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They have some restrictions as opposed to traditional unions: in C11, they must be a member of another structure or union, [2] and in C++, they can not have methods or access specifiers. Simply omitting the class-name portion of the syntax does not make a union an anonymous union.
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.
The above rules also apply to all the base classes and to all non-static data members in the class hierarchy; It has no base classes of the same type as the first defined non-static data member; A class/struct/union is considered POD if it is trivial, standard-layout, and all of its non-static data members and base classes are PODs.
For instance, in Eclipse's content assist feature, typing just the upper-case letters of a CamelCase word will suggest any matching class or method name (for example, typing "NPE" and activating content assist could suggest NullPointerException).
Also, C++ defines many new keywords, such as new and class, which may be used as identifiers (for example, variable names) in a C program. Some incompatibilities have been removed by the 1999 revision of the C standard ( C99 ), which now supports C++ features such as line comments ( // ) and declarations mixed with code.
C++ uses the three modifiers called public, protected, and private. [3] C# has the modifiers public, protected,internal, private, protected internal, private protected, and file. [4] Java has public, package, protected, and private; package is the default, used if no other access modifier keyword is specified. The meaning of these modifiers may ...
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.
In C++, classes can be forward-declared if you only need to use the pointer-to-that-class type (since all object pointers are the same size, and this is what the compiler cares about). This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer (or a reference) to another class.