6.2 Formal Parameter Modes
parameter is passed either by copy
or by reference
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.
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
type is a by-reference type
if it is a descendant of one of the
- a task or protected type;
- a nonprivate type with the reserved
word limited in its declaration;
- a composite type with a subcomponent
of a by-reference type;
- a private type whose full type is
a by-reference type.
A parameter of a by-reference type is passed by reference.
Each value of a by-reference type has an associated
object. For a parenthesized expression, qualified_expression
this object is the one associated with the operand.
For parameters of other types,
it is unspecified whether the parameter is passed by copy or by reference.
Bounded (Run-Time) Errors
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 name
are considered distinct access paths
. If an object is of a type
for which the parameter passing mechanism is not specified, 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
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.
5 A formal parameter of mode in
is a constant view (see 3.3
); it cannot be
updated within the subprogram_body