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Varargs
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Subject: Delphi: Example of variable number of parameters
program VarPar;
{ A simple program to demonstrate use of type-safe variable number of
parameters in Delphi.
Written Mars 1995 by Hallvard Vassbotn
hallvard@falcon.no
}
uses WinCrt, SysUtils;
{ These are predefined in System:
const
vtInteger = 0;
vtBoolean = 1;
vtChar = 2;
vtExtended = 3;
vtString = 4;
vtPointer = 5;
vtPChar = 6;
vtObject = 7;
vtClass = 8;
type
TVarRec = record
case Integer of
vtInteger: (VInteger: Longint; VType: Byte);
vtBoolean: (VBoolean: Boolean);
vtChar: (VChar: Char);
vtExtended: (VExtended: PExtended);
vtString: (VString: PString);
vtPointer: (VPointer: Pointer);
vtPChar: (VPChar: PChar);
vtObject: (VObject: TObject);
vtClass: (VClass: TClass);
end;
}
const
TypeNames : array [vtInteger..vtClass] of PChar =
('Integer', 'Boolean', 'Char', 'Extended', 'String',
'Pointer', 'PChar', 'Object', 'Class');
{
According to the on-line docs (search for TVarRec), array of const
parameters are treated like array of TVarRec by the compiler.
This example will work just as well if you change the declaration of
TestMultiPar to:
procedure TestMultiPar(const Args: array of TVarRec);
This would make the implementation of the routine cleaner (no absolute
variable declaration), but the interface would be less understandable
to the user of the routine.
The compiler looks at the parameters and builds the array directly on the
stack. For each item in the array it also sets the VType field to one
of the pre-defined constants vtXXXX. The actual value is always sent as
four bytes of information. For the Boolean and Char types, only the first
byte contains useful information.
So, go ahead, now you can write all those neat routines with variable
number of parameters - and still keep the type safety!
}
function PtrToHex(P: pointer): string;
begin
Result := IntToHex(Seg(P^), 4) + ':' + IntToHex(Ofs(P^), 4);
end;
procedure TestMultiPar(const Args: array of const);
var
ArgsTyped : array [0..$fff0 div sizeof(TVarRec)] of TVarRec absolute Args;
i : integer;
begin
for i := Low(Args) to High(Args) do
with ArgsTyped[i] do
begin
Write('Args[', i, '] : ', TypeNames[VType], ' = ');
case VType of
vtInteger: writeln(VInteger);
vtBoolean: writeln(VBoolean);
vtChar: writeln(VChar);
vtExtended: writeln(VExtended^:0:4);
vtString: writeln(VString^);
vtPointer: writeln(PtrToHex(VPointer));
vtPChar: writeln(VPChar);
vtObject: writeln(PtrToHex(Pointer(VObject)));
vtClass: writeln(PtrToHex(Pointer(VClass)));
end;
end;
end;
var
MyObj : TObject;
begin
Writeln('Test of type-safe variable number of parameters in Delphi:');
MyObj := TObject.Create;
TestMultiPar([123, 45.67, PChar('ASCIIZ'), 'Hello, world!', true, 'X',
@ShortDayNames, TObject, MyObj]);
MyObj.Free;
{ To verify that the type-safety is used in the supplied formatting routines,
try this: }
writeln(Format('%d', ['hi']));
{ The supplied parameter is not of the type expected. The '%d' format string
signals that the parameter should be an integer value, but instead we
send a string. At run-time this will generate a exception, and if you
have enabled IDE-trapping of exceptions, Delphi will show you the offending
line. Using c-type sprintf funtions like this will result in undefined
behaviour (read: system crash, GP or whatever) }
end.
--
Hallvard Vassbotn | Falcon AS (a Reuters company) | Without programmers,
hallvard@falcon.no | Stranden 1, 0250 OSLO, Norway | the world would stop!
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