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-=-=-=-=-=-=-
-Date: 2 Apr 1995 05:20:04 GMT
-From: jon@armory.com (Jon Shemitz)
-Newsgroups: comp.lang.pascal
-Subject: comp.lang.pascal FAQ, ver 0.43
I haven't had the time to do ANYTHING with this since January, when I took
a contract job that left me away from home (and w/out Net access) five days
a week. While that was happening, we bought a house, so I've been busy
settling in for the past month. I'll try to start an official release process
on this soon, so it IS on rtfm.mit.edu, and then maybe start collecting
Delphi questions.
_________________________________________________________________
COMP.LANG.PASCAL FAQ, VERSION 0.43
Table of Contents
Preface
1. Passing Parameters To Programs (Reading The Command Line)
1.1 How do I read command line parameters?
1.2 How do I read the whole command line?
2. Logarithms, Trigonometry, and Other Numerical Calculations
2.1 How do I calculate X^Y (aka X**Y)?
2.2 How do I take 'un-natural' logarithms?
2.3 Why do the trig functions like Sin() and Cos() give the wrong
answers?
2.4 How do I calculate ArcSin() or ArcCos()?
2.5 Can I trap (and recover from) floating point overflow errors?
2.6 Why do I always get the same random numbers every time I run
my program?
3. Strings and Numbers
3.1 How do I convert a string to a number?
3.2 How do I convert a number to a string?
4. Breaking The 64K Limit
4.1 I have 64K of global data and can't add anymore - what do I
do?
4.2 Can I have more than 64K of global data in my Windows program?
4.3 How can I build an array bigger than 64K?
4.4 How do I handle .BMP and/or .WAV files with more than 64K of
data?
5. Procedural Types
5.1 Can I make an indirect call to an object's method, using a
pointer or a procedural type?
6. Windows Sound Programming
6.1 Where can I find documentation on Windows sound programming?
7. Other Pascal Resources
_________________________________________________________________
Preface
This is a still-rather-rough draft of a FAQ file for
comp.lang.pascal. (I'm still not thrilled with the ASCII formatting
I'm getting from lynx -dump, but the hanging indents and extra blank
lines seem to be lynx bugs - the HTML Validation Service at
http://www.hal.com/%7Econnolly/html-test/service/validation-form.htm
l likes my HTML just fine.) Comments are more than welcome; when
people are happy with the content (and I'm happy with the
formatting) I'll submit it as an official news.answers/rtfm.mit.edu
FAQ.
I have only included answers to questions that seem to me to be
genuinely frequently asked. I have no desire to write a Pascal primer,
nor do I want to produce a intimidatingly large document. On the other
hand, if this FAQ does get out of hand, it's being written and
maintained as an HTML file, so you can use Mosaic or Lynx to jump
straight from the Table Of Contents to the answer to a particular
question. I'll post the ASCII version periodically to
comp.lang.pascal; you can always get the latest version via the World
Wide Web at <http://www.armory.com/~jon/clp-faq.html>.
Most of the readers of comp.lang.pascal use Turbo/Borland Pascal.
Certainly most of the FAQ's are about BP. Thus, this FAQ file deals
primarily with Turbo/Borland Pascal. Some of the questions and answers
may apply to other Pascal's, but it's up to you to decide if this is
the case. In any case, this FAQ file is a free public service, and
comes with no warranty as to accuracy. Use any information you find
here with discretion and caution.
CONTRIBUTORS
* At this point, most of the material in this FAQ is by me, Jon
Shemitz, aka [jds] or jon@armory.com.
* Some of the material is borrowed from Professor Timo Salmi's
(ts@uwasa.fi) Turbo Pascal FAQ file, which may be downloaded
from ftp://garbo.uwasa.fi/pc/link/tsfaqp.zip. Such material is
generally 'signed' [ts].
* Duncan Murdoch, dmurdoch@mast.queensu.ca, contributed the material
signed [dm].
_________________________________________________________________
Passing Parameters To Programs (Reading The Command Line)
1.1: HOW DO I READ COMMAND LINE PARAMETERS?
The standard function ParamCount: word will return the number of
command line parameters, while function ParamStr(N: word): string will
return the N-th parameter. (Under DOS 3.0 or greater, the 0-th
parameter (ie, ParamStr(0)) is the path and file name of the current
program.) Thus,
function CommandLine: string;
var
Idx: word;
Result: string;
begin
Result := '';
for Idx := 1 to ParamCount do
begin
if Idx > 1 then Result := Result + ' ';
Result := Result + ParamStr(Idx);
end;
CommandLine := Result;
end;
will return the whole command line, with any embedded whitespace
(spaces or tabs) converted to single spaces. If you care about the
amount or type of whitespace, or you want commas, semicolons, and
equal signs to count as parameter separators (as per ancient versions
of DOS manuals), see the next question:
1.2: HOW DO I READ THE WHOLE COMMAND LINE?
ParamCount and ParamStr are for parsed parts of the command line and
cannot be used to get the command line exactly as it was. If you try
to capture
"Hello. I'm here"
you'll end up with a false number of blanks. For obtaining the command
line unaltered use
type CommandLines = string[127];
function CommandLine: CommandLines;
type
CommandLinePtr = ^CommandLines;
begin
CommandLine := CommandLinePtr( Ptr(PrefixSeg, $80) )^;
end;
A warning. If you want to get this correct (the same goes for TP's own
ParamStr and ParamCount) apply them early in your program, before any
disk I/O takes place.
For the contents of the Program Segment Prefix (PSP) see a DOS
Technical Reference Manual (available on the Microsoft DevNet CD) or
Tischer, Michael (1992), PC Intern System Programming, p. 753.
- Based on [ts]'s Turbo Pascal FAQ
_________________________________________________________________
Logarithms, Trigonometry, and Other Numerical Calculations
I often find it convenient to define floating-point functions in terms
of a type float = {$ifopt N+} double {$else} real {$endif}, rather
than either explicitly using real or double. This way, the same
function will automatically use the 'best' argument and result type
for either the N+ (80x87) or N- state, without any changes and without
obscuring its logic with lots of {$ifdef}s.
2.1: HOW DO I CALCULATE X^Y (X**Y)?
Pascals do not have an inbuilt power function. You have to write one
yourself. The common, but non-general method is defining
function POWERFN(number, exponent: float): float;
begin
PowerFn := Exp(Exponent*Ln(Number));
end;
To make it general use:
(* Generalized power function by [ts] *)
{ Some modifications by jds }
function GenPowFn(Number, Exponent: float): float;
begin
if (Exponent = 0.0)
then GenPowFn := 1.0
else if Number = 0.0
then GenPowFn := 0.0
else if Abs(Exponent*Ln(Abs(Number))) > 87.498
then RunError(205) {Floating point overflow}
else if Number > 0.0
then GenPowFn := Exp(Exponent*Ln(Number))
else if (Number < 0.0) and (Frac(Exponent) = 0.0)
then if Odd(Round(Exponent))
then GenPowFn := -GenPowFn(-Number, Exponent)
else GenPowFn := GenPowFn(-Number, Exponent)
else RunError(207); {Invalid float-op}
end; (* genpowfn *)
On the lighter side of things, here's an extract from an answer of
mine [TS] in the comp.lang.pascal UseNet newsgroup:
> anyone point out why X**Y is not allowed in Turbo Pascal?
The situation in TP is a left-over from standard Pascal. You'll
recall that Pascal was originally devised for teaching
programming, not for something as silly and frivolous as
actually writing programs. :-)
The above is a lightly-edited version of the answer from [ts]'s Turbo
Pascal FAQ. For the common special-case where you're raising a
floating point number to an integral power (eg, X^7 or Y^3), you can
use this fast code:
function RealPower(Base: Float; Power: word): Float;
begin
if Odd(Power)
then if Power = 1
then RealPower := Base
else RealPower := Base * RealPower(Base, Power - 1)
else if Power = 0
then RealPower := 1
else RealPower := Sqr(RealPower(Base, Power shr 1));
end;
2.2: HOW DO I TAKE 'UN-NATURAL' LOGARITHMS?
Just define
function Log(X, Base: float): float;
begin
Log := Ln(X) / Ln(Base);
end;
This result is based on some elementary math. By definition y = log(x)
in base B is equivalent to x = B^y (where the ^ indicates an
exponent). Thus ln(x) = y ln(B) and hence y = ln(x) / ln(B).
- A lightly-edited version of the answer from [ts]'s Turbo Pascal FAQ
2.3: WHY DO THE TRIG FUNCTIONS LIKE SIN() AND COS() GIVE THE WRONG ANSWERS?
While most people express angles in degrees, the trig functions expect
their arguments to be in radians. For historical reasons, a complete
rotation is 360 degrees; for more cosmological reasons, the same
complete rotation is 2 * Pi radians (the circumference of a circle
with a radius of 1). Thus, to convert degrees to radians, just divide
by 180 / Pi.
2.4: HOW DO I CALCULATE ARCSIN() OR ARCCOS()?
Borland Pascal does not have ArcSin or ArcCos functions. It does have
an ArcTan function, and the online help for that function gives the
following conversion formulae:
ArcSin(x) = ArcTan (x/sqrt (1-sqr (x)))
ArcCos(x) = ArcTan (sqrt (1-sqr (x)) /x)
Dr. John Stockton, jrs@merlin.dclf.npl.co.uk, points out that ArcTan
will always return a value between -Pi / 2 and Pi / 2. Also, there are
two angles in the range from 0 to 2 pi for any given sine or cosine
value, even though the formulae above will only give you one of them.
2.5: CAN I TRAP (AND RECOVER FROM) FLOATING POINT OVERFLOW ERRORS?
Duncan Murdoch [dm] writes:
Because the floating point processor operates in parallel to the
integer processor, this is generally quite tricky. The best approach
is not to trap the errors, but just to mask them, and at the end of
a calculation check whether they have occurred by examining the
coprocessor status word.
Be aware that Borland's string-conversion routines (used in Str,
Write and Writeln) clear the FPU's status word. If you do any I/O of
floating point values in between status checks, you may miss seeing
signs of errors.
I would add that it's probably best to leave the floating point
overflow exception on (unmasked) in the vast majority of your code,
and that you should only mask it off around the few calculations where
you expect and can handle a possible overflow. That is, a runtime
error is probably better than allowing the program to continue with
unnoticed math errors!
The following demo code may be helpful:
function Get87CtrlWord: word; assembler;
var
CtrlWord: word;
asm
fstcw [CtrlWord]
mov ax,[CtrlWord]
end;
procedure Set87CtrlWord(NewCtrlWord: word); assembler;
asm
fldcw [NewCtrlWord]
end;
function Get87StatusWord: word; assembler;
var
StatusWord: word;
asm
fstsw [StatusWord]
mov ax,[StatusWord]
end;
const
InvalidOp = $01;
DenormalOp = $02;
ZeroDivide = $04;
Overflow = $08;
Underflow = $10;
Precision = $20;
var
Ctrl, Status: word;
X, Y: double;
begin
Ctrl := Get87CtrlWord;
Set87CtrlWord(Ctrl or Overflow); {Setting a bit masks the exception}
X := 1e308; Y := X * X;
Status := Get87StatusWord;
Set87CtrlWord(Ctrl);
if (Status and Overflow) 0 {A set bit indicates an exception}
then WriteLn('Overflow flag set')
else WriteLn('Overflow flag clear');
end.
[dm] adds
const
StackFault = $40;
StackOverflow = $200;
These bits are supported in the 387 and up; together they indicate a
stack overflow (both set) or stack underflow (just StackFault).
2.6: WHY DO I ALWAYS GET THE SAME RANDOM NUMBERS EVERY TIME I RUN MY PROGRAM?
Software random number generators apply a function to a RandSeed which
cycles the seed through its possible values in a quasi-random way.
Each call to the random number generator does one iteration of the
function, and returns a result based on the new seed value.
When your program loads, this seed will have some default value
(probably 0). If you do not change the seed, a series of calls to the
random number generator will yield the same series of "random" numbers
every time your program is run. (Obviously, this will make it easier
to track down the bugs!) Typically, the system clock is used to
provide a value for the random number seed: Even if a given task is
always run at the same time of day, a difference of a few milliseconds
is enough to put a good random number generator in an entirely
different part of its sequence.
In Borland Pascal, the command to randomize the seed is (surprise!)
Randomize; in Think Pascal for the Mac, the equivalent command is
QwertyUiop. Note that you should only call this routine once per
program, when it first loads, or at the very least at times separated
by minutes or hours - calling it on every timer tick (say) will just
reset the 'random' sequence several times a second!
_________________________________________________________________
Strings and Numbers
3.1: HOW DO I CONVERT A STRING TO A NUMBER?
In Turbo/Borland Pascal, you can use the standard Val() procedure. The
first argument is a string, and the second argument is a numeric
variable which will be set to the numeric value represented by the
string. The result variable can be any of the numeric types - from
byte or ShortInt to comp or extended - the compiler will automagically
pass the variable's type to the Val() procedure.
Val() is a procedure, not a function, which means that it does not
return any sort of error code to indicate that it couldn't set the
result variable because eg the string was 'Four score and seven' not
'87', or because the string was '3.14159' and the result variable was
a word, or even because '12261958' is too big for a word variable.
Obviously, you do want to be able to find out whether Val() was able
to decode the string. This is where the third parameter - an integer
(or word) variable which will receive a result code - comes in. If the
result code is 0, then the string represents a number which could be
(and was) placed in the result variable; a nonzero result code means
that the string does not represent a compatible number: the result
variable has not been changed, and the error code is the index of the
first illegal character in the string.
Note that Val() and ReadLn() can handle hexadecimal numbers, using the
$1234 format.
3.2: HOW DO I CONVERT A NUMBER TO A STRING?
In Turbo/Borland Pascal, you can use the standard Str() procedure. The
first argument is a number, and the second is a string variable which
will be set to the formatted value. Just as with Write() and
WriteLn(), the number may (but does not have to) be followed by a
colon and a width specifier and (for float point numbers) a second
colon and a number of digits after the decimal point.
Str() is a procedure, not a function, which makes it a lot harder to
use than, say, WriteLn(). Obviously, it's pretty trivial to put a
'wrapper' around it so that you can write string expressions like
Fmt(Month,2) + DateSep + Fmt(Day,2) + DateSep + Fmt(Year,2):
type NString = string[20];
function Fmt(N: LongInt; Width: word): NString;
var Result: NString;
begin
Str(N: Width, Result); {A width of 0 is the same as no width at all}
Fmt := Result;
end;
While Val() can read hex strings, there's no way to force Str() to
produce a hex string. For that, you can use
type HexString = string[8];
function HexFmt(Int: LongInt; Width: integer): _HexFmtResult_;
const Hex: array[0..$000F] of char = '0123456789ABCDEF';
begin
if (Width <= 1) and (Int and $FFFFFFF0 = 0)
then HexFmt := Hex[Int]
else HexFmt := HexFmt(Int shr 4, Width-1) + Hex[Int and $000F];
end;
_________________________________________________________________
Breaking The 64K Limit
4.1: I HAVE 64K OF GLOBAL DATA AND CAN'T ADD ANYMORE - WHAT DO I DO?
What you have to do is to find the largest data structures. A good
way to do this is to look at a .map file with the addresses of
"public" names, and look for large jumps between names. Often, a big
jump flags a big variable. However, the .map file will not show any
variables that are private to a unit, so a big jump may just indicate
a lot of private variables within a unit. In that case, you'll have to
look at the unit's source code to see what's taking up so much space.
When you find your largest data structures, look at how they're used.
(The cross-reference tool in the BP7 IDE is an excellent way to do
this.) If they're only used in a single function or procedure, you can
move them into that procedure's local variables section, which will
get them out of the global data segment and onto the stack.
Of course, this may now cause the stack to be too big. If so, or if
the data are used in more than one place, you'll have to move the data
structure onto the heap. To do this, you'll have to do three things:
1. Replace a declaration like
var BigVar: BigType;
with
var SmallVar: ^BigType;
2. Change all references to BigVar to SmallVar^. Because BP is so
fast, it's usually quite practical to just keep compiling until
you get no more 'BigVar unknown' or 'need a ^' errors.
3. Somewhere in your program initialization, before you ever execute
any code that refers to SmallVar^, do a New(SmallVar); to allocate
space on the heap.
It's a good idea to get in the habit of using Dispose() whenever
you no longer need the space you've allocated, but it's not
strictly necessary, here: neither DOS nor Windows will 'lose' any
memory that you allocate but don't free when your program
terminates.
One thing to keep in mind is that reading or writing (dereferencing) a
pointer is slower than reading or writing a global or local variable,
especially in protected mode. Generally, you shouldn't worry about
this sort of thing except in bottleneck code but, where efficiency
does matter, you should definitely replace a series of dereferences
of the same pointer with a single with statement. (BP7 is
significantly better at "peephole" optimization than its predecessors,
but even in BP7 there are cases where a with PtrVar^ statement
produces better code than the equivalent series of 'raw' pointer
statements. Certainly replacing multiple instances of Row[Idx]^ or
even RecordPtr^.RecordField with a single with will result in smaller
and faster code.)
Finally, a word of admonition: You should use global data very
sparingly. If you are pushing the 64K limit, and it's not all in a
handful of large buffers, you're almost certainly using too many
global variables. Obviously, global variables that are only used in
one place waste space; less obviously, using global variables tends to
produce bugs. A seemingly innocuous change to a variable here can have
effects way over there. You can fight this sort of potentially
crippling interdepence by explicitly passing parameters to a function,
and explicitly returning results. It can be impossible to totally
avoid writing code that has side-effects, but such code should be
avoided as much as possible -- and always documented.
4.2: CAN I HAVE MORE THAN 64K OF GLOBAL DATA IN MY WINDOWS PROGRAM?
Yes and no. Your .exe module can only have a single 64K
data-and-stack segment, but each .dll has its own, up-to-64K data
segment. Thus, if you run out of room in your global data segment, you
can move some units and their global data into a DLL.
There are two things to keep in mind if you do this:
1. A call to a DLL is more expensive than a call within an EXE. Each
call into a DLL swaps data segments on the way in and on the way
out.
2. If a user is running two or more copies of your program at the
same time, each copy shares the code, but has its own copy of the
.exe's global data. However, there will only be one copy of the
.dll's global data, no matter how many copies of your application
(or how many different applications) are using it.
That is, any global variables in your .exe are a task resource;
any global variables in a .dll are a system resource.
4.3: HOW CAN I BUILD AN ARRAY BIGGER THAN 64K?
Break the array into two (types of) pieces: an array of row ptrs, and
a set of rows on the heap. That is, replace
type BigArrayType = array[0..Rows, 0..Cols] of DataType;
var BigArray: BigArrayType;
with
type BigArrayRow = array[0..Cols] of DataType;
BigArrayType = array[0..Rows] of ^ BigArrayRow;
var BigArray: BigArrayType
and replace any references to BigArray[Row, Col] with references to
BigArray[Row]^[Col]. (As per the answer to 4.1, don't forget to
allocate memory for each row!)
If performance isn't a major issue, you might well want to
encapsulate the array behavior in an object. That is, instead
of replacing BigArray[Row, Col] with BigArray[Row]^[Col], you
would replace it with calls to BigArray.Get(Row, Col) or
BigArray.Set(Row, Col, NewVal). While this is a bit slower than
direct array references, the object (compiled) code is smaller
and the source code is both easier to read and easier to
change. If you need to add virtual memory (swapping to and from
disk, EMS, or XMS) or to move your code to a 32-bit compiler,
you would only have to change the object definitions, not every
reference to your big array.
(One way to maintain both modifiability and good performance is
to replace the array of row pointers with a function that
returns a row pointer. Since you only need to call this once,
no matter how many operations you're doing on the row, using
your huge array might not be much slower than a normal array.
Since the function can do anything from simply looking up a
pointer in an array to swapping out the Least Recently Used row
and swapping in the one you need, you maintain flexibility.)
If the array has a lot of rows, and the rows are not some multiple of
8 (or 16 bytes) long, you might end up wasting a lot of space at the
end of each row. If so, and if you're just barely running out of room,
you might want to New() a "multi-row" type, that consists of an array
of 8 (or 16) individual rows. This will eliminate the wasted pad
bytes, and will only complicate your setup and teardown code: since
the row pointer array will still point to the start of each individual
row, access to any individual array element will be no different than
if each row is its own heap block.
Of course, decomposing an array in this way only works if the array
has two (or more) dimensions. See the next question if you have to
deal with a one-dimensional data stream that's larger than 64K.
4.4: HOW DO I HANDLE .BMP AND/OR .WAV FILES WITH MORE THAN 64K OF DATA?
There are two issues here: allocating a single heap block that's
bigger than 64K, and accessing it. Both are significantly different in
real mode than in protected mode.
Real mode:
In real mode, the primary issue is allocating the space: BP's
heap manager won't let you allocate blocks bigger than 63 and a
bit K-bytes. One approach is to simply rely on the current
implementation's behavior and break your large heap request
into multiple subrequests. If previous allocation and
deallocation hasn't left the heap fragmented, back-to-back
allocations will all be contiguous: The last byte of one will
be just below the first byte of the next. This behavior is not
guaranteed by Borland, though, and may change in future
releases; similarly, if the heap is fragmented, back-to-back
allocations will probably not be contiguous. A better approach
is to restrict the size of the heap, and to use DOS services to
allocate memory outside of your .exe's memory map.
However you allocate it, you will still have to access it. In
real mode, this is simply a matter of understanding that the
segment part of a pointer is multiplied by 16 and added to the
offset part to obtain a linear address within the first meg of
memory. You can thus use a function like
function RealPtrTo(Base: pointer; Offset: LongInt): pointer;
{Note: This function has NOT been pulled from existing code
and has NOT been tested. JDS, 30 September 1994}
var Normal: record Segment, Offset: word; end;
begin
Normal.Segment := Seg(Base^) + Ofs(Base^) shr 4;
Normal.Offset := Ofs(Base^) and $000F;
{Normalise the Base pointer}
Inc(Offset, Normal.Offset); {# bytes from start of Base seg}
RealPtrTo := Ptr(Normal.Segment + Offset shr 4,
Normal.Offset + Offset and $0000000F);
end;
to generate a pointer to any byte within your huge data structure.
This pointer can then be used just as you use any other 16-bit
pointer, to address an up-to-64K window within your huge data.
Protected mode:
In protected mode, allocation is easy, and access is hard. Just
use GlobalAlloc() or GlobalAllocPtr() to allocate however much
space you need. (Well, if your program is running in Windows
standard mode, this had better be less than or equal to one
meg.)
Once you've allocated it, life continues to be easy - if you're
using 386 Enhanced Mode and a 32-bit compiler or assembler. The
selector you get from GlobalAllocPtr can be used with any
32-bit offset within the declared size of the heap block. If
you're using Standard Mode, or 16-bit compilers like TPW or
BP7, though, you'll have to work a bit.
To start with, you can't do segment arithmetic: segments have
been replaced with selectors, which are essentially indices
into a system-global table of allocated segments. Doing
arithmetic with selectors will either result in value that
points to the wrong selector or (more likely) produce an
invalid selector. In either case, using the resulting pointer
will probably produce a General Protection Fault.
This is a complicated subject that I've already covered
elsewhere (see http://www.armory.com/~jon/pubs/huge-model.html
for my PC Techniques article on huge model programming): All
I'll say here is that
1. You have to use SelectorInc to 'step' the selector from one
64K window to the next, and
2. You can't make a single reference that starts in one 64K
window and ends in the next.
_________________________________________________________________
Procedural Types
5.1: CAN I MAKE AN INDIRECT CALL TO AN OBJECT'S METHOD, USING A POINTER OR A
PROCEDURAL TYPE?
Yes, but you'll need to make aggressive use of casting, and to have
a bit of background on just what a method call is. While method
calls look and act very differently than normal calls -- the call
looks like a reference to one of the object's fields, and there's
the implicit with Self do that lets us refer to the object's fields
as if they were global variables -- at the level of words on the
stack they're not all that different from a normal procedure or
function call. All methods have an `invisible', or implicit,
parameter, var Self, after any regular, or explicit, parameters;
constructors and destructors also add an implicit word parameter
(the 16-bit VMT pointer) between the explicit parameters and Self.
Also, while constructors act as if they return a boolean, they
actually return a pointer which contains @ Self if Fail was not
called, and Nil if it was.
The implicit parameters and the special handling of constructor
results are the only differences between method calls and normal
calls: there's no magic involved. If we simply define a procedural
type, ProcType, that explicitly declares the method's implicit
parameters after any normal parameters, we can then use ProcType to
cast any pointer variable to a procedural variable. Once it's cast,
the pointer acts just like a normal procedural variable; we can
assign it to another procedural variable or use it to call a
procedure. Just as with a normal procedural type, the only
difference between a direct and indirect call lies in the way we
make the call: The parameters are pushed and popped in the same way;
the called code operates just the same; and indirectly called
methods have the same full access to their object's fields (through
the Self pointer) as directly called methods do.
Thus, if we have a method with no arguments and no results, we would
simply make the declaration type Niladic = procedure (var Self);. To
use it, we remember that we can only cast pointer variables, not
pointer expressions, and so do something like PtrVar := @
ObjectType.Method; Niladic(PtrVar)(Self); Now, while there is
something strange looking about a cast (in parentheses) followed by
an argument list (in parentheses), indirect method calls are
typically rare and concentrated in a few key routines, even in
programs that rely heavily on them. (My typical uses for indirect
method calls involve things like executing a list of object/method
pairs on every timer tick, or calling a window object's message
handler after DMT lookup reveals that it does have a handler.)
What's more, the strange look of an indirect method call does not
translate into strange object code: Using a cast to a procedural
type generates the exact same code as using an normal procedural
type, and that's both a little smaller and only slightly slower than
a normal, direct procedure or function call.
Methods that require parameters or that return results are only
slightly different than our Niladic example above. We simply have to
remember to put any explicit parameters before the implicit
parameter(s). Thus, we might use type SimplePredicate = function
(var Self): boolean; for a method that takes no arguments and
returns a boolean, and type UntypedDyadic = procedure (var A, B; var
Self); for a method that requires two untyped memory references.
Just as with a normal procedure call, the compiler will not let us
make an indirect method call with the wrong number or type of
arguments. This is obviously desirable behavior, but it's tempered
with a bit of a caveat: When we make a cast, we are effectively
telling the compiler that we know exactly what we are doing. If we
accidentally use a pointer to a UntypedDyadic method as a Niladic,
the compiler will neither require nor accept the two var parameters
to the UntypedDyadic method but the procedure will probably use them
and the result will not be pretty! Similarly, the compiler will not
complain if you cast a data pointer or the address of a near routine
into a procedural type: it will blithely generate code that will (at
best!) crash your computer.
This answer is based on my [jds] article Three Myths About Procedural
Variables which originally appeared in the Dec/Jan 1993 issue of PC
Techniques.
_________________________________________________________________
Windows Sound Programming
6.1: WHERE CAN I FIND DOCUMENTATION ON WINDOWS SOUND PROGRAMMING?
For a start, try MMSYSTEM.HLP, in your \bp\bin directory.
(The BP7 install program creates an icon for this in the
Borland Pascal program group, but many people simply copy the
BPW icon to a working group, and zap the BP group. Then, when
they want to do some sound programming, they find there is no
mention of it in the online help file, and they don't know
where to turn.)
Unfortunately, MMSYSTEM.HLP only contains part of material in the
SDK's Multimedia Programmer's Reference. Piecing together the sequence
of steps to properly open a MIDI or WAV device from the help file can
be tough! I strongly recommend the SDK - or, better, the DevNet CD.
_________________________________________________________________
Other Pascal Resources
* Professor Timo Salmi (ts@uwasa.fi) of the University of Vaasa in
Finland maintains a Turbo Pascal archives, which includes both
source code and a Turbo Pascal FAQ file. You can find these by
ftp-ing to garbo.uwasa.fi and looking in the /pc/ts directory.
* I have written a number of articles on Turbo Pascal esoterica. An
online bibliography is available via the World Wide Web at
http://www.armory.com/~jon/Publications.html. Some of the actual
articles are also available online and I can post most of the
others on request.
* I heartily recommend Microsoft's DevNet CD. It's not exactly a
Pascal resource, but many of the questions on comp.lang.pascal
these days have to do with Windows programming. The CD includes
not only the reference material in the .HLP files but also
electronic copies of the SDK's programmer's guides; Petzold's
Programming Windows 3.1; DOS and hardware references; and much,
much more. Best of all, it's extensively indexed, so it's very
easy to find the answers to your questions. If you're at all
serious about Windows programming, get a CD-ROM drive and this CD.
(For somewhat more info, you can see the on-line version of the
review I wrote for PC Techniques at
http://www.armory.com/~jon/pubs/DevNet-CD.html).
_________________________________________________________________
Jon Shemitz - jon@armory.com - 30-Sep-94..7-Jan-95
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