Rationale for Ada 2005

John Barnes

6.1 Ada Issues: Exceptions, generics, etc

The areas mentioned in this chapter are not specifically mentioned in the WG9 guidance document [1] other than under the request to remedy shortcomings and improve interfacing.

The following Ada Issues cover the relevant changes and are described in detail in this chapter.

161

Preelaborable initialization

216

Unchecked unions – variants without discriminant

224

pragma Unsuppress

241

Testing for null occurrence

251

Abstract interfaces to provide multiple inheritance

257

Restrictions for implementation defined entities

260

Abstract formal subprograms & dispatching constructors

267

Fast float to integer conversion

286

Assert pragma

317

Partial parameter lists for formal packages

329

pragma No_Return – procedures that never return

340

Mod attribute

361

Raise with message

364

Fixed point multiply and divide

368

Restrictions for obsolescent features

381

New Restrictions identifier – No_Dependence

394

Redundant Restrictions identifiers and Ravenscar

398

Parameters of formal packages given at most once

400

Wide and wide-wide images

414

pragma No_Return for overriding procedures

417

Lower bound of functions in Ada.Exceptions etc

419

Limitedness of derived types

420

Resolution of universal operations in Standard

423

Renaming, null exclusion and formal objects

These changes can be grouped as follows.

First there are some minor changes to exception handling. There are neater means for testing for null occurrence and raising an exception with a message (241, 361) and also wide and wide-wide versions of some procedures (400, 417).

The numerics area has a number of small but important changes. They are the introduction of an attribute Mod to aid conversion between signed and unsigned integers (340); changes to the rules for fixed point multiplication and division which permit user-defined operations (364, 420); and an attribute Machine_Rounding which can be used to aid fast conversions from floating to integer types (267).

A number of new pragmas and Restrictions identifiers have been added. These generally make for more reliable programming. The pragmas are: Assert, No_Return, Preelaborable_Initialization, Unchecked_Union, and Unsuppress (161, 216, 224, 286, 329, 414). The restrictions identifiers are No_Dependence, No_Implementation_Pragmas, No_Implementation_Restrictions, and No_Obsolescent_Features (257, 368, 381). Note that there are also other new pragmas and new restrictions identifiers concerned with tasking as described in the previous chapter (see 5.4). However, the introduction of No_Dependence means that the identifiers No_Asynchronous_Control, No_Unchecked_Conversion and No_Unchecked_Deallocation are now obsolescent (394).

Finally there are changes in generic units. There are changes in generic parameters which are consequences of changes in other areas such as the introduction of interfaces and dispatching constructors as described in the chapter on the object oriented model (parts of 251 and 260); there are also changes to formal access and derived types (419, 423). Also, it is now possible to give just some parameters of a formal package in the generic formal part (317, 398).