6.2 Formal Parameter Modes
1
Static Semantics
2
A
parameter is passed either
by copy or
by reference. [When
a parameter is passed by copy, the formal parameter denotes a separate
object from the actual parameter, and any information transfer between
the two occurs only before and after executing the subprogram. When a
parameter is passed by reference, the formal parameter denotes (a view
of) the object denoted by the actual parameter; reads and updates of
the formal parameter directly reference the actual parameter object.]
3/3
{
AI05-0142-4}
{
AI05-0262-1}
A type is a
by-copy type if it is an elementary
type, or if it is a descendant of a private type whose full type is a
by-copy type. A parameter of a by-copy type is passed by copy
,
unless the formal parameter is explicitly aliased.
4
A
type is a
by-reference type if it is a descendant of one of the
following:
5
a tagged type;
6
a task or protected type;
7/3
{
AI05-0096-1}
an explicitly limited record type a
nonprivate type with the reserved word limited in its declaration;
7.a/3
This paragraph
was deleted.Ramification: {
AI05-0096-1}
A limited private type is by-reference only if
it falls under one of the other categories.
8
a composite type with a subcomponent of a by-reference
type;
9
a private type whose full type is a by-reference
type.
10/3
10.a
Ramification: By-reference parameter
passing makes sense only if there is an object to reference; hence, we
define such an object for each case.
10.b
Since tagged types are by-reference types, this
implies that every value of a tagged type has an associated object. This
simplifies things, because we can define the tag to be a property of
the object, and not of the value of the object, which makes it clearer
that object tags never change.
10.c
We considered simplifying things even more by
making every value (and therefore every expression) have an associated
object. After all, there is little semantic difference between a constant
object and a value. However, this would cause problems for untagged types.
In particular, we would have to do a constraint check on every read of
a type conversion (or a renaming thereof) in certain cases.
10.d/2
{
AI95-00318-02}
We do not want this definition to depend on the view of the type; privateness
is essentially ignored for this definition. Otherwise, things would be
confusing (does the rule apply at the call site, at the site of the declaration
of the subprogram, at the site of the
return statement return_statement?),
and requiring different calls to use different mechanisms would be an
implementation burden.
10.e
C.6, “
Shared
Variable Control” says that a composite type with an atomic
or volatile subcomponent is a by-reference type, among other things.
10.f
Every value of a limited
by-reference type is the value of one and only one limited object. The
associated object of a value of a limited by-reference type is
the object whose value it represents.
Two values
of a limited by-reference type are the
same if and only if they
represent the value of the same object.
10.g
We say “by-reference” above because
these statements are not always true for limited private types whose
underlying type is nonlimited (unfortunately).
11/3
{
AI05-0240-1}
For
other parameters
of other types, it is unspecified whether the parameter is passed
by copy or by reference.
11.a/3
Discussion: {
AI05-0005-1}
There is no need to incorporate the discussion of AI83-00178, which requires
pass-by-copy for certain kinds of actual parameters, while allowing pass-by-reference
for others. This is because we explicitly indicate that a function creates
an anonymous constant object for its result
, unless
the type is a return-by-reference type (see
6.5).
We also provide a special dispensation for instances of Unchecked_Conversion
to return by reference
, even if the result type
is not a return-by-reference type (see
13.9).
Bounded (Run-Time) Errors
12/3
{
AI05-0240-1}
If
one
name denotes
a part of a formal parameter, and a second
name
denotes a part of a distinct formal parameter or an object that is not
part of a formal parameter, then the two
names
are considered
distinct access paths. If an object is of a type
for which the parameter passing mechanism is not specified
and is not an explicitly aliased parameter, then it is a bounded
error to assign to the object via one access path, and then read the
value of the object via a distinct access path, unless the first access
path denotes a part of a formal parameter that no longer exists at the
point of the second access [(due to leaving the corresponding callable
construct).]
The possible consequences are that Program_Error
is raised, or the newly assigned value is read, or some old value of
the object is read.
12.a
Discussion: For example, if we call “P(X
=> Global_Variable, Y => Global_Variable)”, then within P,
the names “X”, “Y”, and “Global_Variable”
are all distinct access paths. If Global_Variable's type is neither pass-by-copy
nor pass-by-reference, then it is a bounded error to assign to Global_Variable
and then read X or Y, since the language does not specify whether the
old or the new value would be read. On the other hand, if Global_Variable's
type is pass-by-copy, then the old value would always be read, and there
is no error. Similarly, if Global_Variable's type is defined by the language
to be pass-by-reference, then the new value would always be read, and
again there is no error.
12.b
Reason: We are saying assign here,
not update, because updating any subcomponent is considered to
update the enclosing object.
12.c
The “still exists” part is so that
a read after the subprogram returns is OK.
12.d
If the parameter is of a by-copy type, then
there is no issue here — the formal is not a view of the actual.
If the parameter is of a by-reference type, then the programmer may depend
on updates through one access path being visible through some other access
path, just as if the parameter were of an access type.
12.e
Implementation Note: The implementation
can keep a copy in a register of a parameter whose parameter-passing
mechanism is not specified. If a different access path is used to update
the object (creating a bounded error situation), then the implementation
can still use the value of the register, even though the in-memory version
of the object has been changed. However, to keep the error properly bounded,
if the implementation chooses to read the in-memory version, it has to
be consistent -- it cannot then assume that something it has proven about
the register is true of the memory location. For example, suppose the
formal parameter is L, the value of L(6) is now in a register, and L(6)
is used in an
indexed_component
as in “A(L(6)) := 99;”, where A has bounds 1..3. If the implementation
can prove that the value for L(6) in the register is in the range 1..3,
then it need not perform the constraint check if it uses the register
value. However, if the memory value of L(6) has been changed to 4, and
the implementation uses that memory value, then it had better not alter
memory outside of A.
12.f
Note that the rule allows the implementation
to pass a parameter by reference and then keep just part of it in a register,
or, equivalently, to pass part of the parameter by reference and another
part by copy.
12.g
Reason: We
do not want to go so far as to say that the mere presence of aliasing
is wrong. We wish to be able to write the following sorts of things in
standard Ada:
12.h
procedure Move ( Source : in String;
Target : out String;
Drop : in Truncation := Error;
Justify : in Alignment := Left;
Pad : in Character := Space);
-- Copies elements from Source to Target (safely if they overlap)
12.i
This is from the standard string handling package.
It would be embarrassing if this couldn't be written in Ada!
12.j
The “then” before “read”
in the rule implies that the implementation can move a read to an earlier
place in the code, but not to a later place after a potentially aliased
assignment. Thus, if the subprogram reads one of its parameters into
a local variable, and then updates another potentially aliased one, the
local copy is safe — it is known to have the old value. For example,
the above-mentioned Move subprogram can be implemented by copying Source
into a local variable before assigning into Target.
12.k
For an
assignment_statement
assigning one array parameter to another, the implementation has to check
which direction to copy at run time, in general, in case the actual parameters
are overlapping slices. For example:
12.l
procedure Copy(X : in out String; Y: String) is
begin
X := Y;
end Copy;
12.m
It would be wrong for the compiler to assume
that X and Y do not overlap (unless, of course, it can prove otherwise).
13
6 A formal parameter of mode
in
is a constant view (see
3.3); it cannot be
updated within the
subprogram_body.
Extensions to Ada 83
13.a
The value of an
out
parameter may be read. An
out parameter is treated like a declared
variable without an explicit initial expression.
Wording Changes from Ada 83
13.b
13.c
The concept of a by-reference type is new to
Ada 95.
13.d
We now cover in a general way in
3.7.2
the rule regarding erroneous execution when a discriminant is changed
and one of the parameters depends on the discriminant.
Wording Changes from Ada 2005
13.e/3
{
AI05-0096-1}
Correction: Corrected so that limited derived
types are by-reference only if their parent is.13.f/3
{
AI05-0142-4}
Defined that explicitly aliased parameters (see
6.1) are always passed by reference.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
