Last updated：2026-10-12

C++ union

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A union is a space-saving special kind of class: a union can have multiple data members, but at any moment only one of them can have a value. When some member is assigned, the other members become undefined. Unions have the following characteristics:

• The default access specifier is public
• They may have constructors and destructors
• They may not have members of reference type
• They may not inherit from other classes and may not serve as base classes
• They may not have virtual functions
• The members of an anonymous union can be accessed directly in the scope where the union is defined
• An anonymous union may not contain protected or private members
• A global anonymous union must be static

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