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6.2 Formal Parameter Modes

A parameter_specification declares a formal parameter of mode in, in out, or out.

Static Semantics

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.
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.
A type is a by-reference type if it is a descendant of one of the following: 
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, or type_conversion, 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

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, 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. 
5  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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