous. The result object is an object of subtype Integer containing the value 0. The assignment to X is required to do a constraint check; the fact that the conversion is unchecked does not obviate the need for subsequent checks required by the language rules.
{AI95-00167-01} {AI95-00426-01} The reason for delaying erroneous execution until the object is used is so that the invalid representation can be tested for validity using the Valid attribute (see 13.9.2) without causing execution to become erroneous. Note that this delay does not imply an exception will not be raised; an implementation could treat both conversions in the example in the same way and raise Constraint_Error.
{AI05-0279-1} The rules are defined in terms of the result object, and thus the name used to reference that object is irrelevant. That is why we don't need any special rules to describe what happens when the function result is renamed.
Implementation Note: If an implementation wants to have a “friendly” mode, it might always assign an uninitialized scalar a default initial value that is outside the object's subtype (if there is one), and check for this value on some or all reads of the object, so as to help detect references to uninitialized scalars. Alternatively, an implementation might want to provide an “unsafe” mode where it presumed even uninitialized scalars were always within their subtype. 
Ramification: The above rules imply that it is a bounded error to apply a predefined operator to an object with a scalar subcomponent having an invalid representation, since this implies reading the value of each subcomponent. Either Program_Error or Constraint_Error is raised, or some result is produced, which if composite, might have a corresponding scalar subcomponent still with an invalid representation.
Note that it is not an error to assign, convert, or pass as a parameter a composite object with an uninitialized scalar subcomponent. In the other hand, it is a (bounded) error to apply a predefined operator such as =, <, and xor to a composite operand with an invalid scalar subcomponent. 
 {AI05-0054-2} 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.]
Ramification: The above mentioned Unchecked_... features are not the only causes of such access values. For example, interfacing to other languages can also cause the problem.
{AI05-0054-2} We permit the use of access-to-variable values that designate constant objects so long as they originate from an aliased variable view of a controlled or immutably limited constant, such as during the initialization of a constant (both via the “current instance” and during a call to Initialize) or during an assignment (during a call to Adjust).
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. 

Wording Changes from Ada 83

In order to reduce the amount of erroneousness, we separate the concept of an undefined value into objects with invalid representation (scalars only) and abnormal objects.
Reading an object with an invalid representation is a bounded error rather than erroneous; reading an abnormal object is still erroneous. In fact, the only safe thing to do to an abnormal object is to assign to the object as a whole. 

Wording Changes from Ada 95

{AI95-00167-01} The description of erroneous execution for Unchecked_Conversion and imported objects was tightened up so that using the Valid attribute to test such a value is not erroneous.
{AI95-00426-01} Clarified the definition of objects that can become abnormal; made sure that all of the possibilities are included. 

Wording Changes from Ada 2005

{AI05-0054-2} Correction: Common programming techniques such as squirreling away an access to a controlled object during initialization and using a self-referencing discriminant (the so-called “Rosen trick”) no longer are immediately erroneous if the object is declared constant, so these techniques can be used portably and safely. Practically, these techniques already worked as compilers did not take much advantage of this rule, so the impact of this change will be slight. 
{AI05-0279-1} Correction: The description of erroneous execution for Unchecked_Conversion and imported objects was adjusted to clarify that renaming such an object is not, by itself, erroneous. 

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