3.5.4 Integer Types

Discussion: We don't call this a range_constraint, because it is rather different — not only is it required to be static, but the associated overload resolution rules are different than for normal range constraints. A similar comment applies to real_range_specification. This used to be integer_range_specification but when we added support for modular types, it seemed overkill to have three levels of syntax rules, and just calling these signed_integer_range_specification and modular_range_specification loses the fact that they are defining different classes of types, which is important for the generic type matching rules. 

Reason: For a 2's-complement machine, supporting nonbinary moduli greater than System.Max_Int can be quite difficult, whereas essentially any binary moduli are straightforward to support, up to 2*System.Max_Int+2, so this justifies having two separate limits. 

Implementation Note: {AI95-00114-01} The base range of a signed integer type might be much larger than is necessary to satisfy the above aboved requirements. 

Reason: Integer is a constrained subtype, rather than an unconstrained subtype. This means that on assignment to an object of subtype Integer, a range check is required. On the other hand, an object of subtype Integer'Base is unconstrained, and no range check (only overflow check) is required on assignment. For example, if the object is held in an extended-length register, its value might be outside of Integer'First .. Integer'Last. All parameter and result subtypes of the predefined integer operators are of such unconstrained subtypes, allowing extended-length registers to be used as operands or for the result. In an earlier version of Ada 95, Integer was unconstrained. However, the fact that certain Constraint_Errors might be omitted or appear elsewhere was felt to be an undesirable upward inconsistency in this case. Note that for Float, the opposite conclusion was reached, partly because of the high cost of performing range checks when not actually necessary. Objects of subtype Float are unconstrained, and no range checks, only overflow checks, are performed for them. 

Discussion: This implicit derivation is not considered exactly equivalent to explicit derivation via a derived_type_definition. In particular, integer types defined via a derived_type_definition inherit their base range from their parent type. A type defined by an integer_type_definition does not necessarily inherit its base range from root_integer. It is not specified whether the implicit derivation from root_integer is direct or indirect, not that it really matters. All we want is for all integer types to be descendants of root_integer.

{8652/0099} {AI95-00152-01} Note that this derivation does not imply any inheritance of subprograms. Subprograms are inherited only for types derived by a derived_type_definition (see 3.4), or a private_extension_declaration (see 7.3, 7.3.1, and 12.5.1). 

Implementation Note: It is the intent that even nonstandard integer types (see below) will be descendants of root_integer, even though they might have a base range that exceeds that of root_integer. This causes no problem for static calculations, which are performed without range restrictions (see 4.9). However for run-time calculations, it is possible that Constraint_Error might be raised when using an operator of root_integer on the result of 'Val applied to a value of a nonstandard integer type. 

Discussion: Constraint_Error is never raised for operations on modular types, except for divide-by-zero (and rem/mod-by-zero). 

Implementation defined: The predefined integer types declared in Standard.

Implementation defined: Any nonstandard integer types and the operators defined for them.

Reason: On a one's complement machine, the natural full word type would have a modulus of 2**Word_Size–1. However, we would want to allow the all-ones bit pattern (which represents negative zero as a number) in logical operations. These permissions are intended to allow that and the natural modulus value without burdening implementations with supporting expensive modulus values. 

Implementation Note: To promote portability, implementations should explicitly declare the integer (sub)types Integer and Long_Integer in Standard, and leave other predefined integer types anonymous. For implementations that already support Byte_Integer, etc., upward compatibility argues for keeping such declarations in Standard during the transition period, but perhaps generating a warning on use. A separate package Interfaces in the predefined environment is available for pre-declaring types such as Integer_8, Integer_16, etc. See B.2. In any case, if the user declares a subtype (first or not) whose range fits in, for example, a byte, the implementation can store variables of the subtype in a single byte, even if the base range of the type is wider. 

Reason: Modular types provide bit-wise "and", "or", "xor", and "not" operations. It is important for systems programming that these be available for all integer types of the target hardware.

Ramification: Note that on a one's complement machine, the largest supported modular type would normally have a nonbinary modulus. On a two's complement machine, the largest supported modular type would normally have a binary modulus. 

Implementation Note: Supporting a nonbinary modulus greater than Integer'Last can impose an undesirable implementation burden on some machines. 

An implementation is allowed to support any number of distinct base ranges for integer types, even if fewer integer types are explicitly declared in Standard.

Modular (unsigned, wrap-around) types are new.

Ada 83's integer types are now called "signed" integer types, to contrast them with "modular" integer types.

Standard.Integer, Standard.Long_Integer, etc., denote constrained subtypes of predefined integer types, consistent with the Ada 95 model that only subtypes have names.

We now impose minimum requirements on the base range of Integer and Long_Integer.

We no longer explain integer type definition in terms of an equivalence to a normal type derivation, except to say that all integer types are by definition implicitly derived from root_integer. This is for various reasons.

First of all, the equivalence with a type derivation and a subtype declaration was not perfect, and was the source of various AIs (for example, is the conversion of the bounds static? Is a numeric type a derived type with respect to other rules of the language?)

Secondly, we don't want to require that every integer size supported shall have a corresponding named type in Standard. Adding named types to Standard creates nonportabilities.

Thirdly, we don't want the set of types that match a formal derived type "type T is new Integer;" to depend on the particular underlying integer representation chosen to implement a given user-defined integer type. Hence, we would have needed anonymous integer types as parent types for the implicit derivation anyway. We have simply chosen to identify only one anonymous integer type — root_integer, and stated that every integer type is derived from it.

Finally, the “fiction” that there were distinct preexisting predefined types for every supported representation breaks down for fixed point with arbitrary smalls, and was never exploited for enumeration types, array types, etc. Hence, there seems little benefit to pushing an explicit equivalence between integer type definition and normal type derivation. 

{AI95-00340-01} The Mod attribute is new. It eases mixing of signed and unsigned values in an expression, which can be difficult as there may be no type which can contain all of the values of both of the types involved. 

{8652/0003} {AI95-00095-01} Corrigendum: Added additional permissions for modular types on one's complement machines.