4.10 Image Attributes
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An image of a value is a string representing
the value in display form. The attributes Image,
Wide_Image, and Wide_Wide_Image are available to produce the image of
a value as a String, Wide_String, or Wide_Wide_String (respectively).
User-defined images for a given type can be implemented by overriding
the default implementation of the attribute Put_Image. Static Semantics
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For every subtype S of a type T other than universal_real
or universal_fixed, the following type-related operational attribute
is defined:S'Put_Image
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S'Put_Image denotes
a procedure with the following specification:
procedure S'Put_Image
(Buffer : in out Ada.Strings.Text_Buffers.Root_Buffer_Type'Class;
Arg : in T);
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The default implementation of S'Put_Image writes
(using Wide_Wide_Put) an image of the value of Arg.Aspect Description
for Put_Image: Procedure
to define the image of a given type.
Discussion: In
contrast, the Image, Wide_Image, and Wide_Wide_Image attributes and their
associated aspects can not be specified. The behavior of any of these
attributes is defined in terms of calls to the corresponding Put_Image
procedure, so changes in their behavior may be accomplished via a Put_Image
specification.
In earlier versions of
Ada, Image and related attributes were defined only for scalar types.
The definition of these attributes is now very different, but there should
be no change in the behavior of existing programs as a result of these
changes.
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The behavior of the default implementation of S'Put_Image
depends on the class of T. For an elementary type, the implementation
is equivalent to: procedure Scalar_Type'Put_Image
(Buffer : in out Ada.Strings.Text_Buffers.Root_Buffer_Type'Class;
Arg : in Scalar_Type) is
begin
Buffer.Wide_Wide_Put (<described below>);
end Scalar_Type'Put_Image;
where
the Wide_Wide_String value written out to the stream is defined as follows:
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For an integer type, the image written out is the
corresponding decimal literal, without underlines, leading zeros, exponent,
or trailing spaces, but with a single leading character that is either
a minus sign or a space.Implementation Note:
If the machine supports negative zeros for signed integer types,
it is not specified whether " 0" or "-0" should be
returned for negative zero. We don't have enough experience with such
machines to know what is appropriate, and what other languages do. In
any case, the implementation should be consistent.
Discussion: We
allow S'Put_Image when S is universal_integer or root_integer,
because the details of the desired string do not depend on properties
of an integer type. While S'Put_Image cannot be called directly for these
types (as they cannot be named), it can be called as part of evaluating
an Image attribute. Note that other rules of the language ensure that
an implementation can evaluate any universal_integer attribute
using type root_integer; therefore, Constraint_Error could be
raised by the evaluation of an Image attribute if the value of the prefix
is outside of the range of root_integer.
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For an enumeration type, the image written out
is either the corresponding identifier in upper case or the corresponding
character literal (including the two apostrophes); neither leading nor
trailing spaces are included. For a nongraphic character (a value
of a character type that has no enumeration literal associated with it),
the value is a corresponding language-defined name in upper case (for
example, the image of the nongraphic character identified as nul
is "NUL" — the quotes are not part of the image).Implementation Note:
For an enumeration type T that has "holes" (caused by an
enumeration_representation_clause), T'Put_Image should raise Program_Error
if the value is one of the holes (which is a bounded error anyway, since
holes can be generated only via uninitialized variables and similar things).
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For a floating point type, the image written out
is a decimal real literal best approximating the value (rounded away
from zero if halfway between) with a single leading character that is
either a minus sign or a space, a single digit (that is nonzero unless
the value is zero), a decimal point, S'Digits-1 (see 3.5.8)
digits after the decimal point (but one if S'Digits is one), an upper
case E, the sign of the exponent (either + or -), and two or more digits
(with leading zeros if necessary) representing the exponent. If S'Signed_Zeros
is True, then the leading character is a minus sign for a negatively
signed zero.To be honest: Leading
zeros are present in the exponent only if necessary to make the exponent
at least two digits.
Reason: This image
is intended to conform to that produced by Text_IO.Float_IO.Put in its
default format.
Implementation Note:
The rounding direction is specified here to ensure portability of
output results.
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We do not allow S'Put_Image when S is universal_real,
as the details of the desired string depend on the properties of the
(specific) type of S. Specifically, universal_real does not have
a defined value for S'Digits.
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For a fixed point type, the image written out is
a decimal real literal best approximating the value (rounded away from
zero if halfway between) with a single leading character that is either
a minus sign or a space, one or more digits before the decimal point
(with no redundant leading zeros), a decimal point, and S'Aft (see 3.5.10)
digits after the decimal point.Reason: This image
is intended to conform to that produced by Text_IO.Fixed_IO.Put.
Implementation Note:
The rounding direction is specified here to ensure portability of
output results.
For a machine that supports
negative zeros, it is not specified whether " 0.000" or "-0.000"
is returned. See corresponding comment above about integer types with
signed zeros.
Reason: {
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We do not allow S'Put_Image when S is universal_fixed,
as the details of the desired string depend on the properties of the
(specific) type of S. Specifically, universal_fixed does not have
a defined value for S'Aft.
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For an access type (named or anonymous), the image
written out depends on whether the value is null. If it is null,
then the image is "NULL". Otherwise the image is a
left parenthesis followed by "ACCESS", a space, and
a sequence of graphic characters, other than space or right parenthesis,
representing the location of the designated object, followed by a right
parenthesis, as in "(ACCESS FF0012AC)".To be honest: {
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S'Put_Image is defined for universal_access,
but it can never be called (as no legal prefix
of Image has that type, and that type cannot be named preventing direct
calls).
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For an array type T, the default implementation
of T'Put_Image generates an image based on named (not positional) array
aggregate syntax (with '[' and ']' as the delimiters) using calls to
the Put_Image procedures of the index type(s) and the element type to
generate images for values of those types.Discussion: In
general, the default implementation of T'Put_Image for a composite type
will involve some sequence of calls to Wide_Wide_String'Write and calls
to the Put_Image procedures of component types and, in the case of an
array type, index types. The Wide_Wide_String'Write calls may pass in
either literal values (e.g., "(", ")", "'(",
" => ", or ", "), or other things (such as component
names for record values, task_id images for tasks, or the Wide_Wide_Expanded_Name
of the tag in the class-wide case).
Discussion:
An array type might generate an image such as:
[ 1 => [ 1 => [ 123 => True, 124 => False]
2 => [ 123 => False, 124 => False]],
2 => [ 1 => [ 123 => True, 124 => True],
2 => [ 123 => True, 124 => False]]]
although perhaps with
different white space and/or line breaking.
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The case of a null array is handled specially,
using ranges for index bounds and "<>" as a
syntactic component-value placeholder.Discussion: This
might generate an image such as "[ 1 .. 3 => [ 1 .. 0 =>
[ 1 .. 5 => <>]]]", where the use of "<>"
(among other things) indicates that the overall array is a null array
and has no actual elements.
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The order in which components are written for a
composite type is the same canonical order in which components of a composite
type T are written out by the default implementation of T'Write. [This
is also the order that is used in determining the meaning of a positional
aggregate of type T.]{
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For a class-wide type, the default implementation
of T'Put_Image generates an image based on qualified expression syntax.
Wide_Wide_String'Write is called with Wide_Wide_Expanded_Name of Arg'Tag.
Then S'Put_Image is called, where S is the specific type identified by
Arg'Tag.Implementation Note:
This will typically require a dispatching call.
Discussion:
This might generate an image such as:
SOME_PACKAGE.SOME_TAGGED_TYPE'
(COMPONENT_1 => 123, COMPONENT_2 => 456)
The parentheses in this
case are generated by the call to Some_Tagged_Type'Put_Image.
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For a type extension, the default implementation
of T'Put_Image depends on whether there exists a noninterface ancestor
of T (other than T itself) for which the Put_Image aspect has been [explicitly]
specified. If so, then T'Put_Image will generate an image based on extension
aggregate syntax where the ancestor type of the extension aggregate is
the nearest ancestor type whose Put_Image aspect has been specified.Discussion: This
might generate an image such as "(This Text Was User-Generated
with C1 => 123, C2 => 456)" where the "This Text
was User-Generated" portion of the text was generated by the call
to the user-specified Put_Image routine.
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If no such ancestor exists, then the default implementation
of T'Put_Image is the same as described below for an untagged record
type.{
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For an untagged record type, a specific tagged
record type other than a type extension which meets the criteria described
in the previous paragraph, or a protected type, the default implementation
of T'Put_Image generates an image based on named (not positional) record
aggregate syntax (except that for a protected type, the initial left
parenthesis is followed by "PROTECTED with "). Component
names are displayed in upper case, following the rules for the image
of an enumeration value. Component values are displayed via calls to
the component type's Put_Image procedure.Discussion:
This might generate an image such as:
"(FOO => [1 => 'c', 2 => 'a', 3 => 't'], BAR => TRUE)"
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The image written out for a record having no components
(including any interface type) is "(NULL RECORD)".
The image written out for a componentless protected type is "(PROTECTED NULL RECORD)".
In the case of a protected type T, a call to the default implementation
of T'Put_Image begins only one protected (read-only) action.Implementation Note:
The expected, but not required, implementation model for generating
the image of a protected record involves the compiler producing a "helper"
protected function which T'Put_Image would call. The result type of this
function might be a null record; it is only a function because it does
not need a write-lock, not because it returns a meaningful result.
The
assertion in the following example should succeed:
type T1 (D1, D2 : Positive) is record ... end record; -- untagged
type T2 (D : Positive) is new T1 (D1 => D, D2 => D);
X : T2 (D => 123) := ... ;
pragma Assert (X'Image /= T1(X)'Image);
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For an undiscriminated task type, the default implementation
of T'Put_Image generates an image of the form "(TASK <task_id_image>)"
where <task_id_image> is the result obtained by calling Task_Identification.Image
with the id of the given task and then passing that String to Characters.Conversions.To_Wide_Wide_String.{
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For a discriminated task type, the default implementation
of T'Put_Image also includes discriminant values, as in: "(TASK <task_id_image> with D1 => 123, D2 => 456)"
Ramification: If
T is an unchecked union type, then the default implementation of T'Put_Image
will raise Program_Error.
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For every subtype S of a type T, the following
attributes are defined:S'Wide_Wide_Image
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S'Wide_Wide_Image denotes a function with the following
specification:
function S'Wide_Wide_Image(Arg : S'Base)
return Wide_Wide_String
S'Wide_Wide_Image
calls S'Put_Image passing Arg (which will typically store a sequence
of character values in a text buffer) and then returns the result of
retrieving the contents of that buffer with Wide_Wide_Get. The
lower bound of the result is one.
S'Wide_Image
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S'Wide_Image denotes a function with the following
specification:
function S'Wide_Image(Arg : S'Base)
return Wide_String
S'Wide_Image calls
S'Put_Image passing Arg (which will typically store a sequence
of character values in a text buffer) and then returns the result of
retrieving the contents of that buffer with Wide_Get. The
lower bound of the result is one.
S'Image
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S'Image denotes a function with the following specification:
function S'Image(Arg : S'Base)
return String
S'Image calls S'Put_Image
passing Arg (which will typically store a sequence of character
values in a text buffer) and then returns the result of retrieving the
contents of that buffer with Get. The lower
bound of the result is one.
X'Wide_Wide_Image
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X'Wide_Wide_Image
denotes the result of calling function S'Wide_Wide_Image with Arg
being X, where S is the nominal subtype of X.
X'Wide_Image
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X'Wide_Image denotes
the result of calling function S'Wide_Image with Arg being X,
where S is the nominal subtype of X.
X'Image
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X'Image denotes the
result of calling function S'Image with Arg being X, where S is
the nominal subtype of X.
Implementation Permissions
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An implementation may transform the image generated
by the default implementation of S'Put_Image for a composite subtype
S in the following ways:If S is a composite subtype,
the leading character of the image of a component value or index value
is a space, and the immediately preceding character is an open parenthesis
or bracket, then the space may be omitted. The same transformation is
also permitted if the leading character of the component image is a space
(in which case one of the two spaces may be omitted).
Discussion: This
means that it is permitted to generate "[1 => 123, 2 =>
456]" instead of "[ 1 => 123, 2 => 456]".
If S is an array subtype,
the low bound of the array in each dimension equals the low bound of
the corresponding index subtype, and the array value is not a null array
value, then positional array aggregate syntax may be used.
Discussion: This
means that it is permitted to generate "[TRUE, TRUE, FALSE]"
instead of "[ 1 => TRUE, 2 => TRUE, 3 => FALSE]"
if the low bound of the index subtype is one.
If S is an array subtype
and the given value can be displayed using named_array_aggregate
syntax where some discrete_choice_list
identifies more than one index value by identifying a sequence of one
or more ranges and values separated by vertical bars, then this image
may be generated instead; this may involve the reordering of component
values.
Discussion: This
means that it is permitted to generate "[ 1 .. 2 | 5 =>
TRUE, 3 .. 4 => FALSE]" instead of "[ 1 =>
TRUE, 2 => TRUE, 3 => FALSE, 4 => FALSE, 5 => TRUE]".
Similarly, if S is a record
subtype (or a discriminated type) and the given value can be displayed
using named component association syntax where the length of some component_choice_list
is greater than one, then this image may be generated instead; this may
involve the reordering of component values.
Discussion: This
means that it is permitted to generate "(F1 | F2 => TRUE)"
instead of "(F1 => TRUE, F2 => TRUE)".
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Additional spaces (Wide_Wide_Characters with position
32), and calls to the New_Line operation of a text buffer, may be inserted
to improve readability of the generated image.{
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For each language-defined nonscalar type T, T'Put_Image
may be specified.Discussion: This
permission applies, in particular, to nonscalar types declared in language-defined
generic packages, and to any language-defined private type, even if an
implementation chooses to complete it as a scalar type.
Ramification: For
any language-defined scalar type T, T'Put_Image should not be specified;
the Image attribute needs to return the language-defined image for such
types. This is important for compatibility: the Image attribute has been
available for scalar types for many Ada revisions, and programs can (and
do!) depend on its behavior.
Implementation Requirements
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For each language-defined container type T (that
is, each of the Vector, List, Map, Set, Tree, and Holder types defined
in the various children of Ada.Containers), T'Put_Image shall be specified
so that T'Image produces a result consistent with array aggregate syntax
(using '[' and ']' as delimiters) as follows:Vector images shall be consistent
with the default image of an array type with the same index and component
types.
Discussion: In
particular, this means that the format is that of a named array aggregate.
We have no recommendation on how to handle empty elements; if the implementation
can identify them, it may wish to display them specially, but otherwise,
they're just uninitialized elements.
Map images shall be consistent
with named array aggregate syntax, using key value images in place of
discrete choice names. For example, [Key1 => Value1, Key2 =>
Value2].
Discussion: There
is no recommendation about the order in which key/element pairs occur
within a map image. In the case of multiple key values whose corresponding
element values have the same image, there is no recommendation about
factoring (that is, generating Key1 | Key2 => Some_Value
instead of Key1 => Some_Value, Key2 => Some_Value).
Set, List, and Holder images
shall be consistent with positional array aggregate syntax. List elements
shall occur in order within an image of a list. The image of an empty
holder shall be [].
Discussion: There
is no recommendation about the order in which set elements occur within
the image of a set.
Tree images (and images of
subtrees of trees) shall be consistent with positional array aggregate
syntax. For example, [[1, 2], [111, 222, 333]].
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For each language-defined nonscalar type T that
has a primitive language-defined Image function whose profile is type
conformant with that of T'Image (for example, Ada.Numerics.Float_Random.State
has such an Image function), T'Put_Image shall be specified so that T'Image
yields the same result as that Image function.Implementation Advice
For each language-defined private
type T, T'Image should generate an image that would be meaningful based
only on the relevant public interfaces, as opposed to requiring knowledge
of the implementation of the private type.
Implementation Advice:
each language-defined private type T,
T'Image should generate an image that would be meaningful based only
on the relevant public interfaces.
Extensions to Ada 2012
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Attribute Put_Image is new.
Attributes Image, Wide_Image, and Wide_Wide_Image now can be used with
any type, and are defined in terms of Put_Image so that they can be redefined.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
