4.2.1 User-Defined Literals
Using one or more of the aspects defined below, a
type may be specified to allow the use of one or more kinds of literals
as values of the type.
Static Semantics
The following type-related
operational aspects (collectively known as
user-defined literal aspects)
may be specified for a type
T:
Integer_Literal
This aspect is specified by a
function_name
that statically denotes a function with a result type of
T and
one
in parameter that is of type String and is not explicitly
aliased.
Real_Literal
This aspect is specified by a
function_name
that statically denotes a function with a result type of
T and
one
in parameter that is of type String and is not explicitly
aliased, and optionally a second function (overloading the first) with
a result type of
T and two
in parameters of type String
that are not explicitly aliased.
String_Literal
This aspect is specified by a
function_name
that statically denotes a function with a result type of
T and
one
in parameter that is of type Wide_Wide_String and is not explicitly
aliased.
User-defined literal aspects are nonoverridable (see
13.1.1).
When a numeric literal is interpreted as a value
of a non-numeric type
T or a
string_literal
is interpreted as a value of a type
T that is not a string type
(see
4.2), it is equivalent to a call to the
subprogram denoted by the corresponding aspect of
T: the Integer_Literal
aspect for an integer literal, the Real_Literal aspect for a real literal,
and the String_Literal aspect for a
string_literal.
The actual parameter of this notional call is a
string_literal
representing a sequence of characters that is the same as the sequence
of characters in the original numeric literal, or the sequence represented
by the original string literal.
Such a literal is said to be a
user-defined literal.
When a named number that denotes a value of type
universal_integer is interpreted as a value of a non-numeric type
T, it is equivalent to a call to the function denoted by the Integer_Literal
aspect of T. The actual parameter of this notional call is a String
having a textual representation of a decimal integer literal optionally
preceded by a minus sign, representing the same value as the named number.
When a named number that denotes a value of type
universal_real is interpreted as a value of a non-numeric type
T, it is equivalent to a call to the two-parameter function denoted
by the Real_Literal aspect of T, if any. The actual parameters
of this notional call are each a String with the textual representation
of a decimal integer literal, with the first optionally preceded by a
minus sign, where the first String represents the same value as the numerator,
and the second the same value as the denominator, of the named number
when represented as a rational number in lowest terms, with a positive
denominator.
Legality Rules
The Integer_Literal or Real_Literal aspect shall
not be specified for a type T if the full view of T is
a numeric type. The String_Literal aspect shall not be specified for
a type T if the full view of T is a string type.
For a nonabstract type, the function directly specified
for a user-defined literal aspect shall not be abstract.
For a tagged type with a partial view, a user-defined
literal aspect shall not be directly specified on the full type.
If a nonabstract tagged type inherits any user-defined
literal aspect, then each inherited aspect shall be directly specified
as a nonabstract function for the type unless the inherited aspect denotes
a nonabstract function, or functions, and the type is a null extension.
If a named number that denotes a value of type universal_integer
is interpreted as a value of a non-numeric type T, T shall
have an Integer_Literal aspect. If a named number that denotes a value
of type universal_real is interpreted as a value of a non-numeric
type T, T shall have a Real_Literal aspect, and the aspect
shall denote a function that has two in parameters, both of type
String, with result of type T.
In addition to the places where Legality Rules normally
apply (see
12.3), these rules also apply in
the private part of an instance of a generic unit.
Bounded (Run-Time) Errors
It is a bounded error if the
evaluation of a literal or named number that has an expected type with
a specified user-defined literal aspect propagates an exception. Either
Program_Error or the exception propagated by the evaluation is raised
at the point of use of the value of the literal or named number. If it
is recognized prior to run time that evaluation of such a literal or
named number will inevitably (if executed) result in such a bounded error,
then this may be reported as an error prior to run time.
Examples
Examples of the
specification and use of user-defined literals:
subtype Roman_Character is Wide_Wide_Character
with Static_Predicate =>
Roman_Character in 'I' | 'V' | 'X' | 'L' | 'C' | 'D' | 'M';
Max_Roman_Number : constant := 3_999; -- MMMCMXCIX
type Roman_Number is range 1 .. Max_Roman_Number
with String_Literal => To_Roman_Number;
function To_Roman_Number (S : Wide_Wide_String) return Roman_Number
with Pre => S'Length > 0 and then
(for all Char of S => Char in Roman_Character);
function To_Roman_Number (S : Wide_Wide_String)
return Roman_Number
is
(
declare
R :
constant array (Integer
range <>)
of Roman_Number :=
(
for D
in S'Range => Roman_Digit'Enum_Rep
(Roman_Digit'Wide_Wide_Value (''' & S(D) & ''')));
--
See 3.5.2 and 13.4
begin
[
for I
in R'Range =>
(
if I < R'Last
and then R(I) < R(I + 1)
then -1
else 1) * R(I)]
'Reduce("+", 0)
);
X : Roman_Number := "III" * "IV" * "XII"; -- 144 (that is, CXLIV)
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