3.1 Declarations
There are several forms of declaration. A
basic_declaration
is a form of declaration defined as follows.
Syntax
Static Semantics
{Declaration}
[Glossary Entry]
A
declaration is a language
construct that associates a name with (a view of) an entity.
{explicit
declaration} {implicit
declaration} A declaration may appear
explicitly in the program text (an
explicit declaration), or may
be supposed to occur at a given place in the text as a consequence of
the semantics of another construct (an
implicit declaration).
Discussion: An implicit declaration generally
declares a predefined or inherited operation associated with the definition
of a type. This term is used primarily when allowing explicit declarations
to override implicit declarations, as part of a type declaration.
{view}
{definition}
All declarations contain a
definition for
a
view of an entity. A view consists of an identification of the
entity (the entity
of the view), plus view-specific characteristics
that affect the use of the entity through that view (such as mode of
access to an object, formal parameter names and defaults for a subprogram,
or visibility to components of a type). In most cases, a declaration
also contains the definition for the entity itself (a
renaming_declaration
is an example of a declaration that does not define a new entity, but
instead defines a view of an existing entity (see
8.5)).
Glossary entry: {View} A view
of an entity reveals some or all of the properties of the entity. A single
entity may have multiple views.
Discussion: Most declarations define
a view (of some entity) whose view-specific characteristics are unchanging
for the life of the view. However, subtypes are somewhat unusual in that
they inherit characteristics from whatever view of their type is currently
visible. Hence, a subtype is not a view of a type; it is more
of an indirect reference. By contrast, a private type provides a single,
unchanging (partial) view of its full type.
{Definition}
[Glossary Entry]
{scope (informal
definition) [partial]} For each declaration,
the language rules define a certain region of text called the
scope
of the declaration (see
8.2). Most declarations
associate an
identifier
with a declared entity. Within its scope, and only there, there are places
where it is possible to use the
identifier
to refer to the declaration, the view it defines, and the associated
entity; these places are defined by the visibility rules (see
8.3).
{name (of (a view of) an entity)}
At such places the
identifier
is said to be a
name of the entity (the
direct_name
or
selector_name);
{denote (informal definition) [partial]}
the name is said to
denote the declaration,
the view, and the associated entity (see
8.6).
{declare} The
declaration is said to
declare the name, the view, and in most
cases, the entity itself.
Dynamic Semantics
{execution [distributed]}
The process by which a construct achieves its run-time
effect is called
execution.
{elaboration
[distributed]} {evaluation
[distributed]} This process is also called
elaboration for declarations and
evaluation for expressions.
One of the terms execution, elaboration, or evaluation is defined by
this International Standard for each construct that has a run-time effect.
Glossary entry: {
Execution} The
process by which a construct achieves its run-time effect is called
execution.
{
elaboration}
{
evaluation}
Execution
of a declaration is also called
elaboration. Execution of an expression
is also called
evaluation.
To be honest: The term elaboration is
also used for the execution of certain constructs that are not declarations,
and the term evaluation is used for the execution of certain constructs
that are not expressions. For example,
subtype_indications
are elaborated, and
ranges
are evaluated.
For bodies, execution and elaboration are both
explicitly defined. When we refer specifically to the execution of a
body, we mean the explicit definition of execution for that kind of body,
not its elaboration.
Discussion: Technically, "the execution
of a declaration" and "the elaboration of a declaration"
are synonymous. We use the term "elaboration" of a construct
when we know the construct is elaborable. When we are talking about more
arbitrary constructs, we use the term "execution". For example,
we use the term "erroneous execution", to refer to any erroneous
execution, including erroneous elaboration or evaluation.
When we explicitly define evaluation or elaboration
for a construct, we are implicitly defining execution of that construct.
We also use the term "execution" for
things like
statements,
which are executable, but neither elaborable nor evaluable. We considered
using the term "execution" only for non-elaborable, non-evaluable
constructs, and defining the term "action" to mean what we
have defined "execution" to mean. We rejected this idea because
we thought three terms that mean the same thing was enough — four
would be overkill. Thus, the term "action" is used only informally
in the standard (except where it is defined as part of a larger term,
such as "protected action").
Glossary entry: {Elaboration}
The process by which a declaration achieves its run-time effect is called
elaboration. Elaboration is one of the forms of execution.
Glossary entry: {Evaluation} The
process by which an expression achieves its run-time effect is called
evaluation. Evaluation is one of the forms of execution.
To be honest: {
elaborable}
A
construct is
elaborable if elaboration is defined for it. {
evaluable}
A construct is
evaluable if evaluation is
defined for it. {
executable}
A construct is
executable if execution is defined for it.
Discussion: Don't confuse “elaborable”
with “preelaborable” (defined in
10.2.1).
Intuitively, an executable construct
is one that has a defined run-time effect (which may be null). Since
execution includes elaboration and evaluation as special cases, all elaborable
and all evaluable constructs are also executable. Hence, most constructs
in Ada are executable. An important exception is that the constructs
inside a generic unit are not executable directly, but rather are used
as a template for (generally) executable constructs in instances of the
generic.
1
{declare}
At compile time, the declaration of an entity
declares
the entity.
{create} At
run time, the elaboration of the declaration
creates the entity.
Ramification: Syntactic categories for
declarations are named either entity_declaration
(if they include a trailing semicolon) or entity_specification
(if not).
{
entity}
The various
kinds of named entities that can be declared are as follows: an object
(including components and parameters), a named number, a type (the name
always refers to its first subtype), a subtype, a subprogram (including
enumeration literals and operators), a single entry, an entry family,
a package, a protected or task unit (which corresponds to either a type
or a single object), an exception, a generic unit, a label, and the name
of a statement.
Identifiers are also associated with names of
pragmas, arguments to pragmas, and with attributes, but these are not
user-definable.
Wording Changes from Ada 83
It becomes a
direct_name,
in usage occurrences where the usage is required (in Section 8) to be
directly visible;
It becomes a
selector_name,
in usage occurrences where the usage is required (in Section 8) to be
visible but not necessarily directly visible;
For declarations that come in “two parts”
(program unit declaration plus body, private or incomplete type plus
full type, deferred constant plus full constant), we consider both to
be defining occurrences. Thus, for example, the syntax for
package_body
uses
defining_identifier
after the reserved word
body, as opposed to
direct_name.
The defining occurrence of a statement name
is in its implicit declaration, not where it appears in the program text.
Considering the statement name itself to be the defining occurrence would
complicate the visibility rules.
The phrase “visible by selection”
is not used in Ada 95. It is subsumed by simply “visible”
and the Name Resolution Rules for
selector_names.
(Note that in Ada 95, a declaration is visible
at all places where one could have used a
selector_name,
not just at places where a
selector_name
was actually used. Thus, the places where a declaration is directly visible
are a subset of the places where it is visible. See Section 8 for details.)
We use the term “declaration” to
cover
_specifications that declare (views
of) objects, such as
parameter_specifications.
In Ada 83, these are referred to as a “form of declaration,”
but it is not entirely clear that they are considered simply “declarations.”
RM83 contains an incomplete definition of "elaborated"
in this clause: it defines "elaborated" for declarations,
declarative_parts,
declarative_items
and
compilation_units,
but "elaboration" is defined elsewhere for various other constructs.
To make matters worse, Ada 95 has a different set of elaborable constructs.
Instead of correcting the list, it is more maintainable to refer to the
term "elaborable," which is defined in a distributed manner.
RM83 uses the term “has no other effect”
to describe an elaboration that doesn't do anything except change the
state from not-yet-elaborated to elaborated. This was a confusing wording,
because the answer to “other than what?” was to be found
many pages away. In Ada 95, we change this wording to “has no effect”
(for things that truly do nothing at run time), and “has no effect
other than to establish that so-and-so can happen without failing the
Elaboration_Check” (for things where it matters).
We make it clearer that the term "execution"
covers elaboration and evaluation as special cases. This was implied
in RM83. For example, "erroneous execution" can include any
execution, and RM83-9.4(3) has, "The task designated by any other
task object depends on the master whose execution creates the task object;"
the elaboration of the master's
declarative_part
is doing the task creation.
Wording Changes from Ada 95
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