6.3.1 Conformance Rules
[When
subprogram profiles are given in more than one place, they are required
to conform in one of four ways: type conformance, mode conformance, subtype
conformance, or full conformance.]
Static Semantics
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8652/0011}
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[As explained in
B.1,
“
Interfacing Aspects”, a
convention
can be specified for an entity.] Unless this International Standard states
otherwise, the default convention of an entity is Ada. [For a callable
entity or access-to-subprogram type, the convention is called the
calling
convention.] The following conventions are defined by the language:
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The default calling convention
for any subprogram not listed below is
Ada. [The Convention aspect
may be specified to override the default calling convention (see
B.1)].
Ramification: See also the rule about
renamings-as-body in
8.5.4.
The
Intrinsic calling convention represents subprograms that are “built
in” to the compiler. The default calling convention is Intrinsic
for the following:
an enumeration literal;
a "/=" operator declared
implicitly due to the declaration of "=" (see
6.6);
any other implicitly declared subprogram
unless it is a dispatching operation of a tagged type;
an inherited subprogram of a generic
formal tagged type with unknown discriminants;
Reason: Consider:
package P is
type Root is tagged null record;
procedure Proc(X: Root);
end P;
generic
type Formal(<>) is new Root with private;
package G is
...
end G;
package body G is
...
X: Formal := ...;
...
Proc(X); -- This is a dispatching call in Instance, because
-- the actual type for Formal is class-wide.
...
-- Proc'Access would be illegal here, because it is of
-- convention Intrinsic, by the above rule.
end G;
type Actual is new Root with ...;
procedure Proc(X: Actual);
package Instance is new G(Formal => Actual'Class);
-- It is legal to pass in a class-wide actual, because Formal
-- has unknown discriminants.
Within Instance, all calls to Proc will be dispatching
calls, so Proc doesn't really exist in machine code, so we wish to avoid
taking 'Access of it. This rule applies to those cases where the actual
type might be class-wide, and makes these Intrinsic, thus forbidding
'Access.
an attribute that is a subprogram;
Reason: The profile of a prefixed view
is different than the “real” profile of the subprogram (it
doesn't have the first parameter), so we don't want to be able to take
'Access of it, as that would require generating a wrapper of some sort.
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We except prefixed views that have synchronization kind By_Protected_Procedure
so that they can be used with an access-to-protected-procedure type.
These don't require special wrappers (this is the normal form for a protected
subprogram call). The By_Entry part is just for consistency (there is
no access-to-entry type in Ada).
[The Access attribute is not allowed for
Intrinsic subprograms.]
Ramification: The Intrinsic calling convention
really represents any number of calling conventions at the machine code
level; the compiler might have a different instruction sequence for each
intrinsic. That's why the Access attribute is disallowed. We do not wish
to require the implementation to generate an out of line body for an
intrinsic.
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Whenever we wish to disallow the Access attribute in order to ease implementation,
we make the subprogram Intrinsic. Several language-defined subprograms
have “
with Convention => Intrinsic;”. An implementation
might actually implement this as “
with Import => True,
Convention => Intrinsic;”, if there is really no body, and the
implementation of the subprogram is built into the code generator.
Subprograms declared in protected_bodies
will generally have a special calling convention so as to pass along
the identification of the current instance of the protected type. The
convention is not protected since such local subprograms need
not contain any “locking” logic since they are not callable
via “external” calls; this rule prevents an access value
designating such a subprogram from being passed outside the protected
unit.
The “implicitly declared subprogram”
above refers to predefined operators (other than the "=" of
a tagged type) and the inherited subprograms of untagged types.
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The default calling convention
is
protected for a protected subprogram, for a prefixed view of
a subprogram with a synchronization kind of By_Protected_Procedure, and
for an access-to-subprogram type with the reserved word
protected
in its definition.
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The default calling convention
is
entry for an entry and for a prefixed view of a subprogram
with a synchronization kind of By_Entry.
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The calling convention for an anonymous access-to-subprogram parameter
or anonymous access-to-subprogram result is
protected if the reserved
word
protected appears in its definition; otherwise, it is the
convention of the subprogram that contains the parameter.
Ramification: The calling convention
for other anonymous access-to-subprogram types is Ada.
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8652/0011}
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[If not specified above as Intrinsic, the calling convention for any
inherited or overriding dispatching operation of a tagged type is that
of the corresponding subprogram of the parent type.] The default calling
convention for a new dispatching operation of a tagged type is the convention
of the type.
Reason: The first rule is officially
stated in
3.9.2. The second is intended to
make interfacing to foreign OOP languages easier, by making the default
be that the type and operations all have the same convention.
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Of these four conventions, only Ada and Intrinsic are allowed as a
convention_identifier
in the specification of a Convention aspect.
Discussion: {
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The names of the
protected and
entry calling conventions
cannot be used in the specification of Convention. Note that
protected
and
entry are reserved words.
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Two profiles are
type conformant
if they have the same number of parameters, and both have a result if
either does, and corresponding parameter and result types are the same,
or, for access parameters or access results, corresponding designated
types are the same, or corresponding designated profiles are type conformant.
Discussion: {
AI95-00409-01}
For anonymous access-to-object parameters, the designated types have
to be the same for type conformance, not the access types, since in general
each access parameter has its own anonymous access type, created when
the subprogram is called. Of course, corresponding parameters have to
be either both access parameters or both not access parameters.
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Similarly, for anonymous access-to-subprogram parameters, the designated
profiles of the types, not the types themselves, have to be conformant.
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corresponding parameters have identical modes and both or neither are
explicitly aliased parameters; and
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for corresponding access parameters and any access result type, the designated
subtypes statically match and either both or neither are access-to-constant,
or the designated profiles are subtype conformant.
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Two profiles are
subtype conformant
if they are mode conformant, corresponding subtypes of the profile statically
match, and the associated calling conventions are the same. The profile
of a generic formal subprogram is not subtype conformant with any other
profile.
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Two profiles are
fully conformant
if they are subtype conformant, if they have access-to-subprogram results
whose designated profiles are fully conformant, and for corresponding
parameters:
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for access-to-subprogram parameters, the designated profiles are fully
conformant.
Ramification: Full conformance requires
subtype conformance, which requires the same calling conventions. However,
the calling convention of the declaration and body of a subprogram or
entry are always the same by definition.
Reason: {
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The part about
null_exclusions
is necessary to prevent controlling parameters from having different
exclusions, as such a parameter is defined to exclude null whether or
not an exclusion is given.
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The parts about access-to-subprogram parameters and results is necessary
to prevent such types from having different
default_expressions
in the specification and body of a subprogram. If that was allowed, it
would be undefined which
default_expression
was used in a call of an access-to-subprogram parameter.
Two
expressions are
fully conformant if, [after replacing each use
of an operator with the equivalent
function_call:]
each constituent construct of one corresponds to
an instance of the same syntactic category in the other, except that
an expanded name may correspond to a
direct_name
(or
character_literal)
or to a different expanded name in the other; and
Ramification: Note that it doesn't say
“respectively” because a
direct_name
can correspond to a
selector_name,
and vice-versa, by the previous bullet. This rule allows the
prefix
of an expanded name to be removed, or replaced with a different
prefix
that denotes a renaming of the same entity. However, it does not allow
a
direct_name
or
selector_name
to be replaced with one denoting a distinct renaming (except for
direct_names
and
selector_names
in
prefixes
of expanded names). Note that calls using operator notation are equivalent
to calls using prefix notation.
Given the following
declarations:
package A is
function F(X : Integer := 1) return Boolean;
end A;
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with A;
package B
is
package A_View
renames A;
function F_View(X : Integer := 9999)
return Boolean
renames A.F;
end B;
with A, B; use A, B;
procedure Main is ...
Within Main, the expressions “F”,
“A.F”, “B.A_View.F”, and “A_View.F”
are all fully conformant with one another. However, “F” and
“F_View” are not fully conformant. If they were, it would
be bad news, since the two denoted views have different
default_expressions.
each
primary
that is a literal in one has the same value as the corresponding literal
in the other.
Ramification: The literals may be written
differently.
Ramification: Note that the above definition
makes full conformance a transitive relation.
Two
known_discriminant_parts
are
fully conformant if they have the same number of discriminants,
and discriminants in the same positions have the same names, statically
matching subtypes, and
default_expressions
that are fully conformant with one another.
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The
prefixed view profile of a subprogram
is the profile obtained by omitting the first parameter of that subprogram.
There is no prefixed view profile for a parameterless subprogram. For
the purposes of defining subtype and mode conformance, the convention
of a prefixed view profile is considered to match that of either an entry
or a protected operation.
Discussion: This definition is used to
define how primitive subprograms of interfaces match operations in task
and protected type definitions (see
9.1 and
9.4).
Reason: The weird rule about conventions
is pretty much required for synchronized interfaces to make any sense.
There will be wrappers all over the place for interfaces anyway. Of course,
this doesn't imply that entries have the same convention as protected
operations.
Implementation Permissions
An implementation may declare an operator declared
in a language-defined library unit to be intrinsic.
Extensions to Ada 83
The rules for full conformance
are relaxed — they are now based on the structure of constructs,
rather than the sequence of lexical elements. This implies, for example,
that "(X, Y: T)" conforms fully with "(X: T; Y: T)",
and "(X: T)" conforms fully with "(X:
in T)".
Wording Changes from Ada 95
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8652/0011}
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AI95-00117-01}
Corrigendum: Clarified that the default convention is Ada. Also
clarified that the convention of a primitive operation of a tagged type
is the same as that of the type.
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Defined the conformance of anonymous access-to-subprogram parameters.
Incompatibilities With Ada 2005
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AI05-0046-1}
Correction: Now require
null_exclusions
to match for full conformance. While this is technically incompatible
with Ada 2005 as defined by Amendment 1, it is a new Ada 2005 feature
and it is unlikely that users have been intentionally taking advantage
of the ability to write mismatching exclusions. In any case, it is easy
to fix: add a
null_exclusion
where needed for conformance.
{
AI05-0134-1}
Correction: Now require full conformance of anonymous access-to-subprogram
parameters and results for full conformance. This is necessary so that
there is no confusion about the default expression that is used for a
call. While this is technically incompatible with Ada 2005 as defined
by Amendment 1, it is a new Ada 2005 feature and it is unlikely that
users have been intentionally taking advantage and writing different
default expressions. In any case, it is easy to fix: change any default
expressions that don't conform so that they do conform.
{
AI05-0207-1}
Correction: Now include the presence or absence of
constant
in access parameters to be considered when checking mode conformance.
This is necessary to prevent modification of constants. While this is
technically incompatible with Ada 2005 as defined by Amendment 1, it
is a new Ada 2005 feature and it is unlikely that users have been intentionally
taking advantage and writing mismatching access types.
Wording Changes from Ada 2005
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Explicitly aliased parameters are included as part of mode conformance
(since it affects the parameter passing mechanism).
Extensions to Ada 2012
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Corrigendum: We now define that a prefixed
view of a subprogram with synchronization kind By_Protected_Procedure
can be used as the prefix of 'Access for an access-to-protected type.
We consider this a correction as it certainly appears that it ought to
work, but in original Ada 2012 it would have had a convention mismatch.
Wording Changes from Ada 2012
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Corrigendum: We now define how two expressions containing quantified
expressions can fully conform. This isn't incompatible, as the original
Ada 2012 never allowed such expressions to conform (the declarations
in each formally being different). Neither is it an extension as one
would expect these to conform.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
