Last-modified: 22 May 1996
Last-posted: 23 April 1996
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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 4-part posting; part 1 contains the table of contents. Part 2 begins with question 5. Part 3 begins with question 6. Part 4 begins with question 9. 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.Kempe@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? + 4.3: Is there a contest with fame and money for good Ada programmers? * 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? + 7.2: How does Ada compare to other languages in efficiency of code? * 8: Advanced Programming Techniques with Ada + 8.1: How can I redefine the assignment operation? + 8.2: Does Ada have automatic constructors and destructors? + 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: Credits * 14: Copying this FAQ _________________________________________________________________ 1: Recent changes to this FAQ * 960522: "aimap" located (12.3). * 960320: comparing the efficiency of Ada and other languages (7.2) (submitted by D. Wheeler) * 960228: references for redefining the assignment operation (8.1) (switched questions 8.1 and 8.2). * 960130: another game written in Ada (3.13). * 960123: ARA contest for good Ada (4.3). * 950925: removed a couple of non-programming questions. * 950828: the change logs now indicate the section/question number. * 950819: non-blocking, keystroke-at-a-time reading (3.14). * 950819: some URL updates: AdaIC files keep moving around. * 950630: corrected answers on task switching (3.6) and conversion vs. qualifier (3.8). * 950517: "general" class-wide access type (5.9). * 950413: variant records vs. tagged types (5.2), submitted by K. Shillington. * 950406: why Ada does not and should not have macros (8.7). * 950320: more on DIS (10.1). * 950315: Ada is good at numerics (6.3), by J. Parker. * 950309: drinking and driving example (8.6). * 950306: added counting SLOC (12.4) and measuring (12.5). * 950222: fixed some typos. * 950207: revised introduction. * 950202: updated discussion of C++ STL and Smalltalk library (9.8). * 950126: advantages of code sharing for generics (7.1); Pascal to Ada tool (9.1). * 950125: why define new integer types (3.11), and why use the predefined Integer type (3.12), submitted by J. Parker. * 950124: approved for posting in *.answers. * 950116: converting Ada 83 code to Ada 9X (9.2). * 950106: lengthy code sections extracted and put on FTP server. * 950105: printf solution (9.9); update on exception traces and vgrind. * 950104: links from TOC to all questions. 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/ 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.Kempe@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! There is also "program Small", a tiny text adventure program, that you can expand; it is documented at URL http://lglwww.epfl.ch/Ada/Tutorials/Lovelace/small.htm 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/ _________________________________________________________________ 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. 4.3: Is there a contest with fame and money for good Ada programmers? Yes, they should enter the Ada Lovelace Programming Contest sponsored by the Ada Resource Association (ARA). The Ada contest seeks to recognize the most readable, original, reusable, and clear working Ada programs. Like the Ada programming language, the contest is named in honor of the first programmer in history, Lady Ada Lovelace. Every three months, the ARA will pay US$ 750 to the best Ada code segment submitted. Submissions must be received by the 15th (midnight) of the "contest month" and the award will be announced at the end of the second month. A submission is made by emailing the source code to firstname.lastname@example.org The first contest closed December 15th, 1995. (The next contest month is March 1996.) The rules and guidelines of the contest are available from the Ada Contest WWW Home at http://lglwww.epfl.ch/Ada/Contest/announce.html This contest is open to all. Sharpen your designs, code, comments, and demos; show the world how good and unobfuscated your Ada code is, and win the prize!