3
Is there any reason why I can’t leave a Delphi constructor as private?
Contrary to what you mentioned, you can declare a private constructor in Delphi. See, the code below simply works:
program Project2;
{$APPTYPE CONSOLE}
uses
SysUtils,
Unit1 in 'Unit1.pas';
//var Foo: Tfoo2;
begin
try
// Foo := Tfoo2.CreatePrivado(1);
TFoo2.FA := 'hello world';
writeln(Foo.FA);
readln;
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.
unit Unit1;
interface
type Tfoo2 = class(TObject)
private
constructor CreatePrivado(i:integer);
public
class var FA: string;
// constructor Create(bar:Boolean);
end;
implementation
uses
SysUtils;
//constructor Tfoo2.Create(bar:Boolean);
//begin
// inherited Create;
// FA := 'bar';
//end;
constructor Tfoo2.CreatePrivado(i: integer);
begin
inherited Create;
FA := IntToStr(i);
end;
end.
In Delphi, constructors can be inherited. This does not happen in Java, C# or C++ for example. In addition, a class can have multiple constructors and they can also have different names. Usually called Create. But this is just a convention and not a rule.
Further, all classes in Delphi ultimately inherit from the class TObject. This class contains a constructor without parameters called Create.
Thus, it is easy to understand why all classes in Delphi have the constructor without parameters called Create.
If you need to hide the builder Create already mentioned, try an answer to one of the following questions on Soen:
https://stackoverflow.com/a/14003208/460775
https://stackoverflow.com/q/5392107/460775
https://stackoverflow.com/q/14003153/460775
Part of the answer is based on: http://www.yanniel.info/2011/08/hide-tobject-create-constructor-delphi.html
How will you build the object if the constructor is private?
– EMBarbosa
@Embarbosa, in C++ the visibility of the constructor can be private, and internally I see no reason for this feature not to be allowed in Delphi/pascal, this hinders a lot when we need to create a Singleton object, because the constructor is always exposed, of course we use tricks to circumvent it but it is by no means elegant.
– Wellington Silva Ribeiro
I don’t know of a situation where this could get in the way of... But you may want to check this question and its answers here http://stackoverflow.com/a/5392736/460775
– EMBarbosa
Ahh... now that I saw that I had already asked the question on the Soen and received basically the same answer. http://stackoverflow.com/q/36535704/460775
– EMBarbosa
@Embarbosa, If you read the comments of my question in stackoverflow-en will come to the conclusion that there is no possibility of hiding builders in Delphi, you can do tricks using overloading, but the create method is always available, exactly what I questioned is why I cannot change the visibility of the constructor, I can do this in java, c++ but not in pascal. What I meant to say is that "disturbs", is that you have to use tricks to be able to make a Singleton, cool the example you posted but using interfaces and critical session?
– Wellington Silva Ribeiro
@Embarbosa looks at C++: class Logger{ public: Static Logger* Instance(); bool openLogFile(Std::string logfile); void writeToLogFile(); bool closeLogFile(); private: Logger(){}; // Private so that it can not be called Logger(Logger const&){}; // copy constructor is private Logger& Operator=(Logger const&){}; // assignment Operator is private Static Logger* m_pInstance; };
– Wellington Silva Ribeiro