Ada FAQ: Programming with Ada (part 1 of 3)

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Magnus Kempe

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Aug 19, 1995, 3:00:00 AM8/19/95
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Archive-name: computer-lang/Ada/programming/part1
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Last-modified: 19 August 1995
Last-posted: 19 July 1995

Ada Programmer's
Frequently Asked Questions (FAQ)

IMPORTANT NOTE: No FAQ can substitute for real teaching and
documentation. There is an annotated list of Ada books in the
companion comp.lang.ada FAQ.

Recent changes to this FAQ are listed in the first section after the table
of contents. This document is under explicit copyright.

This is part 1 of a 3-part posting.
Part 2 begins with question 5.5.
Part 3 begins with question 8.3.
They should be the next postings in this thread.


Introduction

Ada is an advanced, modern programming language, designed and
standardized to support and strongly encourage widely recognized
software engineering principles: reliability, portability, modularity,
reusability, programming as a human activity, efficiency,
maintainability, information hiding, abstract data types, genericity,
concurrent programming, object-oriented programming, etc.

All validated Ada compilers (i.e. a huge majority of the commercial
Ada compilers) have passed a controlled validation process using an
extensive validation suite. Ada is not a superset or extension of any
other language. Ada does not allow the dangerous practices or effects
of old languages, although it does provide standardized mechanisms to
interface with other languages such as Fortran, Cobol, and C.

Ada is recognized as an excellent vehicle for education in programming
and software engineering, including for a first programming course.

Ada is defined by an international standard (the language reference
manual, or LRM), which has been revised in 1995. Ada is taught and
used all around the world (not just in the USA). Ada is used in a very
wide range of applications: banking, medical devices,
telecommunications, air traffic control, airplanes, railroad
signalling, satellites, rockets, etc.

The latest version of this FAQ is always accessible through WWW as
http://lglwww.epfl.ch/Ada/FAQ/programming.html#title

Maintenance

This FAQ is maintained on an individual volunteer basis, by Magnus
Kempe (Magnus...@di.epfl.ch). [Note: This is done as a hobby, not
in my capacity as an employee at the Swiss Federal Institute of
Technology. --MK]

The coding style used in most of the example Ada code is my own, and
you'll have to live with it (you may want to adopt it :-).


_________________________________________________________________

Opinions (if any) expressed are those of the submitters and/or
maintainer.
_________________________________________________________________

Table of Contents:

* 1: Recent changes to this FAQ

* 2: Information about this document

* 3: Elementary Questions
+ 3.1: How do I make operations directly visible without
"use"ing a package?
+ 3.2: How do I assign to an array of length 1?
+ 3.3: How do I create a C-style nul-terminated string?
+ 3.4: How can I create an array of strings of various length?
+ 3.5: I know an exception is raised, but my program quits with
no warning. Why?
+ 3.6: I have only one task in my program, but it doesn't seem
to run. Why?
+ 3.7: How do I increase the stack size for a task?
+ 3.8: What's the difference between a type conversion and a
qualifier?
+ 3.9: How do I avoid the potential space in front of
Integer'Image?
+ 3.10: Why is an exception raised when giving a default
discriminant?
+ 3.11: When I want an Integer type, what's wrong with just
using the predefined type Integer or Long_Integer? Why would
I ever want to declare new Integer types?
+ 3.12: Since I can always declare my own portable integer
types, why would I ever want to use the predefined type
Integer?
+ 3.13: I am learning Ada. Can I experiment with a game
program?
+ 3.14: How can I do a non-blocking, keystroke-at-a-time read
from the terminal?


* 4: Advantages of Ada
+ 4.1: Why use Ada?
+ 4.2: Ada seems large and complex, why is it this way?


* 5: Object-Oriented Programming with Ada
+ 5.1: Why does Ada have "tagged types" instead of classes?
+ 5.2: Variant records seem like a dead feature now. When
should I use them instead of tagged types?
+ 5.3: What is meant by "interface inheritance" and how does
Ada support it?
+ 5.4: How do you do multiple inheritance in Ada 9X?
+ 5.5: Why are Controlled types so, well, strange?
+ 5.6: What do "covariance" and "contravariance" mean, and does
Ada support either or both?
+ 5.7: What is meant by upcasting/expanding and
downcasting/narrowing?
+ 5.8: How does Ada do "narrowing"?
+ 5.9: What is the difference between a class-wide access type
and a "general" class-wide access type?


* 6: Ada Numerics
+ 6.1: Where can I find anonymous ftp sites for Ada math
packages? In particular where are the random number
generators?
+ 6.2: How can I write portable code in Ada 83 using predefined
types like Float and Long_Float? Likewise, how can I write
portable code that uses Math functions like Sin and Log that
are defined for Float and Long_Float?
+ 6.3: Is Ada any good at numerics, and where can I learn more
about it?
+ 6.4: How do I get Real valued and Complex valued math
functions in Ada 95?
+ 6.5: What libraries or public algorithms exist for Ada?


* 7: Efficiency of Ada Constructs
+ 7.1: How much extra overhead do generics have?


* 8: Advanced Programming Techniques with Ada
+ 8.1: Does Ada have automatic constructors and destructors?
+ 8.2: How can I redefine assignment operations?
+ 8.3: Should I stick to a one package, one type approach while
writing Ada software?
+ 8.4: What is the "Beaujolais Effect"?
+ 8.5: What about the "Ripple Effect"?
+ 8.6: How to write an Ada program to compute when one has had
too much alcohol to legally drive?
+ 8.7: Does Ada have macros?


* 9: Ada and Other Programming Languages
+ 9.1: Where can I find programs that will translate from [some
language] to Ada?
+ 9.2: How can I convert Ada 83 sources to Ada 9X?
+ 9.3: I hear that Ada is slower than Fortran or C, is that
true?
+ 9.4: Isn't Ada less "elegant" than Eiffel?
+ 9.5: Are there any papers detailing the differences between
Ada and C++?
+ 9.6: I keep hearing that Ada is a "strongly typed language",
but it seems different from what's meant in C++. Are they
different?
+ 9.7: I'm told Ada does all sorts of static type checking, but
can't you get the same effect using a tool like "lint" with
C?
+ 9.8: Does Ada have something like the Standard Template
Library (STL) in C++, or like the components one finds in
Smalltalk environments?
+ 9.9: Where can I find the equivalent of "printf" in Ada?


* 10: Interfacing with Ada
+ 10.1: I am writing software that used the Distributed
Interactive Simulation (DIS) interface, does an interface
exist in Ada?
+ 10.2: Is there any support for Common Object Request Broker
Architecture (CORBA) for Ada 9X?


* 11: Finding Additional Information
+ 11.1: Where can I find Ada books?
+ 11.2: Are there other Ada-related FAQs?
+ 11.3: What is the "HBAP WWW Server"?


* 12: Pretty-printing and Measuring Ada Source Code
+ 12.1: Is there software that generates a pretty PostScript
file from Ada source code?
+ 12.2: I use vgrind to do "pretty printing" of my source. Is
there a vgrind definition for Ada?
+ 12.3: How about a source code reformatter?
+ 12.4: How can I count source lines of code (SLOC)?
+ 12.5: Can I measure other things?


* 13: Common Confusions
+ 13.1: Wasn't Ada designed by some committee? What kind of a
language could you possibly get from a committee?
+ 13.2: I've heard the DoD is dropping all Military standards
to reduce costs, doesn't that mean the mandate to use Ada
goes away too?


* 14: Credits

* 15: Copying this FAQ


_________________________________________________________________

1: Recent changes to this FAQ

* 950819: non-blocking, keystroke-at-a-time reading.
* 950819: some URL updates: AdaIC files keep moving around.
* 950630: corrected answers to questions 3.6 (task switching) and
3.8 (conversion vs. qualifier).
* 950517: "general" class-wide access type.
* 950413: variant records vs. tagged types, submitted by
K. Shillington.
* 950406: why Ada does not and should not have macros.
* 950320: more on DIS.
* 950315: Ada is good at numerics, by J. Parker.
* 950309: drinking and driving example.
* 950306: added counting SLOC and measuring.
* 950222: fixed some typos.
* 950207: revised introduction.
* 950202: updated discussion of C++ STL and Smalltalk library.
* 950126: advantages of code sharing for generics; Pascal to Ada
tool.
* 950125: why define new integer types, and why use the predefined
Integer type, submitted by J. Parker.
* 950124: approved for posting in *.answers.
* 950116: converting Ada 83 code to Ada 9X.
* 950106: lengthy code sections extracted and put on FTP server.
* 950105: printf solution; update on exception traces and vgrind.
* 950104: links from TOC to all questions.
* 941222: pretty-printing solutions.
* 941220: numbered all questions; various updates.
* 941215: one-element aggregates.
* 941212: additional elementary questions, submitted by P. Pukite.
* 941212: examining the so-called "elegance" of Eiffel (vs. Ada), by
T. Taft.
* 941212: integrated "Ada Numerics" section contributed by
J. Parker.
* 941208: added default discriminant and "Beaujolais effect."
* December 1994: more re-organization, HTML'ized.
* October 1994: FAQ format by Magnus Kempe.
* September 1994: first draft by Dave Weller.


What's important and missing:
* everything, life, and 42


_________________________________________________________________


2: Information about this document

This file is posted monthly to comp.lang.ada, comp.answers, and
news.answers.

This document has a home on the Home of the Brave Ada Programmers
(HBAP) WWW Server, in hypertext format, URL
http://lglwww.epfl.ch/Ada/FAQ/programming.html

It is available --as posted in *.answers-- on rtfm.mit.edu, which
archives all FAQ files posted to *.answers; see
ftp://rtfm.mit.edu/pub/usenet-by-group/news.answers/computer-lang/Ada

The text-only version is also available in directory
ftp://lglftp.epfl.ch/pub/Ada/FAQ

Magnus Kempe maintains this document; it's a hobby, not a job.
Feedback (corrections, suggestions, ideas) about it is to be sent via
e-mail to Magnus...@di.epfl.ch
Thanks.

In all cases, the most up-to-date version of the FAQ is the version
maintained on the HBAP WWW Server. Please excuse any formatting
inconsistencies in the posted version of this document, as it is
automatically generated from the on-line version.

_________________________________________________________________


3: Elementary Questions


3.1: How do I make operations directly visible without "use"ing the package?

In Ada 83, you can rename the operations in your scope.

-- Say you have an integer type called Int in package Types
function "<" (Left, Right : Types.Int)
return Boolean
renames Types."<";
-- Make sure the profiles of the first and last "<" match!


For operators, Ada 95 introduces the "use type" clause:

use type Types.Int; -- makes operators directly visible


3.2: How do I assign to an array of length 1?

Because of ambiguity of parentheses, named notation must be used for
one-element aggregates (or, under a different angle: a positional
aggregate must have more than one component).

See [RM9X 4.3.3(7)] as well as the syntax rule of
positional_array_aggregate in [RM9X 4.3.3]; historians see [RM83
4.3(4)].

declare
Array_of_One : array (1..1) of Float;
begin
-- Array_of_One := (10.0); -- Won't work, parsed as an expression
-- within parentheses

Array_of_One := (1 => 10.0); -- No ambiguity here
end;


You can't write a one-element positional aggregate in Ada. Nor a
zero-element aggregate. The reason for this restriction is that it
would be difficult for compilers to determine whether:

( exp )

is a parenthesized expression of some type, or an aggregate of an
array type. If Ada had used some other notation for aggregates (say,
"[...]"), then this problem would not exist.

Apparently the original requirements for Ada forbade using certain
ASCII characters, like '[' and ']', because those characters were not
available on all hardware. Also, certain characters are used for
different purposes and glyphs in countries that need additional
letters not present in ASCII.


3.3: How do I create a C-style nul-terminated string?

In a declaration block, append an ASCII.NUL to create a constant Ada
string.

declare
Str_Nul : constant String := Str & ASCII.NUL;
begin
Call_Requiring_C_String (Str_Nul (Str_Nul'First)'Address);
end;

-- or --

function Nul_Terminate (Str : String)
return String is
Str_Nul : constant String := Str & ASCII.NUL;
begin
return Str_Nul;
end Nul_Terminate;


3.4: How can I create an array of strings of various length?

In Ada 83, you have to use string access types and "new" to get
"ragged" arrays:

type String_Access is
access String;

Strings : constant array (Positive range 1..3) of String_Access
:= ( 1 => new String'("One"),
2 => new String'("Two"),
3 => new String'("Three")
);


In Ada 95, the process is simplified by using aliased constants:

type String_Access is
access constant String;

One : aliased constant String := "One";
Two : aliased constant String := "Two";
Three : aliased constant String := "Three";

Strings : constant array (Positive range <>) of String_Access
:= ( 1 => One'Access,
2 => Two'Access,
3 => Three'Access
);


3.5: I know an exception is raised, but my program quits with no warning. Why?


On some Ada compilers, you have to manually "with" Text_IO before
exception information is diplayed to the terminal.

On other Ada compilers, you must set an environment variable flag in
order to cause the exception information trace to be displayed.


3.6: I have only one task in my program, but it doesn't seem to run. Why?

In Ada, the main procedure is automatically designated as a task. This
task may be running forever, thus starving your other task(s), because
round-robin scheduling (time-slicing) is not required (pre-emptive
scheduling applies to tasks with different levels of priority).

If the task in question is getting starved, it's a programmer problem,
not an Ada problem. The programmer has to use an Ada compiler that
supports pragma Time_Slice, or do the scheduling himself (by changing
the implementation of his Ada program to ensure that no task starves
another).

One solution is to explicitly put the main task to sleep within a loop
construct in order to avoid starvation of the other task(s), as in:

procedure Main is
task Test;
task body Test is
begin
loop
delay 1.0;
Text_IO.Put_Line ("Test");
end loop;
end Test;
begin
loop
delay 20.0;
Text_IO.Put_Line ("Sleeping then writing");
end loop;
end Main;


3.7: How do I increase the stack size for a task?

Define the task as a "task type" and then use a pragma representation
clause.

task type A_Task_Type;
for A_Task_Type'STORAGE_SIZE use 10_000;
-- 10K bytes allocated to instances of A_Task_Type
A_Task : A_Task_Type;


3.8: What's the difference between a type conversion and a qualifier?

Use a qualifier (tick) to tell the compiler what type it can expect;
this is strictly a compile-time issue: a qualifier "hints" the type,
usually to remove an ambiguity. Use a conversion to tell the compiler
to convert an expression from one type to another (usually within one
derivation hierarchy); this operation may require a change of
representation at run-time (e.g. in case of a representation clause
applying exclusively to the source type).

A : Integer := Integer'(1); -- this is a qualifier: same as ":= 1;"
B : Integer := Integer (1); -- this is a conversion


3.9: How do I avoid the potential space in front of Integer'Image?

Use the function Trim from package Ada.Strings.Fixed (you can
actually trim strings in many other useful ways):

function My_Image (I : Integer)
return String is
begin -- My_Image
return Ada.Strings.Fixed.Trim (Integer'Image (I), Ada.Strings.Left);
end My_Image;

... My_Image (12) = "12" ...


In Ada 83, code a function that accepts a string and strips the
leading blank:

function Strip_Leading_Blank (Str : String)
return String is
begin -- Strip_Leading_Blank
if Str (Str'First) = ' ' then
return Str (1+Str'First .. Str'Last);
else
return Str;
end if;
end Strip_Leading_Blank;

...

function My_Image (I : Integer)
return String is
begin -- My_Image
return Strip_Leading_Blank (Integer'Image (I));
end My_Image;

... My_Image (12) = "12" ...


3.10: Why is an exception raised when giving a default discriminant?

Let's assume you would like to model varying-length strings:

type V_String (Size : Natural := 0) is
record
S : String (1 .. Size);
end record;


(from Robert Dewar)

When you give a default discriminant, then one method (I actually
think it is the preferred method) of implementation is to allocate the
maximum possible length. Since your discriminant is of type Natural,
this clearly won't work!

GNAT may compile it, but it won't run it, and indeed I consider it a
GNAT bug (on the todo list) that no warning is issued at compile time
for this misuse.

Some compilers, notably Alsys and RR, have at least partially "solved"
this problem by introducing hidden pointers, but this to me is an
undesirable implementation choice.

First, it means there is hidden heap activity, which seems
undesirable. In a language where pointers are explicit, it is
generally a good idea if allocation is also explicit, and certainly
for real-time work, hidden anything is worrisome.

Second, it is not easy to do uniformly. Alsys ends up introducing
arbitrary restrictions on the composition of such types (try making an
array of them), and RR introduces non-contiguous representations,
which are legal but troublesome.

To "solve" the problem yourself, just declare a reasonable maximum
length, and use a subtype representing this length as the subtype of
the discriminant:

Max_Length : constant := 200;

subtype Index is
Natural range 0 .. Max_Length;

type V_String (Size : Index := 0) is
record
S : String (1 .. Size);
end record;


3.11: When I want an Integer type, what's wrong with just using the predefined
type Integer or Long_Integer? Why would I ever want to declare new Integer
types?

If you declare 2 distinct integer types, for example,

type Data_Index is range 1..100;
type Time_Series_Index is range 0..2**15-1;


then objects of type Data_Index can't be assigned (directly) to
variables of type Time_Series_Index, and vice-versa. Likewise,
variables of these 2 types can't be mixed in arithmetical expressions
(without explicit type conversions). This may seem like a source of
endless irritation, but on the contrary, good progammers use it to
improve the clarity of their code, to make it more robust, and more
portable. The first 2 examples discuss this. The third example
discusses the declaration of machine-portable 32-bit integers.
Declaring objects of type Integer can be highly non-portable, and of
course type Long_Integer may not exist on some compilers.

Example 1.

Suppose you declare arrays using the above indices:

type Time_Series is array (Time_Series_Index) of Float;
type Y_Axis_Data is array (Data_Index) of Float;

Measurement : Time_Series;


Now if you mistakenly try to iterate over one array with the index of
the other, the compiler can catch the error at compile time:

for I in Data_Index loop
Sum := Sum + Measurement(I); -- compilation error
end loop;


Example 2.

This is lifted from Tucker Taft's brief introduction to Ada 95 in the
contributed papers section of the Ada World Wide Web homepage. Here
Tucker uses the Ada 95 unsigned integers, called modular types, in the
implementation of a protected type, which defines a disk control unit.
Modular types are integer types with "and", "or" and "xor" defined, so
systems programmers are likely to use them as bit masks. Just as the
array indices of the 2 arrays defined above are never meant to be
mixed, the modular integer types used to implement the disk control
unit are never meant to be mixed. To make sure the compiler enforces
this, they are declared as distinct types:

type Flags is mod 2**4; -- a 4-bit flags field
type Control is mod 2**4; -- A 4-bit control field

Status_Mask : constant Flags := 2#1001#; -- Set first and last bits.
Status_Ready : constant Flags := 2#1000#; -- Status = Ready

Start_Xfr : constant Control := 2#0001#; -- Initiate xfr command


Now if someone attempts to apply a Flag variable where a Control
variable should be used (or vice-versa) the compiler will catch the
error. This is especially important when the code is maintained by
programmers who did not write it.

Remarks on Examples 1 and 2.

1. Notice that in both examples the programmer was able to state his
intentions rather forcefully in the code - intentions that otherwise
might have been expressed much less forcefully in comment statements.
Because of Ada's strong typing model, the compiler was able to catch
errors at compile-time when the programmer's intentions were violated.


2. Notice also that the Integer declarations in the 2 examples are
machine portable, unlike Integer and Long_Integer. A compiler will
typically map these integer types onto the most efficient base type
that is available on the target machine.

Example 3.

Although the examples given above are good ones, it is not necessarily
a common practice to define a large number of distinct integer types.
In many cases it is appropriate to use (say) a 32-bit integer (or a
small number of such types) and declare appropriate subtypes of it
(them). To declare a portable 32-bit integer (or more accurately, the
most efficient integer that is at least 32-bits):

type Int_tmp is range -2**31+1 .. 2**31-1;
type Integer_32 is range Int_tmp'Base'First..Int_tmp'Base'Last;


A compiler may reject this declaration if no suitable base type is
available, but this is rare. What happens is this: in order to
implement Int_tmp, the compiler chooses as the base type of Int_tmp an
integer type that is available on the target machine. This base type
is usually the most efficient integer that accomodates the range of
Int_tmp, which in turn is usually the machine's 32-bit integer. (It
might even be a 64-bit integer on some machines, in which case
Integer_32'Size = 64, and Integer_32'Last = 2**63-1. Maybe we should
not call it Integer_32!)


3.12: Since I can always declare my own portable integer types, why would I
ever want to use the predefined type Integer?

The language itself provides some guidance here. The predefined type
Integer is used by Ada in the implementation of a number of convenient
services. The following examples describe some of these services.
Notice that in most of the following examples, it is unlikely that it
will ever matter whether or not the predefined type Integer is
16-bits, 32-bits, 48-bits, or 64-bits.

a) The exponentiation of X (written X**N) is defined by the language
for any floating point or integer X, provided N is of type Integer. (N
should be non-negative for integer X though.)

b) Ada's predefined String type (really just a packed unconstrained
array of characters) uses an index of subtype Positive (i.e. type
Integer).

c) The array index in the following "short-hand" array declaration is
implicitly defined to be type Integer:

A : array(10..40) of Float;


d) The loop parameter I in the following for loop is implicitly
declared type Integer:

for I in 10..40 loop
...
end loop;


This application of type Integer is the one most likely to get you
into portability trouble. If you write: "for I in 1..2**17 loop", then
you get a constraint error on compilers that make Integer 16-bits,
because 2**17 is out of range of any Ada 16-bit integer.


3.13: I am learning Ada. Can I experiment with a game program?

Of course. The Public Ada library (FTP wuarchive.wustl.edu) has a
portable Ada Tetris program in the languages/ada/misc/games directory.
It uses tasking, keyboard input, and ANSI screen graphics. Have fun!


3.14: How can I do a non-blocking, keystroke-at-a-time read from the terminal?

Use the procedure Text_IO.Get_Immediate [RM95 A.10.7(11)].

If you don't have an Ada 95 compiler but have a POSIX binding, there
is a package using POSIX services that provides non-blocking,
keystroke-at-a-time access to the terminal. It is available by FTP in
file ftp://lglftp.epfl.ch/pub/Ada/FAQ/inkey.ada

_________________________________________________________________

4: Advantages of Ada

4.1: Why use Ada?

Think of it like this: We're the kid on the street corner, licking
that tasty ice cream cone on a hot summer day; an impish grin
decorates our face as we consume our cool confection. Meanwhile, other
kids gather round, noticing our pleasure. It matters not a whit that
they've just had a drink, or had their fill with supper -- they now
want ice cream. We offer no lecture on how good the ice cream is, we
simply demonstrate that we are happy, and let their memories carry
them to the nearest ice cream truck.

(Sorry, I got a little carried away --DW).


4.2: Ada seems large and complex, why is it this way?

(Robert Dewar, lead designer of the GNU Ada compiler, responds):

During the Ada 9X development process we have often had fierce
arguments over the need to simplify proposals, and I pointed out some
time ago that the idea of simplicity is heavily overloaded:
* simple to implement
* simple to describe informally
* simple to describe formally
* results in simple programs
* simple to understand and/or remember
* short to describe


None of these goals are quite the same, and often they severely
conflict.

If you listen to programming language design types, especially from
universities, they often have very little experience in programming,
and especially little experience in writing large delivered,
maintained software. That doesn't mean they know nothing about
programming languages, but it does tend to mean that their view of
complexity is skewed, and in particularly concentrates on the
simplicity of the language itself, rather than on the simplicity of
resulting programs.

A lot of the creative tension in the 9X design process arose from this
same fundamental dichotomy. The design team tended to have a high
tolerance for language complexity (partly because they were very good
at understanding language details), but had a lot of experience in
actual large scale programming, and so their idea of simplicity was
biased heavily to simplifying Ada programs. The opposite voice,
worried about the simplicity of the language itself, represented by a
section of the DR's and ISO group (who, being a larger more diverse
group tended to reflect a wider view), considered that the design team
had gone too far in this direction. If you want to get a feel for the
transitions, look at the early versions of the 9X ILS, particularly
version 1.0.

In retrospect, I think we came up with what is at least very close the
optimal balance. Tuck can speak for himself here more clearly than I
can speak for him, but I would guess that he and the other members of
the team recognize that you have to be able to sell the resulting
design as an acceptably simple whole, and thus must step back from the
most extensive proposals, while on the other hand, the more
conservative KISS sentiments were convinced to accept more features
than they originally felt comfortable with because of convincing
programming examples and discussions of resulting programming
complexity. The third wing of opinion ("I don't care what you think,
but if we can't implement it, then it's not much use!") was also
effectively fed in from the user-implementor teams.

Is the result too complex? Time will tell, but I think the balance is
a successful blend.

_________________________________________________________________


5: Object-Oriented Programming with Ada


5.1: Why does Ada have "tagged types" instead of classes?

(Tucker Taft responds):

Someone recently asked me to explain the difference between the
meaning of the term "class" in C++ and its meaning in Ada 9X. Here is
a synopsis of the answer:

In C++, the term "class" refers to three different, but related
things:
* a language construct, that encapsulates the definitions of data
members, member functions, nested types, etc.;

* a particular kind of type, defined by a class construct (or by
"struct" which is a special case of "class");

* a set of types consisting of a type and all of its derivatives,
direct and indirect.


In Ada 9X, the term "class" refers only to the third of the above
definitions. Ada 9X (and Ada 83) has three different terms for the
concepts corresponding to the above three things:
* a "package" encapsulates the definitions of types, objects,
operations, exceptions, etc which are logically related. (The
operations of a type defined immediately within the package where
the type is declared are called, in 9X, the "primitive operations"
of the type, and in some sense, define the "primitive" semantics
of the type, especially if it is a private type.)

* a "type" is characterized by a set of values and a set of
primitive operations (there are a million definitions of "type,"
unfortunately, but you know what I mean...);

* a "class" is a set of types with similar values and operations; in
particular, a type and and all of its derivatives, direct and
indirect, represents a (derivation) class. Also, the set of
integer types form the integer "class," and so on for the other
language-defined classes of types in the language.


Some OOP languages take an intermediary position. In CLOS, a "class"
is not an encapsulating construct (CLOS has "packages"). However, a
"class" is both a type and a set of types, depending on context.
(Methods "float" freely.)

The distinction Ada 9X makes between types and classes (= set of
types) carries over into the semantic model, and allows some
interesting capabilities not present in C++. In particular, in Ada 9X
one can declare a "class-wide" object initialized by copy from a
"class-wide" formal parameter, with the new object carrying over the
underlying type of the actual parameter. For example:

procedure Print_In_Bold (X : T'Class) is
-- Copy X, make it bold face, and then print it.
Copy_Of_X : T'Class := X;
begin
Make_Bold (Copy_Of_X);
Print (Copy_Of_X);
end P;


In C++, when you declare an object, you must specify the "exact" class
of the object -- it cannot be determined by the underlying class of
the initializing value. Implementing the above procedure in a general
way in C++ would be slightly more tedious.

Similarly, in Ada 9X one can define an access type that designates
only one specific type, or alternatively, one can define one that can
designate objects of any type in a class (a "class-wide" access type).
For example:

type Fancy_Window_Ptr is access Fancy_Window;
-- Only points at Fancy Windows -- no derivatives allowed
type Any_Window_Ptr is access Window'Class;
-- Points at Windows, and any derivatives thereof.


In C++, all pointers/references are "class-wide" in this sense; you
can't restrict them to point at only one "specific" type.

In other words, C++ makes the distinction between "specific" and
"class-wide" based on pointer/reference versus object/value, whereas
in Ada 9X, this distinction is explicit, and corresponds to the
distinction between "type" (one specific type) and "class" (set of
types).

The Ada 9X approach we believe (hope ;-) gives somewhat better control
over static versus dynamic binding, and is less error prone since it
is type-based, rather than being based on reference vs. value.

In any case, in Ada 9X, C++, and CLOS it makes sense to talk about
"class libraries," since a given library will generally consist of a
set of interrelated types. In Ada 9X and CLOS, one could alternatively
talk about a set of "reusable packages" and mean essentially the same
thing.


5.2: Variant records seem like a dead feature now. When should I use them
instead of tagged types?

This is an instance of a much more general question: "When should I
use what kind of type?" The simple answer is: "When it makes sense to
do so." The real key to chosing a type in Ada is to look at the
application, and pick the type that most closely models the problem.

For instance, if you are modelling data transmission where the message
packets may contain variable forms of data, a variant record --not a
hierarchy of tagged types-- is an appropriate model, since there may
be no relationship between the data items other than their being
transmitted over one channel. If you choose to model the base type of
the messages with a tagged type, that may present more problems than
it solves when communicating across distinct architectures.

[More to be said about variant programming vs. incremental
programming.]


5.3: What is meant by "interface inheritance" and how does Ada support it?

This answer intentionally left blank.


5.4: How do you do multiple inheritance in Ada 9X?

There is a lengthy paper in file
ftp://sw-eng.falls-church.va.us/public/AdaIC/flyers/9xm-inh.txt

That document describes several mechanisms for achieving MI in Ada. It
is not unusual, however, to find complaints about the syntax and the
perceived burden it places on the developer. This is what Tucker Taft
had to say when responging to such a criticism on comp.lang.ada:

Coming up with a syntax for multiple inheritance was not the
challenge. The challenge was coming up with a set of straightforward
yet flexible rules for resolving the well known problems associated
with multiple inheritance, namely:
* If the same type appears as an ancestor more than once, should all
or some of its data components be duplicated, or shared? If any
are duplicated, how are they referenced unambiguously?

* If the same-named (including same parameter/result profile)
operation is inherited along two paths, how is the ambiguity
resolved? Can you override each with different code? How do you
refer to them later?

* Etc.


For answers, you can look at the various languages that define a
built-in approach to multiple inheritance. Unfortunately, you will
generally get a different answer for each language -- hardly a
situation that suggests we will be able to craft an international
consensus. Eiffel uses renaming and other techniques, which seem quite
flexible, but at least in some examples, can be quite confusing (where
you override "B" to change what "A" does in some distant ancestor).
C++ has both non-virtual and virtual base clases, with a number of
rules associated with each, and various limitations relating to
downcasting and virtual base classes. CLOS uses simple name matching
to control "slot" merging. Some languages require that all but one of
the parent types be abstract, data-less types, so only interfaces are
being inherited; however if the interfaces happen to collide, you
still can end up with undesirable and potentially unresolvable
collisions (where you really want different code for same-named
interfaces inherited from different ancestors).

One argument is that collisions are rare to begin with, so it doesn't
make much different how they are resolved. That is probably true, but
the argument doesn't work too well during an open language design
process -- people get upset at the most unbelievably trivial and
rarely used features if not "correctly" designed (speaking from
experience here ;-).

Furthermore, given that many of the predominant uses of MI (separation
of interface inheritance from implementation inheritance, gaining
convenient access to another class's features, has-a relationships
being coded using MI for convenience, etc.) are already handled very
well in Ada 9X, it is hard to justify getting into the MI language
design fray at all. The basic inheritance model in Ada 9X is simple
and elegant. Why clutter it up with a lot of relatively ad-hoc rules
to handle one particular approach to MI? For the rare cases where MI
is really critical, the last thing the programmer wants in the
language is the "wrong" MI approach built in.

So the basic answer is that at this point in the evolution of OO
language design, it seemed wiser to provide MI building blocks, rather
than to foist the wrong approach on the programmer, and be regretting
it and working around it for years to come.

Perhaps [Douglas Arndt] said it best...

Final note: inheritance is overrated, especially MI. ...


If the only or primary type composition mechanism in the language is
based on inheritance, then by all means, load it up. But Ada 9X
provides several efficient and flexible type composition mechanisms,
and there is no need to overburden inheritance with unnecessary and
complicated baggage.

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