4.9 Static Expressions and Static Subtypes
Certain expressions of a scalar or string type are
defined to be static. Similarly, certain discrete ranges are defined
to be static, and certain scalar and string subtypes are defined to be
static subtypes. [ Static
at compile time, using the declared properties or values of the program
Discussion: As opposed to more elaborate
data flow analysis, etc.
Language Design Principles
For an expression to be static, it has to be
calculable at compile time.
Only scalar and string expressions are static.
To be static, an expression cannot have any
nonscalar, nonstring subexpressions (though it can have nonscalar constituent
static scalar expression cannot have any nonscalar subexpressions. There
is one exception — a membership test for a string subtype can be
static, and the result is scalar, even though a subexpression is nonscalar.
The rules for evaluating static expressions
are designed to maximize portability of static calculations.
static expression is [a scalar or string expression that is] one of the
is always a static expression, even if its expected type is not that
of a static subtype. However, if its value is explicitly converted to,
or qualified by, a nonstatic subtype, the resulting expression is nonstatic.
Ramification: That is, the constrained
subtype defined by the index range of the string is static. Note that
elementary values don't generally have subtypes, while composite values
do (since the bounds or discriminants are inherent in the value).
that denotes the declaration of a named number or a static constant;
Note that enumeration literals
are covered by the function_call
statically denotes a static function, and whose actual parameters, if
any (whether given explicitly or by default), are all static expressions;
This includes uses of operators
that are equivalent to function_call
Ramification: Note that this does not
include the case of an attribute that is a function; a reference to such
an attribute is not even an expression. See above for function calls.
An implementation may define the staticness
and other properties of implementation-defined attributes.
Reason: Adding qualification to an expression
shouldn't make it nonstatic, even for strings.
Clearly, we should allow membership
tests in exactly the same cases where we allow qualified_expression
a short-circuit control form both of whose relation
are static expressions;
a static expression enclosed in parentheses.
we talk about a static value
. When we do, we mean a value specified
by a static expression.
an entity if it denotes the entity and:
is one of the following:
Ramification: These are the functions
whose calls can be static expressions.
a predefined operator whose parameter and result
types are all scalar types none of which are descendants of formal scalar
a predefined concatenation operator whose result
type is a string type;
an enumeration literal;
a language-defined attribute that is a function,
if the prefix
denotes a static scalar subtype, and if the parameter and result types
In any case, a generic formal subprogram is not a
A static constant
constant view declared by a full constant declaration or an object_renaming_declaration
with a static nominal subtype, having a value defined by a static scalar
expression or by a static string expression whose value has a length
not exceeding the maximum length of a string_literal
in the implementation.
Ramification: A deferred constant is
not static; the view introduced by the corresponding full constant declaration
can be static.
The reason for restricting the length of static string constants is so
that compilers don't have to store giant strings in their symbol tables.
Since most string constants will be initialized from string_literal
the length limit seems pretty natural. The reason for avoiding nonstring
types is also to save symbol table space. We're trying to keep it cheap
and simple (from the implementer's viewpoint), while still allowing,
for example, the aspect_definition
for a Link_Name aspect link name of a pragma
to contain a concatenation.
The length we're talking about is the maximum
number of characters in the value represented by a string_literal
not the number of characters in the source representation; the quotes
A static range
is a range
whose bounds are static expressions, [or a range_attribute_reference
that is equivalent to such a range
A static discrete_range
is one that is a static range or is a subtype_indication
that defines a static scalar subtype. The base range of a scalar type
is a static range, unless the type is a descendant of a formal scalar
A static subtype
is either a static scalar
or a static string subtype
static scalar subtype is an unconstrained scalar subtype whose type is
not a descendant of a formal scalar
or a constrained scalar subtype formed by imposing a compatible static
constraint on a static scalar subtype.
A static string
subtype is an unconstrained string subtype whose index subtype and component
subtype are static (and whose type is not a descendant
of a formal array type)
, or a constrained string subtype formed
by imposing a compatible static constraint on a static string subtype.
In any case, the subtype of a generic formal object of mode in out
and the result subtype of a generic formal function, are not static. Also, a subtype is not static if any Dynamic_Predicate specifications
apply to it.
Ramification: String subtypes are the
only composite subtypes that can be static.
part about generic formal objects of mode in out is necessary
because the subtype of the formal is not required to have anything to
do with the subtype of the actual. For example:
subtype Int10 is Integer range 1..10;
F : in out Int10;
procedure G is
case F is
when 1..10 => null;
X : Integer range 1..20;
procedure I is new G(F => X); -- OK.
is illegal, because the subtype of F is not static, so the choices have
to cover all values of Integer, not just those in the range 1..10. A
similar issue arises for generic formal functions, now that function
calls are object names.
different kinds of static constraint
are defined as follows:
A null constraint is always static;
scalar constraint is static if it has no range_constraint
or one with a static range;
An index constraint is static
if each discrete_range
is static, and each index subtype of the corresponding array type is
A discriminant constraint is
static if each expression
of the constraint is static, and the subtype of each discriminant is
In any case, the constraint of the first subtype
of a scalar formal type is neither static nor null.
A subtype is statically constrained
if it is constrained, and its constraint is static. An object is statically
if its nominal subtype is statically constrained, or
if it is a static string constant.
An expression is statically unevaluated
if it is part of:
the right operand of a static short-circuit control
form whose value is determined by its left operand; or
(if N = 0 then Min elsif 10_000/N > Min then 10_000/N else Min)
legal if N and Min are
static and N = 0.
A static expression is evaluated at compile time except when it is statically
unevaluated part of the right operand of
a static short-circuit control form whose value is determined by its
. The compile-time This
evaluation of a static expression
exactly, without performing Overflow_Checks. For a static expression
that is evaluated:
The expression is illegal if its evaluation fails a language-defined
check other than Overflow_Check. For the purposes
of this evaluation, the assertion policy is assumed to be Check.
Assertion policies can control whether checks are
made, but we don't want assertion policies to affect legality. For Ada
2012, subtype predicates are the only checks controlled by the assertion
policy that can appear in static expressions.
If the expression is not part of a larger static expression and the expression is expected to be of a single specific type
then its value shall be within the base range of its expected type. Otherwise,
the value may be arbitrarily large or small.
If the expression is expected to be of a universal
type, or of “any integer type”, there are no limits on the
value of the expression.
If the expression is of type universal_real
and its expected type
is a decimal fixed point type, then its value shall be a multiple of
of the decimal type. This restriction
does not apply if the expected type is a descendant of a formal scalar
type (or a corresponding actual type in an instance).
This means that a numeric_literal
for a decimal type cannot have “extra” significant digits.
The small is not known for a generic formal type,
so we have to exclude formal types from this check.
In addition to the places where
Legality Rules normally apply (see 12.3),
the above restrictions also apply in the private part of an instance
of a generic unit. The last two restrictions
above do not apply if the expected type is a descendant of a formal scalar
type (or a corresponding actual type in an instance).
Discussion: Values outside the base range
are not permitted when crossing from the “static” domain
to the “dynamic” domain. This rule is designed to enhance
portability of programs containing static expressions. Note that this
rule applies to the exact value, not the value after any rounding or
truncation. (See below for the rounding and truncation requirements.)
control forms are a special case:
N: constant := 0.0;
X: constant Boolean := (N = 0.0) or else (1.0/N > 0.5); -- Static.
The declaration of X is legal, since the divide-by-zero
part of the expression is not evaluated. X is a static constant equal
There is no requirement to recheck these rules
in an instance; the base range check will generally be performed at run
For a real static expression that is not part of a larger static expression,
and whose expected type is not a descendant of a formal scalar
type, the implementation shall round or truncate the value (according
to the Machine_Rounds attribute of the expected type) to the nearest
machine number of the expected type; if the value is exactly half-way
between two machine numbers, the any
rounding shall be
implementation-defined away from zero
If the expected type is a descendant of a formal scalar
type, or if the static expression appears
in the body of an instance of a generic unit and the corresponding expression
is nonstatic in the corresponding generic body, then
rounding or truncating is required — normal accuracy rules apply
(see Annex G
Rounding of real static expressions
which are exactly half-way between two machine numbers.
Discarding extended precision enhances portability by ensuring that the
value of a static constant of a real type is always a machine number
of the type. Deterministic rounding of exact halves
also enhances portability.
When the expected type is a descendant of a
formal floating point type, extended precision (beyond that of the machine
numbers) can be retained when evaluating a static expression, to ease
code sharing for generic instantiations. For similar reasons, normal
(nondeterministic) rounding or truncating rules apply for descendants
of a formal fixed point type.
There is no requirement for exact evaluation or
special rounding in an instance body (unless the expression is static
in the generic body). This eliminates a potential contract issue where
the exact value of a static expression depends on the actual parameters
(which could then affect the legality of other code).
Implementation Note: Note that the implementation
of static expressions has to keep track of plus and minus zero for a
type whose Signed_Zeros attribute is True.
Note that the only machine numbers values
of a fixed point type are the multiples of the small, so a static conversion
to a fixed-point type, or division by an integer, must do truncation
to a multiple of small. It is not correct for the implementation to do
all static calculations in infinite precision.
For a real static expression that is not part of
a larger static expression, and whose expected type is not a descendant
of a formal type, the rounding should be the same as the default rounding
for the target system.
A real static expression with a nonformal
type that is not part of a larger static expression should be rounded
the same as the target system.
28 An expression can be static even if
it occurs in a context where staticness is not required.
X : Float := Float'(1.0E+400) + 1.0 - Float'(1.0E+400);
The expression is static, which means that the
value of X must be exactly 1.0, independent of the accuracy or range
of the run-time floating point implementation.
29 A static (or run-time) type_conversion
from a real type to an integer type performs rounding. If the operand
value is exactly half-way between two integers, the rounding is performed
away from zero.
Reason: We specify this for portability.
The reason for not choosing round-to-nearest-even, for example, is that
this method is easier to undo.
The attribute Truncation
) can be used to perform a (static)
truncation prior to conversion, to prevent rounding.
Implementation Note: The value of the
literal 0E999999999999999999999999999999999999999999999 is zero. The
implementation must take care to evaluate such literals properly.
Examples of static
1 + 1 -- 2
abs(-10)*3 -- 30
Kilo : constant := 1000;
Mega : constant := Kilo*Kilo; -- 1_000_000
Long : constant := Float'Digits*2;
Half_Pi : constant
:= Pi/2; -- see 3.3.2
Deg_To_Rad : constant
Rad_To_Deg : constant
:= 1.0/Deg_To_Rad; -- equivalent to 1.0/((3.14159_26536/2)/90)
Extensions to Ada 83
The rules for static expressions
and static subtypes are generalized to allow more kinds of compile-time-known
expressions to be used where compile-time-known values are required,
Membership tests and short-circuit control
forms may appear in a static expression.
The bounds and length of statically constrained
array objects or subtypes are static.
The Range attribute of a statically constrained
array subtype or object gives a static range.
All numeric literals are now static, even
if the expected type is a formal scalar type. This is useful in case_statement
which both now allow a value of a formal scalar type to control the selection,
to ease conversion of a package into a generic package. Similarly, named
array aggregates are also permitted for array types with an index type
that is a formal scalar type.
The rules for the evaluation of static expressions
are revised to require exact evaluation at compile time, and force a
machine number result when crossing from the static realm to the dynamic
realm, to enhance portability and predictability. Exact evaluation is
not required for descendants of a formal scalar type, to simplify generic
code sharing and to avoid generic contract model problems.
are legal even if an intermediate in the expression goes outside the
base range of the type. Therefore, the following will succeed in Ada
95, whereas it might raise an exception in Ada 83:
type Short_Int is range -32_768 .. 32_767;
I : Short_Int := -32_768;
This might raise an exception in Ada 83 because
"32_768" is out of range, even though "–32_768"
is not. In Ada 95, this will always succeed.
Certain expressions involving string operations
(in particular concatenation and membership tests) are considered static
in Ada 95.
The reason for this change is to simplify the
rule requiring compile-time-known string expressions as the link name
in an interfacing pragma, and to simplify the preelaborability rules.
Incompatibilities With Ada 83
An Ada 83 program that uses
an out-of-range static value is illegal in Ada 95, unless the expression
is part of a larger static expression, or the expression is not evaluated
due to being on the right-hand side of a short-circuit control form.
Wording Changes from Ada 83
The existence of static string expressions necessitated
changing the definition of static subtype to include string subtypes.
Most occurrences of "static subtype" have been changed to "static
scalar subtype", in order to preserve the effect of the Ada 83 rules.
This has the added benefit of clarifying the difference between "static
subtype" and "statically constrained subtype", which has
been a source of confusion. In cases where we allow static string subtypes,
we explicitly use phrases like "static string subtype" or "static
(scalar or string) subtype", in order to clarify the meaning for
those who have gotten used to the Ada 83 terminology.
In Ada 83, an
expression was considered nonstatic if it raised an exception. Thus,
Bad: constant := 1/0; -- Illegal!
was illegal because 1/0 was not static. In Ada
95, the above example is still illegal, but for a different reason: 1/0
is static, but there's a separate rule forbidding the exception raising.
Inconsistencies With Ada 95
Rounding of static real expressions is implementation-defined in Ada
2005, while it was specified as away from zero in (original) Ada 95.
This could make subtle differences in programs. However, the original
Ada 95 rule required rounding that (probably) differed from the target
processor, thus creating anomalies where the value of a static expression
was required to be different than the same expression evaluated at run-time.
Wording Changes from Ada 95
The Ada 95 wording that defined static subtypes
unintentionally failed to exclude formal derived types that happen to
be scalar (these aren't formal scalar types); and had a parenthetical
remark excluding formal string types - but that was neither necessary
nor parenthetical (it didn't follow from other wording). This issue also
applies to the rounding rules for real static expressions.
Ada 95 didn't clearly define the bounds of a value
of a static expression for universal types and for "any integer/float/fixed
type". We also make it clear that we do not intend exact evaluation
of static expressions in an instance body if the expressions aren't static
in the generic body.
We clarify that the first subtype of a scalar formal
type has a nonstatic, nonnull constraint.
Wording Changes from Ada 2005
Added wording to prevent subtypes that have dynamic
predicates (see 3.2.4) from being static.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe