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Does the compiler give you an error on return This.P.P.fTest1 ? If so, what? I'm at home and don't have a compiler handy.
โย ajb
Jan 15 '14 at 5:27
Thank you very much. With your edits this is working for me. One question. What happens if I have a SuhClass of the SubClass. A new SubSubClass. This one shouldnt add any new components to the SubClass but it should have his own Make function. My Problem is that SubSubClass.Make returns type SubSubClass. If I add "return SubClass.Make(...)" I am getting the error that it needs a type SubSubClass. If I try SubSubClass (SubClass.Make(..)) I am getting the error that downwards conversion is not allowed.
โย user1058712
Jan 15 '14 at 16:58
You probably want to use an extension aggregate ; my answer had an example of this, and I edited the answer to point it out. return (SubClass.Make(...) with new_component=>new_value, ...) will use SubClass.Make to determine the values of inherited components, and then you use the with ... part to specify the values of new components. If there aren't any new components, then (SubClass.Make(...) with null record) .
โย ajb
Jan 15 '14 at 17:11
What are you actually trying to achieve? What are your requirements for the type?
โย Jacob Sparre Andersen
Jan 16 '14 at 8:25
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I am trying to implement a private type in Ada like it is used in Java or C++. For example I want to have a "private int" in Ada.
As I have found here http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation#Encapsulation:_public.2C_private_and_protected_members I have to implement my variable in the package body. So I have tried to implement this like it is described on the example.
My problem is that I cant create a Create function for creating an object and set the values. My second problem is that I want to have childs of this type. This childs should have a Create function wich sets the values for the masterclass and the subclass.
Here is my code which is not working. I have added comments on the points which is not working.
I cant find any information how to implement this.
The lines which I dont how to implement is the return line in the Make function
Then the line for reading the values
And then the line in the Make function for the Subclass.
ajb's answer is very good, but I hope you'll allow me to "work up" to Ada's OOP facilities because they're actually fairly different than the Java equivalents. -- I'm typing off the top of my head here, so the examples might not be compilable, but should do to get the ideas across.
In the above we are creating a type, but there's a problem here: it exposes the implementation and users can go and change the values directly, which could be disastrous if certain processes/properties need to be ensured. (It also introduces dependance on implementation details.)
In order to address this, we can make the record private and make clients use functions and procedures to alter the internal state -- just like getters and setters (but we haven't touched on any OOP yet).
In the above the change is made so that the implementation is private, this allows us to change the implementation (see below) without forcing clients to recompile. (We could change Test_3 to Test_2 and it would still work.)
So, as you can see private means something different in Ada than it does in Java. Because Ada had this notion of private before OOP was added (in Ada 95) it was carried over to the OOP. (Basically the idea is that changes in a private-type's implementation cannot cause a program to change states becoming legal or illegal as a result.)
As you can see, Ada also had the notion of records and indirected access [private types] and even inheritance [derived types]... but Ada still had the notion of types, and especially distinct types that could be prohibited from interacting (a type for length and another for weight), and subtypes [which were types with added constraints] were different enough that it wouldn't be a good fit, and so they needed some way to distinguish between a type and a "type-or-something-derived-from-it" which is where the 'class attribute comes in: Type_Name'Class denotes a whole branch of an inheritance tree of a tagged-type .
Because of the rule above, a record that had some private components needs to be in the private portion of the spec so that changes to its internal structure don't impact the legality of the clients.
Now we're at a point roughly analogous to what you're used to in Java: class-definitions and inheritance, but with this added idea that private is something dealing with client-use (and packages) rather than, strictly-speaking, the object (or type) itself.
Unlike in Java, you don't need new to create an object. new specifically allocates an object and returns an access to the object. So this:
fails because the function is declared to return a MasterC , not an access MasterC or any type that is declared as access MasterC (or access all MasterC'class or anything of the sort). The function doesn't return an access type at all. So don't use new . Instead, the form will be
The value of P here is a PrivateComponent . This is a subtype of Ada.Finalization.Controlled with an additional component P , so the syntax will look like
The value of that P will be Private_Part_Pointer , which is an access type (to Private_Part ), so that's where you'd use new .
It's really not that complicated. To create a value of a record type, put the values in parentheses with component-name => followed by a value for each component. To create a value of an access type, use new .
When returning a SubC , things get interesting. The SubC has the same components as a MasterC , plus the two you've added. Thus, it has three components: P , fExtra1 , fExtra2 . However, you can't use the name P here, because it's in the private part of MasterClass and is thus invisible to SubClass . To create a SubC , you'll have to call a function to create the MasterC part of the SubC . The result will look something like
[ Note: This is called an extension aggregate .]
This uses MasterClass.Make to create a MasterC , which will have a P component; the SubC function result uses that P component with the fExtra1 and fExtra2 you've specified to create the three components you need for a SubC .
Disclaimer: I haven't tested this (I'm at home and don't have a compiler handy), so I may have gotten some of the syntax wrong, or there may be other subtle errors I missed.
EDIT: Now that I can use a compiler, I've found that the above two changes make your source compile (the second change has to be implemented in two places); except that do is a reserved word and cannot be used as a procedure name, and printNumObjects needs a body. This statement:
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private type - ััะพ... ะงัะพ ัะฐะบะพะต private type ?
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Specifies that one or more declared programming elements are accessible only from within their declaration context, including from within any contained types.
If a programming element represents proprietary functionality, or contains confidential data, you usually want to limit access to it as strictly as possible. You achieve the maximum limitation by allowing only the module, class, or structure that defines it to access it. To limit access to an element in this way, you can declare it with Private .
You can also use the Private Protected access modifier, which makes a member accessible from within that class and from derived classes located in its containing assembly.
Access Level. All code within a declaration context can access its Private elements. This includes code within a contained type, such as a nested class or an assignment expression in an enumeration. No code outside of the declaration context can access its Private elements.
Access Modifiers. The keywords that specify access level are called access modifiers . For a comparison of the access modifiers, see Access levels in Visual Basic .
The Private modifier can be used in these contexts:
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