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
by copy, unless the formal parameter is explicitly aliased.
type is a by-reference type
if it is a descendant of one of the
a tagged type;
a task or protected type;
an explicitly limited record type;
a composite type with a subcomponent of a by-reference
a private type whose full type is a by-reference
A parameter of a by-reference type is passed by
reference, as is an explicitly aliased parameter of any type.
value of a by-reference type has an associated object. For a parenthesized
this object is the one associated with the operand. For a conditional_expression
this object is the one associated with the evaluated dependent_expression
For other parameters, 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 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).
possible consequences are that Program_Error is raised, or the newly
assigned value is read, or some old value of the object is read.
6 A formal parameter of mode in
is a constant view (see 3.3
); it cannot be
updated within the subprogram_body
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