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The memory area storing the value has no intrinsic type (other than just bytes or words of memory), but the value can be treated as one of several abstract data types, having the type of the value that was last written to the memory area. In type theory, a union has a sum type; this corresponds to disjoint union in mathematics.
C# extends the number of them to six, [2] while Java has four access modifiers, but three keywords for this purpose. In Java, having no keyword before defaults to the package-private modifier. When the class is declared as public, it is accessible to other classes defined in the same package as well as those defined in other packages.
One advanced dialect of C, called Cyclone, has extensive built-in support for tagged unions. [1] The enum types in the Rust, Haxe, and Swift languages also work as tagged unions. The variant library from the Boost C++ Libraries demonstrated it was possible to implement a safe tagged union as a library in C++, visitable using function objects.
Friend: C++ supports a mechanism where a function explicitly declared as a friend function of the class may access the members designated as private or protected. [15] Path-based: Java supports restricting access to a member within a Java package, which is the logical path of the file. However, it is a common practice when extending a Java ...
A class in C++ is a user-defined type or data structure declared with any of the keywords class, struct or union (the first two are collectively referred to as non-union classes) that has data and functions (also called member variables and member functions) as its members whose access is governed by the three access specifiers private, protected or public.
Code from other packages is accessed by prefixing the package name before the appropriate identifier, for example class String in package java.lang can be referred to as java.lang.String (this is known as the fully qualified class name). Like C++, Java offers a construct that makes it unnecessary to type the package name (import).
Java has annotations, which allow adding arbitrary custom metadata to classes and metaprogramming via an annotation processing tool. Both Java and C++ distinguish between native types (also termed fundamental or built-in types) and user-defined types (also termed compound types). In Java, native types have value semantics only, and compound ...
For instance, in (defclass x (a b) ()), a class definition, the (a b) is a list of base classes, so these names are looked up in the space of class names, and x isn't a reference to an existing binding, but the name of a new class being derived from a and b. These facts emerge purely from the semantics of defclass.