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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.
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.
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.
In C, declaring a new type with the same name as an existing struct, union or enum is valid, but it is invalid in C++, because in C, struct, union, and enum types must be indicated as such whenever the type is referenced whereas in C++, all declarations of such types carry the typedef implicitly.
A bit field is declared as a structure (or union) member of type int, signed int, unsigned int, or _Bool, [note 4] following the member name by a colon (:) and the number of bits it should occupy. The total number of bits in a single bit field must not exceed the total number of bits in its declared type (this is allowed in C++ however, where ...
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.
This allows also for array members in class-type NTTP. Floating-point types, [133] pointers and references and unions and union-like classes (class types containing anonymous unions) are now allowed as NTTP. Function identity now also includes trailing requires-clauses (P1971) Constrained non-template functions have been removed
In other words, C++ does not have "submodules", meaning the . symbol which may be included in a module name bears no syntactic meaning and is used only to suggest the association of a module. As an example, std.compat is not a submodule of std , but is named so to indicate the association the module bears to the std module (as a "compatibility ...