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9.3 Checking the Elaboration Order ¶

To avoid placing the entire elaboration-order burden on the programmer, Ada provides three lines of defense:

• `Static semantics'

  Static semantic rules restrict the possible choice of elaboration order. For instance, if unit Client `with's unit Server, then the spec of Server is always elaborated prior to Client. The same principle applies to child units - the spec of a parent unit is always elaborated prior to the child unit.

• `Dynamic semantics'

  Dynamic checks are performed at run time, to ensure that a target is elaborated prior to a scenario that invokes it, thus avoiding ABE problems. A failed run-time check raises exception Program_Error. The following restrictions apply:

  • `Restrictions on calls'

    An entry, operator, or subprogram can be called from elaboration code only when the corresponding body has been elaborated.

  • `Restrictions on instantiations'

    A generic unit can be instantiated by elaboration code only when the corresponding body has been elaborated.

  • `Restrictions on task activation'

    A task can be activated by elaboration code only when the body of the associated task type has been elaborated.

  The restrictions above can be summarized by the following rule:

  `If a target has a body, then this body must be elaborated prior to the scenario that invokes the target.'

• `Elaboration control'

  Pragmas are provided for the programmer to specify the desired elaboration order.