How does Ada programming language implement packages? Does it use a struct? a record? and where can I find an example of this implementation if possible?
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- 2It the source code presumably? Note though that "Ada" is a programming language (a concept and a set of standard) which is opposed to an implementation of a programming language, which is a tangible piece of software. GNAT, which source code I referred to is one of such implementations; others to exist though.kostix– kostix2019-03-31 16:49:12 +00:00Commented Mar 31, 2019 at 16:49
- What do you mean by "implement packages"? Do you mean how does a compiler represent internally that something is in a pkg and whether it is visible, in the private part, or in the body? Or do you mean how does a pkg appear in the generated object code?Jeffrey R. Carter– Jeffrey R. Carter2019-04-06 10:42:22 +00:00Commented Apr 6, 2019 at 10:42
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2 Answers
The language doesn't really specify how the underlying implementation should be done. That is left up to the compiler vendor. I doubt they use a struct or record. Any answer you find will be vendor specific and not so much language specific. It's worth noting that the most common compiler, GNAT, is open source, so the source code is available to view and try and make out how they do it. Again, that is just one potential implementation. Other compilers may do it differently.
If you just mean what does an example of a package potentially look like:
empty my_package.ads:
package My_Package is -- Empty package end My_Package; my_package.ads with a "class" like type:
package My_Package is type My_Type is tagged private; function Get_Value(Self : My_Type) return Integer; private type My_Type is tagged record Value : Integer := 2; end record; end My_Package; my_package.adb with definitions of operations:
package body My_Package is function Get_Value(Self : My_Type) return Integer is begin return Self.Value; end Get_Value; end My_Package; As you can see, packages can be empty. They can have types, operations, or both. Packages are where encapsulation is handled (as opposed to the type like C++ and Java do). So you can have private types and operations in a package, but not do any traditional object oriented programming.
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An Ada package is mainly just a namespace, and so has no effect on the executable code emitted by a compiler.
Let me try to illustrate this.
The Ada:
... A := B; ... might emit code such as:
... CPY B, A ... The Ada:
package P is B: Integer; ... end; ... A := P.B; ... might emit exactly the same code. The fact that B is now inside a package does not change the emitted code.
A package can have initialisation code, which will be implemented very like a procedure. An instantiation of a generic package might be implemented like a record (whose components hold the generic parameters), or it might be expanded in-place, in which case the situation is the same as with a non-generic package.
Interesting question! Why do you ask?
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flyx
That's only true for trivial examples. Packages do have effects on the emitted code. For example, package dependencies generate the elaboration order which in turn generates initialization code. That makes them very different from a simple namespace. Then there are of course generic packages which can be instantiated, like you mentioned.