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
static expression is a scalar or string expression that is one of the
that denotes the declaration of a named number or a static constant;
statically denotes a static function, and whose actual parameters, if
any (whether given explicitly or by default), are all static expressions;
a short-circuit control form both of whose relation
are static expressions;
a static expression enclosed in parentheses.
an entity if it denotes the entity and:
is one of the following:
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.
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
a static scalar subtype
or a static string subtype
static scalar subtype is an unconstrained scalar subtype whose type is
not a descendant of a formal type, 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, 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.
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
if it is part of:
the right operand of a static short-circuit control
form whose value is determined by its left operand; or
A static expression
is evaluated at compile time except when it is statically unevaluated.
The compile-time evaluation of a static expression is performed 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.
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 of type universal_real
and its expected type is a decimal fixed point type, then its value shall
be a multiple of the small 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).
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.
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 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 rounding performed is implementation-defined.
If the expected type is a descendant of a formal 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 no special rounding or truncating is required — normal accuracy
rules apply (see Annex G
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.
27 An expression can be static even if
it occurs in a context where staticness is not required.
28 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.
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)
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