10.2.1 Elaboration Control
This subclause defines pragmas
that help control the elaboration order of library_item
Language Design Principles
The rules governing preelaboration are designed
to allow it to be done largely by bulk initialization of statically allocated
storage from information in a “load module” created by a
linker. Some implementations may require run-time code to be executed
in some cases, but we consider these cases rare enough that we need not
further complicate the rules.
It is important that programs be able to declare
data structures that are link-time initialized with aggregate
and concatenations thereof. It is important to be able to write link-time
evaluated expressions involving the First, Last, and Length attributes
of such data structures (including variables), because they might be
initialized with positional aggregate
and we don't want the user to have to count the elements. There is no
corresponding need for accessing discriminants, since they can be initialized
with a static constant, and then the constant can be referred to elsewhere.
It is important to allow link-time initialized data structures involving
discriminant-dependent components. It is important to be able to write
link-time evaluated expressions involving pointers (both access values
and addresses) to the above-mentioned data structures.
The rules also ensure that no Elaboration_Check
need be performed for calls on library-level subprograms declared within
a preelaborated package. This is true also of the Elaboration_Check on
task activation for library level task types declared in a preelaborated
package. However, it is not true of the Elaboration_Check on instantiations.
A static expression should never prevent a library
unit from being preelaborable.
The form of a pragma
Preelaborate is as follows:
Preelaborate is a library unit pragma.
elaborable construct is preelaborable unless its elaboration performs
any of the following actions:
Ramification: A preelaborable
construct can be elaborated without using any information that is available
only at run time. Note that we don't try to prevent exceptions in preelaborable
constructs; if the implementation wishes to generate code to raise an
exception, that's OK.
Because there is no flow of control and there
are no calls (other than to predefined subprograms), these run-time properties
can actually be detected at compile time. This is necessary in order
to require compile-time enforcement of the rules.
A call to a subprogram other than: a static function.
a static function;
an instance of Unchecked_Conversion
a function declared
in System.Storage_Elements (see 13.7.1);
the functions To_Pointer
and To_Address declared in an instance of System.Address_to_Access_Conversions
The parameters of any such function have to pass
the preelaborability rules, so they typically have to be static expressions.
The extra allowed functions are all forms of conversion that the compiler
understands, so there should be little implementation burden. Note that
such a call might raise an exception; preelaborable is not the same as
elaborable with no code.
The evaluation of a primary
that is a name
of an object, unless the name
is a static expression, or statically denotes a discriminant of an enclosing
One can evaluate such a
not as a primary
For example, one can evaluate an attribute of the object. One can evaluate
so long as it does not denote an object, and its prefix
does not disobey any of these rules. For example, Obj'Access, Obj'Unchecked_Access,
and Obj'Address are generally legal in preelaborated library units.
The creation of an object [(including a component)] that is initialized
by default, if its type does not have preelaborable initialization. Similarly,
the evaluation of an extension_aggregate
with an ancestor subtype_mark
denoting a subtype of such a type.
Ramification: One can declare these kinds
of types, but one cannot create objects of those types.
It is also nonpreelaborable to create an object
if that will cause the evaluation of a default expression that will call
a user-defined function. This follows from the rule above forbidding
The elaboration of any elaborable construct that
is not preelaborable.
A generic declaration is preelaborable unless every
instance would perform one of the above actions.
A generic declaration is preelaborable unless there
is no instance that could be declared preelaborated. For instance, a
generic package declaration that directly contains a variable initialized
by a non-static function that is not a formal function is not preelaborable
(and thus would be illegal if pragma Preelaborate were applied to it).
A generic body is preelaborable only if elaboration of a corresponding
instance body would not perform any such actions, presuming that:
the actual for each discriminated formal derived type, formal private
type, or formal private extension declared within the formal part of
the generic unit is a type that does not have preelaborable initialization,
Preelaborable_Initialization has been applied to the formal type;
the actual for each formal object is nonstatic; and
the actual for each formal subprogram is a user-defined subprogram.
This is an “assume-the-worst” rule. The elaboration of a
generic unit doesn't perform any of the actions listed above, because
its sole effect is to establish that the generic can from now on be instantiated.
So the elaboration of the generic itself is not the interesting part
when it comes to preelaboration rules. The interesting part is what happens
when you elaborate “any instantiation” of the generic. For
instance, declaring an object of a limited formal private type might
well start tasks, call functions, and do all sorts of nonpreelaborable
things. We prevent these situations by assuming that the actual parameters
are as badly behaved as possible.
Reason: Without this rule about generics,
we would have to forbid instantiations in preelaborated library units,
which would significantly reduce their usefulness.
Preelaborate (or pragma
Pure — see below) is used to specify that a library unit is preelaborated
namely that the Preelaborate aspect
of the library unit is True; all compilation units of the library unit
are preelaborated. The declaration and body of a preelaborated library
unit, and all subunits that are elaborated as part of elaborating the
library unit, shall be preelaborable. All compilation units of a preelaborated
library unit shall depend semantically only on declared pure or preelaborated
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. [
a library unit is preelaborated, then its declaration, if any, and body,
if any, are elaborated prior to all nonpreelaborated library_item
of the partition.]
Ramification: In a generic body, we assume
the worst about formal private types and extensions.
Subunits of a preelaborated subprogram unit do not need to be preelaborable.
This is needed in order to be consistent with units nested in a subprogram
body, which do not need to be preelaborable even if the subprogram is
preelaborated. However, such subunits cannot depend semantically on nonpreelaborated
units, which is also consistent with nested units.
Aspect Description for Preelaborate:
Code execution during elaboration is avoided for a given package.
The following rules specify which entities have preelaborable
The partial view of a private type or private extension, a protected
type without entry_declaration
a generic formal private type, or a generic formal derived type, has
preelaborable initialization if and only if the pragma
Preelaborable_Initialization has been applied to them. [A protected type
or a task type never has preelaborable initialization.]
A component (including a discriminant) of a record
or protected type has preelaborable initialization if its declaration
includes a default_expression
whose execution does not perform any actions prohibited in preelaborable
constructs as described above, or if its declaration does not include
a default expression and its type has preelaborable initialization.
A derived type has preelaborable initialization if its parent type has
preelaborable initialization and if the noninherited components all have
preelaborable initialization. However, a controlled type with an Initialize
procedure that is not a null procedure does not have preelaborable initialization.
A view of a type has preelaborable initialization if it is an elementary
type, an array type whose component type has preelaborable initialization,
a record type whose components all have preelaborable initialization,
or an interface type.
specifies that a type has preelaborable initialization. This pragma shall
appear in the visible part of a package or generic package.
If the pragma appears in the first list of basic_declarative_item
of a package_specification
then the direct_name
shall denote the first subtype of a composite type, and the type shall
be declared immediately within the same package as the pragma
If the pragma
is applied to a private type or a private extension, the full view of
the type shall have preelaborable initialization. If the pragma
is applied to a protected type, the protected type shall not have entries,
and each component of the protected type shall have preelaborable initialization.
For any other composite type, the type shall have preelaborable initialization.
In addition to the places where Legality Rules normally
apply (see 12.3
), these rules apply also in
the private part of an instance of a generic unit.
The reason why we need the pragma for private types, private extensions,
and protected types is fairly clear: the properties of the full view
determine whether the type has preelaborable initialization or not; in
order to preserve privacy we need a way to express on the partial view
that the full view is well-behaved. The reason why we need the pragma
for other composite types is more subtle: a nonnull override for Initialize
might occur in the private part, even for a nonprivate type; in order
to preserve privacy, we need a way to express on a type declared in a
visible part that the private part does not contain any nasty override
Not only do protected types
and task types not have preelaborable initialization, but they cannot
have pragma Preelaborable_Initialization applied to them.
In an implementation, a type declared in a preelaborated
package should have the same representation in every elaboration of a
given version of the package, whether the elaborations occur in distinct
executions of the same program, or in executions of distinct programs
or partitions that include the given version.
Implementation Advice: A type declared
in a preelaborated package should have the same representation in every
elaboration of a given version of the package.
The form of a pragma
Pure is as follows:
Pure is a library unit pragma.
A pure program compilation
unit is a preelaborable program compilation
unit whose elaboration does not perform any of the following actions:
the elaboration of a variable declaration;
the evaluation of an allocator
of an access-to-variable type; for the purposes of this rule, the partial
view of a type is presumed to have nonvisible components whose default
initialization evaluates such an allocator
Such an allocator
would provide a backdoor way to get a global variable into a pure unit,
so it is prohibited. Most such allocator
are illegal anyway, as their type is required to have Storage_Size =
0 (see the next two rules). But access parameters and access discriminants
don't necessarily disallow allocator
However, a call is also illegal here (by the preelaboration rules), so
access parameters cannot cause trouble. So this rule is really about
prohibiting allocators in discriminant constraints:
type Rec (Acc : access Integer) is record
C : Character;
Not_Const : constant Rec (Acc => new Integer'(2)); -- Illegal in a pure unit.
The second half of the rule is needed because aggregates can specify
the default initialization of a private type or extension using <>
or the ancestor subtype of an extension aggregate. The subtype of a component
could use an allocator
to initialize an access discriminant; the type still could have a pragma
Preelaborable_Initialization given. Ada 95 did not allow such private
types to have preelaborable initialization, so such a default initialization
could not have occurred. Thus this rule is not incompatible with Ada
the elaboration of the declaration of a nonderived named access-to-variable
type unless the Storage_Size of the type has been specified by a static
expression with value zero or is defined by the language to be zero;
A remote access-to-class-wide
type (see E.2.2
) has its Storage_Size defined
to be zero.
We disallow most named access-to-object types because an allocator
has a side effect; the pool constitutes variable data. We allow access-to-subprogram
types because they don't have allocator
We even allow named access-to-object types if they have an empty predefined
pool (they can't have a user-defined pool as System.Storage_Pools is
not pure). In this case, most attempts to use an allocator
are illegal, and any others (in a generic body) will raise Storage_Error.
the elaboration of the declaration of a nonderived named access-to-constant
type for which the Storage_Size has been specified by an expression other
than a static expression with value zero;.
Discussion: We allow access-to-constant
types so long as there is no user-specified nonzero Storage_Size; if
there were a user-specified nonzero Storage_Size restricting the size
of the storage pool, allocators would be problematic since the package
is supposedly ‘stateless’, and the allocated size count for
the storage pool would represent state.
the elaboration of any program unit that is not
A generic declaration is pure unless every instance
would perform one of the above actions.
A generic declaration is pure unless there is no
instance that could be declared pure. For instance, a generic package
declaration that directly contains a variable declaration is not pure
(and thus would be illegal if pragma Pure is applied to it).
A generic body is pure only if elaboration of a corresponding instance
body would not perform any such actions presuming any composite formal
types have nonvisible components whose default initialization evaluates
of an access-to-variable type.
The Storage_Size for an anonymous access-to-variable type declared at
library level in a library unit that is declared pure is defined to be
This makes allocator
illegal for such types (see 4.8
), making a
storage pool unnecessary for these types. A storage pool would represent
Note that access discriminants and access parameters
are never library-level, even when they are declared in a type or subprogram
declared at library-level. That's because they have their own special
accessibility rules (see 3.10.2
Pure is used to specify that a library unit is declared pure
namely that the Pure aspect
the library unit is True; all compilation units of the library unit are
declared pure. In addition, the limited view of any library package is
declared pure. The declaration and body of a declared pure library unit,
and all subunits that are elaborated as part of elaborating the library
unit, shall be pure. All compilation units of a declared pure library
unit shall depend semantically only on declared pure library_item
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. Furthermore, the full
view of any partial view declared in the visible part of a declared pure
library unit that has any available stream attributes shall support external
streaming (see 13.13.2
Discussion: A declared-pure package is
useful for defining types to be shared between partitions with no common
Reason: Note that generic packages are
not mentioned in the list of things that can contain variable declarations.
Note that the Ada 95 rules for deferred constants make them allowable
in library units that are declared pure; that isn't true of Ada 83's
Anonymous access types are allowed.
A limited view is not a library unit, so any rule that starts “declared
pure library unit” does not apply to a limited view. In particular,
the 3rd and last sentences never apply to limited views. However, a limited
view is a library_item
so rules that discuss “declared pure library_item
do include limited views.
Ada 95 didn't allow any access types as these (including access-to-subprogram
types) cause trouble for Annex E
”, because such types allow access values in a shared
passive partition to designate objects in an active partition, thus allowing
inter-address space references. We decided to disallow such uses in the
relatively rare cases where they cause problems, rather than making life
harder for the majority of users. Types declared in a pure package can
be used in remote operations only if they are externally streamable.
That simply means that there is a means to transport values of the type;
that's automatically true for nonlimited types that don't have an access
part. The only tricky part about this is to avoid privacy leakage; that
was handled by ensuring that any private types (and private extensions)
declared in a pure package that have available stream attributes (which
include all nonlimited types by definition) have to be externally streamable.
Aspect Description for Pure: Side
effects are avoided in the subprograms of a given package.
Execution is erroneous if some operation (other
than the initialization or finalization of the object) modifies the value
of a constant object declared at library-level in a pure package.
could be accomplished via a self-referencing pointer or via squirrelling
a writable pointer to a controlled object.
If a library unit is declared pure, then the implementation is permitted
to omit a call on a library-level subprogram of the library unit if the
results are not needed after the call. In addition, the implementation
may omit a call on such a subprogram and simply reuse the results produced
by an earlier call on the same subprogram, provided that none of the
parameters nor any object accessible via access values from the parameters
have any part that is of a type whose full type is an immutably limited
type, and the addresses and values of all by-reference actual parameters,
the values of all by-copy-in actual parameters, and the values of all
objects accessible via access values from the parameters, are the same
as they were at the earlier call. [This permission applies even if the
subprogram produces other side effects when called.]
A declared-pure library_item
has no variable state. Hence, a call on one of its (nonnested) subprograms
cannot normally have side effects. Side effects are still possible via
dispatching calls and via indirect calls through access-to-subprogram
values. Other mechanisms that might be used to modify variable state
include machine code insertions, imported subprograms, and unchecked
conversion to an access type declared within the subprogram; this list
is not exhaustive. Thus, the permissions described in this subclause
may apply to a subprogram whose execution has side effects. The compiler
may omit a call to such a subprogram even if side effects exist, so
the writer of such a subprogram has to keep this in mind.
The form of a pragma
Elaborate, Elaborate_All, or Elaborate_Body is as follows:
“Within a context_clause
allows it to be the last item in the context_clause
It can't be first, because the name
has to denote something mentioned earlier.
Elaborate_Body is a library unit pragma.
Hence, a pragma
Elaborate or Elaborate_All is not elaborated, not that it makes any practical
Note that a pragma
Elaborate or Elaborate_All is neither a program unit pragma, nor a library
If the aspect Elaborate_Body is True for a declaration
[(including when pragma
Elaborate_Body applies)], then the declaration requires a completion
of a pragma
Elaborate or Elaborate_All shall denote a nonlimited view of a library
are intended to prevent elaboration check failures. But a limited view
does not make anything visible that has an elaboration check, so the
do anything useful. Moreover, the pragma
would probably reintroduce the circularity that the limited_with_clause
was intended to break. So we make such uses illegal.
The official statement of the
semantics of these pragma
is given in 10.2
sets the Elaborate_Body representation aspect of the library unit to
which it applies to the value True. [If the Elaborate_Body aspect of
a library unit is True, the body of the library unit is elaborated immediately
after its declaration.
The official statement of the
semantics of this aspect is given in 10.2
The presence of
simplifies the removal of unnecessary Elaboration_Checks. For a subprogram
declared immediately within a library unit to which a pragma
Elaborate_Body applies, the only calls that can fail the Elaboration_Check
are those that occur in the library unit itself, between the declaration
and body of the called subprogram; if there are no such calls (which
can easily be detected at compile time if there are no stub
then no Elaboration_Checks are needed for that subprogram. The same is
true for Elaboration_Checks on task activations and instantiations, and
for library subprograms and generic units.
The fact that the unit
of elaboration is the library_item
means that if a subprogram_body
is not a completion, it is impossible for any library_item
to be elaborated between the declaration and the body of such a subprogram.
Therefore, it is impossible for a call to such a subprogram to fail its
The visibility rules imply
that each library_unit_name
of a pragma
Elaborate or Elaborate_All has to denote a library unit mentioned by
a previous with_clause
of the same context_clause
Aspect Description for Elaborate_Body:
A given package must have a body, and that body is elaborated immediately
after the declaration.
12 A preelaborated library unit is allowed
to have nonpreelaborable children.
But generally not nonpreelaborated subunits. (Nonpreelaborated subunits
of subprograms are allowed as discussed above.)
13 A library unit that is declared pure
is allowed to have impure children.
But generally not impure subunits. (Impure subunits of subprograms are
allowed as discussed above.)
Ramification: Pragma Elaborate is mainly
for closely related library units, such as when two package bodies 'with'
each other's declarations. In such cases, Elaborate_All sometimes won't
Extensions to Ada 83
The concepts of preelaborability
and purity are new to Ada 95. The Elaborate_All, Elaborate_Body, Preelaborate,
and Pure pragma
are new to Ada 95.
Pragmas Elaborate are allowed to be mixed in
with the other things in the context_clause
— in Ada 83, they were required to appear last.
Incompatibilities With Ada 95
The requirement that a partial view with available
stream attributes be externally streamable can cause an incompatibility
in rare cases. If there is a limited tagged type declared in a pure package
with available attributes, and that type is used to declare a private
extension in another pure package, and the full type for the private
extension has a component of an explicitly limited record type, a protected
type, or a type with access discriminants, then the stream attributes
will have to be user-specified in the visible part of the package. That
is not a requirement for Ada 95, but this combination seems very unlikely
in pure packages. Note that this cannot be an incompatibility for a nonlimited
type, as all of the types that are allowed in Ada 95 that would require
explicitly defined stream attributes are limited (and thus cannot be
used as components in a nonlimited type).
Added wording to cover missing cases for
preelaborated generic units. This is incompatible as a preelaborated
unit could have used a formal object to initialize a library-level object;
that isn't allowed in Ada 2005. But such a unit wouldn't really be preelaborable,
and Ada 95 compilers can reject such units (as this is a Binding Interpretation),
so such units should be very rare.
Extensions to Ada 95
The concept of preelaborable
initialization and pragma
Preelaborable_Initialization are new. These allow more types of objects
to be created in preelaborable units, and fix holes in the old rules.
Access-to-subprogram types and access-to-object types with a Storage_Size
of 0 are allowed in pure units. The permission to omit calls was adjusted
accordingly (which also fixes a hole in Ada 95, as access parameters
are allowed, and changes in the values accessed by them must be taken
Wording Changes from Ada 95
The wording was changed so that subunits of a preelaborated
subprogram are also preelaborated.
Incompatibilities With Ada 2005
Corrected a serious unintended
incompatibility with Ada 95 in the new preelaboration wording —
explicit initialization of objects of types that don't have preelaborable
initialization was not allowed. Ada 2012 switches back to the Ada 95
rule in these cases. This is unlikely to occur in practice, as it is
unlikely that a compiler would have implemented the more restrictive
rule (it would fail many ACATS tests if it did).
Added an assume-the-worst rule for generic bodies
(else they would never be checked for purity) and added the boilerplate
so that the entire generic specification is rechecked. Also fixed wording
to have consistent handling for subunits for Pure and Preelaborate. An
Ada 95 program could have depended on marking a generic pure that was
not really pure, although this would defeat the purpose of the categorization
and likely cause problems with distributed programs.
Extensions to Ada 2005
Adjusted wording so that a subunit
can be pure (it is not a library_item
but it is a compilation unit).
Adjusted wording so that the rules for access types
only apply to nonderived types (derived types share their storage pool
with their parent, so if the parent access type is legal, so is any derived
Pure and Preelaborate are now aspects, so they can be specified by an
— although the pragmas are still preferred by the Standard.
Wording Changes from Ada 2005
Added wording so that a limited view is always treated
as pure, no matter what categorization is used for the originating unit.
This was undefined in Ada 2005.
Fixed minor issues with preelaborable initialization
(PI): null Initialize procedures do not make a type non-PI; formal types
with pragma PI can be assumed to have PI; formal extensions are assumed
to not have PI; all composite types can have pragma PI (so that the possibility
of hidden Initialize routines can be handled); added discriminants of
a derived type are not considered in calculating PI.
Clarified that the implementation permission to omit
pure subprogram calls does not apply if any part of the parameters or
any designated object has a part that is immutably limited. The old wording
just said "limited type", which can change via visibility and
thus isn't appropriate for dynamic semantics permissions.
Incompatibilities With Ada 2012
Correction: Added a
rule that a generic declaration is not pure if no instance could be pure
(for instance, because the generic declaration would elaborates a variable.)
This was legal in Ada 2012, but of course no instance could have been
declared pure. A similar rule (with a similar effect) was added for preelaborable
Extensions to Ada 2012
Added some intrinsic conversion
functions to those allowed to be called during the elaboration of a preelaborated
unit. This is necessary to allow a portable Address aspect in a preelaborated
unit, important on small embedded systems.
Wording Changes from Ada 2012
Corrigendum: Explicitly stated that modifying
a library-level constant in a pure package is erroneous. We don't document
this as inconsistent as implementations certainly can still do whatever
they were previously doing (no change is required); moreover, this case
(and many more) were erroneous in Ada 2005 and before, so we're just
restoring the previous semantics.
Correction: Explicitly stated that the pure
and preelaborate rules are recursive; that is, they apply to the contents
of nested packages and generic packages.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe