Please could someone explain me what's the difference between public and published class members in Delphi?
I tried to look at Delphi help and I understand that these members have the same visibility, but I don't understand very well how they differ and when should I use published members instead of public ones.
Thanks a lot.
The compiler generates RTTI (Run-Time Type Information) metadata for published members, but not for public members (by default). The main effect of this is that the published properties of an object will appear in the Object Inspector at design time.
I do not know if you are writing components, but if you do, you probably know that properties and events are normally published, so that they can be set using the Object Inspector.
public
property MyProperty: integer read FMyProperty write FMyProperty
MyProperty will not be visisble in the Object Inspector.
published
property MyProperty: integer read FMyProperty write FMyProperty
MyProperty will be visisble in the Object Inspector.
Runtime Type Informations (RTTI) are only generated for published class members.
Public properties and published properties have the same visibility, as you already stated. Published properties are included in RTTI, public properties aren't.
At run-time, entries in the published and public sections are equally accessible.
The principal difference between them is that published items of a component appear in the Object Inspector at design-time.
This happens because, for fields in published section RTTI is automatically generated.
The Object Inspector picks this up and uses it to identify what to add to its list of properties and events.
As a side note, there is another special thing with published:
The default visibility of class members is published, so check for unsafe code like:
TTopSecret = class(TObject)
Name: string;
Password: string;
function DecryptPassword(const AValue): string;
public
constructor Create(const AName, AEncryptedPassword: string);
end;
Name and Password are visible 'world-wide'.
In addition to the other answers:
Published properties are automatically stored by the streaming system.
For instance if you have a TComponent's descendant instance and write it to a TStream with WriteComponent, all (well, not all, but that is another question) published properties are written to the stream without any further coding.
Of course, the streaming system only can do that because the RTTI is available for those published properties.
It seems there are lots of good answers already, pointing out the Object INspector, RTTI, etc. These are all pieces of the puzzle.
If you take away the published keyword, the entire Delphi RAD tool design would require some way to specify which properties are stored in a DFM, inspected in the component property inspector, and can be reloaded at runtime from a DFM when the form or data module is created.
This, in a word, is what Published is for. It is interesting to me that the designers of QT (originally TrollTech, now part of Nokia) had to emulate this level of RTTI for their C++ RAD library "QT".