Function Pointers with different signatures (example: optional parameter with a default value) - delphi

Is it possible to create a function-pointer with a default parameter, something like
TFunctionPointer = function(sName:AnsiString; tOptional: TObject = nil):smallint;
What I want to achieve:
A function pointer, which can accept a function of type
function A(sName:AnsiString)
or
function B(sName:AnsiString, tOptional: TObject)
How can I achieve this?

Default parameter is just a syntactic sugar - actually function call has two parameters.
But you can use function references and anonymous methods to create such function pointers - function adapters.
type
fnA = function(const sName: AnsiString): integer;
fnB = function(const sName: AnsiString; const tOptional: TObject); integer;
fnRef = reference to function(const sName: AnsiString; const tOptional: TObject): integer;
fnBridge = record
Bridge: fnRef;
class operator Implicit(fn: fnA): fnBridge;
class operator Implicit(fn: fnB): fnBridge;
end;
class operator fnBridge.Implicit(fn: fnA): fnBridge;
begin
Result.Bridge :=
function(const sName: AnsiString; const tOptional: TObject): integer
begin
Result := fn(sName);
end;
end;
class operator fnBridge.Implicit(fn: fnB): fnBridge;
begin
Result.Bridge :=
function(const sName: AnsiString; const tOptional: TObject): integer
begin
Result := fn(sName, tOptional);
end;
end;
function A(const sName: AnsiString): integer;
begin Result := Length(sName) end;
function B(const sName: AnsiString; const tOptional: TObject): integer;
begin Result := Length(sName) - Length(tOptional.ClassName) end;
function Consumer (const Param1, Param2: integer; const Action: fnBridge): integer;
begin
Result := Param1 + Param2 * Action.Bridge('ABCDE', Application);
end;
....
ShowMessage( IntToStr( Consumer(10, 20, A) ));
ShowMessage( IntToStr( Consumer(10, 20, B) ));
PS: since Delphi version was not specified it means that answer for ANY Delphi version suits fine. This method should work wit hany version starting with Delphi 2009 and later.
PPS: references to functions with captured variables are implemented internally as TInterfacedObject descendants. So overall this is just a reduced case of "Strategy pattern" using "higher-order functions"
http://en.wikipedia.org/wiki/Strategy_pattern
http://en.wikipedia.org/wiki/Higher-order_function

That is not possible. In order for a function to be of type TFunctionPointer, it must declare two parameters.
A default parameter is still a parameter. Your TFunctionPointer is a function with two parameters. When you call it and supply only one parameter, the compiler supplies the default parameter at the call site. So two parameters are still passed to the function.
To expand on this. Consider the following:
procedure Foo(Bar: Integer=666);
begin
end;
When you call the procedure like this:
Foo();
it looks as though the procedure has no parameters. But that is not the case. The compiler translates your code into this:
Foo(666);
The conclusion is that if you want to allow receipt of functions with different numbers of parameters, you'll need to provide an explicit mechanism to receive those different function types. For instance:
procedure DoSomething(const Callback: TProc<string, TObject>); overload;
begin
Callback(str, obj);
end;
procedure DoSomething(const Callback: TProc<string>); overload;
begin
DoSomething(
procedure(arg1: string; arg2: TObject)
begin
Callback(arg1);
end
);
end;

Related

Passing parameter value to Function or Procedure with random order

Is there a way in Delphi to pass parameter value to Function or Procedure with random order, so i don't have to make sure that the order is right.
Example:
procedure InsertEmp(ID: Integer;Name: String;Gender: String);
begin
//Content
end
Then I will use this procedure like this:
InsertEmp(1,'Zemmy','Male');
But if at another time i change the function parameter order like this:
procedure InsertEmp(ID: Integer;NickName:String;Name: String;Gender: String);
begin
//Content
end
I have to make position correction to my function as following:
InsertEmp(1,'Jim','Zemmy','Male');
Can i pass the parameter value without correcting the order? Maybe a way like this:
InsertEmp(Gender = 'Male',NickName = 'Jim',ID = 1,Name = 'Zemmy');
Thank you for your help.
You can have multiple overloaded procedures/functions taking parameters that are slightly different.
procedure InsertEmp(id: Integer; name: String; gender: String); overload;
procedure InsertEmp(id: Integer; nickName,name: String; gender: String); overload;
For a random parameter order, there are serializing libraries taking strings as input and then parsing them for input values. Simple key-value pairs or JSON are examples.
Having two or more parameters of the same type, the called procedure cannot determine their meaning.
I think the answer can be a mix of the whosrdaddy's comment an the LU RD's answer.
Using overloading for a record constructor, you can obtain a smart solution.
type
TEmployee = record
ID: Integer;
NickName,
Name,
Gender: String;
constructor Create(const AID: Integer; const ANickname, AName, AGender: String); overload;
constructor Create(const AID: Integer; const AName, AGender: String); overload;
end;
procedure InsertEmp(const AEmployee: TEmployee);
begin
//Content
end;
constructor TEmployee.Create(const AID: Integer; const ANickname, AName, AGender: String);
begin
ID := AID;
Nickname := ANickName
Name := AName;
Gender := AGender;
end;
constructor TEmployee.Create(const AID: Integer; const AName, AGender: String);
begin
Create(AID, '', AName, AGender);
end;
var
myEmployee: TEmployee;
begin
myEmployee := TEmployee.Create(1, 'Zemmy', 'Male');
InsertEmp(myEmployee);
. . .
myEmployee := TEmployee.Create(1, 'Jim', 'Zemmy', 'Male');
InsertEmp(myEmployee);
end.
No you can not. If you want to do something like that you can pass a single string in a Key - Value format
InsertEmp('Gender=Male,NickName=Jim,ID=1,Name=Zemmy');
and then parse it inside you function.
There is another option if you can use only one variable type. Use open arrays. Declare
procedure InsertEmp(values: array of string);
and pass any parameter like this
InsertEmp(['Gender = Male', 'NickName = Jim', 'Name = Zemmy' ...]);

Pass different record types as parameter in a procedure?

Is there a trick to pass records with different type as parameter in a procedure? For example, look at this pseudo-code:
type
TPerson = record
Species: string;
CountLegs: Integer;
end;
TSpider = record
Species: string;
CountLegs: Integer;
Color: TColor;
end;
var
APerson: TPerson;
ASpider: TSpider;
// Is there a trick to pass different record types as parameter in a procedure?:
procedure DoSomethingWithARecord(const ARecord: TAbstractRecord?);
begin
if ARecord is TPerson then
DoSomethingWithThisPerson(ARecord as TPerson)
else if ARecord is TSpider then
DoSomethingWithThisSpider(ARecord as TSpider);
end;
procedure DefineRecords;
begin
APerson.Species := 'Human';
APerson.CountLegs := 2;
ASpider.Species := 'Insect';
ASpider.CountLegs := 8;
ASpider.Color := clBtnFace;
DoSomethingWithARecord(APerson);
DoSomethingWithARecord(ASpider);
end;
Record instances don't contain type information in the same way that classes do. So you would need to pass an extra argument to indicate which type you were working with. For instance:
type
TRecordType = (rtPerson, rtSpider);
procedure DoSomething(RecordType: TRecordType; const ARecord);
begin
case RecordType of
rtPerson:
DoSomethingWithThisPerson(TPerson(ARecord));
rtSpider:
DoSomethingWithThisSpider(TSpider(ARecord));
end;
end;
You might contemplate putting the type code in the first field of each record:
type
TPerson = record
RecordType: TRecordType;
Species: string;
CountLegs: Integer;
end;
TSpider = record
RecordType: TRecordType;
Species: string;
CountLegs: Integer;
Color: TColor;
end;
function GetRecordType(ARecord): TRecordType;
begin
Result := TRecordType(ARecord);
end;
....
procedure DoSomething(const ARecord);
begin
case GetRecordType(ARecord) of
rtPerson:
DoSomethingWithThisPerson(TPerson(ARecord));
rtSpider:
DoSomethingWithThisSpider(TSpider(ARecord));
end;
end;
You could use generics:
type
TMyRecordDispatcher = record
class procedure DoSomething<T: record>(const Value: T); static;
end;
class procedure TMyRecordDispatcher.DoSomething<T>(const Value: T);
begin
if TypeInfo(T) = TypeInfo(TPerson) then
DoSomethingWithThisPerson(PPerson(#Value)^)
else if TypeInfo(T) = TypeInfo(TSpider) then
DoSomethingWithThisSpider(PSpider(#Value)^);
end;
And call the functions like this:
TMyRecordDispatcher.DoSomething(APerson);
TMyRecordDispatcher.DoSomething(ASpider);
This uses generic type inference and so allows you not to explicitly state the type. Although as an example of generics it makes me cringe. Please don't do this.
In my view all of this is messy and brittle. Much of the above reimplements run time method dispatch, polymorphism. Classes are more suited to this. I don't endorse any of the code above.
On the other hand, perhaps this is all needless. What's wrong with:
DoSomethingWithThisPerson(Person);
DoSomethingWithThisSpider(Spider);
Since you know the types at compile time, why opt for anything more complex?
You could use function overloading to make it possible to omit the type from the function name.
procedure DoSomething(const APerson: TPerson); overload;
begin
....
end;
procedure DoSomething(const ASpider: TSpider); overload;
begin
....
end;
....
DoSomething(Person);
DoSomething(Spider);

Pass record parameter without declaring it first as a variable

If I am trying to call a procedure which has a record type (not object) as a parameter, is it possible to somehow pass details of that parameter "inline" without having to declare a variable of that type first?
eg assume I have this simple record type:
type TMyRecord = record
AString: string;
AnInt: Integer;
end;
and this procedure declaration:
procedure MyProcedure(Rec: TMyRecord);
If I want to call MyProcedure do I have to declare a variable of type TMyRecord or can I do something like:
MyProcedure(TMyRecord("Test", 10));
That doesn't work (XE2) (get a compiler error about it expecting a ")").
So, can I do something like that? Or not possible.
Thanks
It is possible using the advanced record structure.
For more information about advanced records, see the Records (advanced) section in Delphi help.
This is a small prototype to see how it works in your case to preinitialize a record in a function/procedure call :
Type
TRecord = record
AString : String;
AnInt : Integer;
Constructor Create( Const s : String; i : Integer);
end;
constructor TRecord.Create(const s: String; i: Integer);
begin
AString := s;
AnInt := i;
end;
procedure DoSomething( theRec : TRecord);
begin
WriteLn(theRec.AString, ' ',theRec.AnInt);
end;
begin
DoSomeThing( TRecord.Create('S',1));
ReadLn;
end.
Looking at the Delphi RTL, see the definitions of the record types TPoint and TRect in unit system.types (XE2).
They define some overloaded Create constructors, which are used in lots of places to preinitialize the record structures in function/procedure calls.
The question you are asking relates to code readability and there is a solution that avoids having to create a variable. The VCL uses this solution with the records TPoint and TRect.
Consider the definition of TPoint:
type
TPoint = record
X,Y integer
end;
To pass a TPoint to a procedure you might do:
var
MyPoint : TPoint;
begin
MyPoint.X := 5;
MyPoint.Y := 7;
DoSomething( MyPoint );
end;
This is fine but takes 3 lines when one is also possible using the factory function Point:
begin
DoSomething( Point(5,7) );
end;
In Delphi, a function has been declared as follows:
function Point( X, Y : integer ) : TPoint;
begin
Result.X := X;
Result.Y := Y;
end;
You can then call this function 'inline' to create the record 'on the fly' to to quickly
You will see the same has been provided for TRect etc. I often put such a factory function together with the record declaration as follows, even if I don't plan to use them yet:
type
TMyRecord = record
A : integer;
B : string;
end;
function MyRecord( A : integer; const B : string ) : TMyRecord;
begin
Result.A := A;
Result.B := B;
end;
Use of this technique can improved the readability of code and also ensures that you don't accidently omit setting a record element.
Just having fun with John Easley's idea:
type TRec = record
X: string;
Y: Integer;
end;
procedure TestRec(const Rec: array of const);
var
R: TRec;
begin
R.X:= string(Rec[0].VUnicodeString);
R.Y:= Rec[1].VInteger;
ShowMessage(R.X + IntToStr(R.Y));
end;
procedure TForm1.Button7Click(Sender: TObject);
begin
TestRec(['Test', 22]);
end;
It is possible to pass record fields as array of const parameters and assign these parameters to local record variable.
It would be nice! But, no.
If passing things inline is really your objective, then perhaps Open Array Parameters would suit you.
Procedure MyProcedure(const Vars: Array of Variant);
begin
ShowMessage(VarToStr(Vars[0])+' '+VarToStr(Vars[1]));
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
MyProcedure(['Test', 12]);
end;
You could also pass an Array of Const, which is basically an array of TVarRec which is a variant record that also includes type information as VType. This is fun stuff..
An excellent article can be found on Rudy's Delphi Corner here:
Rudy's Delphi Corner, Open Array Parameters

Enumerate global methods of a unit using delphi

suppose i have a unit like this
unit sample;
interface
function Test1:Integer;
procedure Test2;
implementation
function Test1:Integer;
begin
result:=0;
end;
procedure Test2;
begin
end;
end.
Is possible enumerate all the procedures and functions of the unit sample in runtime?
No. RTTI is not generated for standalone methods. Hopefully this will be fixed in a later version, (they'd probably need a TRttiUnit type to do that,) but for now it's not available.
You could extract that information from some kind of debug info (TD32, Map file, Jdbg, etc.) using JCL and their great JclDebug.pas.
Try this:
uses
JclDebug;
type
TProc = record
name: string;
addr: Pointer;
end;
TProcArray = array of TProc;
TMapLoader = class
private
FModule: Cardinal;
FProcs: TProcArray;
FMapFileName: string;
FUnitName: string;
procedure HandleOnPublicsByValue(Sender: TObject; const Address: TJclMapAddress; const Name: string);
public
constructor Create(const AFileName: string; AModule: Cardinal; const AUnitName: string);
procedure Scan();
property Procs: TProcArray read FProcs;
end;
constructor TMapLoader.Create(const AFileName: string; AModule: Cardinal; const AUnitName: string);
begin
inherited Create;
FMapFileName := AFileName;
FModule := AModule;
FUnitName := AUnitName;
end;
procedure TMapLoader.HandleOnPublicsByValue(Sender: TObject; const Address: TJclMapAddress; const Name: string);
var
l: Integer;
begin
if Pos(FUnitName + '.', Name) = 1 then
begin
l := Length(FProcs);
SetLength(FProcs, l + 1);
FProcs[l].name := Name;
FProcs[l].addr := Pointer(Address.Offset + FModule + $1000);
end;
end;
procedure TMapLoader.Scan();
var
parser: TJclMapParser;
begin
parser := TJclMapParser.Create(FMapFileName, FModule);
try
parser.OnPublicsByValue := HandleOnPublicsByValue;
parser.Parse;
finally
parser.Free;
end;
end;
I don't think so.
That is a compile-time config, it's used so as the compiler knows which function name is being called or not. As far as I know, there is nothing at runtime which comes close to listing these functions.
Delphi's excellent runtime features come from RTTI, you might want to see what it offers in relation to this. But as I said, I don't think it's possible (know that I've delved in RTTI for quite some time...).
Edit: Oh and by the way, after compilation, functions lose their human-readable names (to addresses). There are some tables which pinpoint those names to addresses, most notably, RTTI and the Debug info.

How to change a generic type value?

In my application, I've created the TList type list, intended to store Integers or Doubles:
TKList<T> = class
private
FItems: TList<T>;
function GetItem(Index: Integer): T;
procedure SetItem(Index: Integer; const Value: T);
function GetMaxValue(): T;
function GetMinValue(): T;
public
constructor Create; overload;
constructor Create(const AKList: TKList<T>); overload;
destructor Destroy; override;
procedure Assign(const AKList: TKList<T>);
function Add(const AValue: T): Integer;
procedure Clear;
function Count: Integer;
procedure Invert;
function ToString: string; override;
function Info: string;
property Values[Index: Integer]: T read GetItem write SetItem; default;
end;
How can I implement Invert() procedure to invert values in generic List?
Thanks in advance.
Assuming you mean to Reverse the array as in you have values 1, 3, 5 after calling this function you want to have 5, 3, 1
Then, you could implement the procedure like this.
procedure TKList<T>.Invert;
var
I: Integer;
begin
for I := 0 to (Count - 1) div 2 do
FItems.Exchange(I, Count - I - 1);
end;
Altho I would suggest Reverse as it's name, since Invert is kind of confusing.
There's no way to specify constraints on generics such that you can require the types to be numbers, so there's no way you can use numeric operators on the values in your list. Craig Stuntz wrote a series of posts describing how to build a generic statistical library, and he came up against the same problem. He solved it by providing additional arguments to his functions so that the caller could provide implementations for the type-specific numeric operations — the template method pattern. Here's how he declared the Average operation:
type
TBinaryOp<T> = reference to function(ALeft, ARight: T): T
TStatistics<T> = class
public
class function Average(const AData: TEnumerable<T>;
AAdder, ADivider: TBinaryOp<T>;
AMapper: TFunc<integer, T>): T; overload;
Callers of that function need to provide their own code for adding, dividing, and "mapping" the generic type. (Mapping is covered in a later post and isn't important here.) You could write your Invert function like this:
type
TUnaryOp<T> = reference to function(Arg: T): T;
TKList<T> = class
procedure Invert(ANegater: TUnaryOp<T>);
procedure TKList<T>.Invert;
var
i: Integer;
begin
for i := 0 to Pred(Count) do
Values[i] := ANegater(Values[i]);
end;
To make it more convenient to call the methods without having to provide the extra arguments all the time, Stuntz showed how to declare a type-specific descendant that provides the right arguments. You could do it like this:
type
TIntKList = class(TKList<Integer>)
private
class function Negate(Arg: Integer): Integer;
public
procedure Invert;
end;
procedure TIntKList.Invert;
begin
inherited Invert(Negate);
end;
You can provide type-specific descendants for the common numeric types, and if consumers of your class need to use other number-like types, they can provide their own implementations for the basic numeric operations without having to re-implement your entire list class.
Thanks Rob, I got it.
What advantages/disadvantages has the following approach:
procedure TKList<T>.Invert;
var
i: Integer;
Val: TValue;
begin
if TTypeInfo(TypeInfo(T)^).Kind = tkInteger then
begin
for i := 0 to FItems.Count - 1 do
begin
Val := TValue.From<T>(FItems[i]);
TValue.From<Integer>(-Val.AsInteger).AsType<T>;
end;
end
else if TTypeInfo(TypeInfo(T)^).Kind = tkFloat then
begin
for i := 0 to FItems.Count - 1 do
begin
Val := TValue.From<T>(FItems[i]);
FItems[i] := TValue.From<Double>(-Val.AsExtended).AsType<T>;
end;
end;
end;

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