From Jason Turner

[class.union.anon]

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+ ### Anonymous unions <a id="class.union.anon">[[class.union.anon]]</a>
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+
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+ A union of the form
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+
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+ is called an *anonymous union*; it defines an unnamed type and an
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+ unnamed object of that type called an *anonymous union object*. Each
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+ *member-declaration* in the *member-specification* of an anonymous union
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+ shall either define a non-static data member or be a
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+ *static_assert-declaration*.
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+
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+ [*Note 1*: Nested types, anonymous unions, and functions cannot be
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+ declared within an anonymous union. — *end note*]
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+
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+ The names of the members of an anonymous union shall be distinct from
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+ the names of any other entity in the scope in which the anonymous union
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+ is declared. For the purpose of name lookup, after the anonymous union
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+ definition, the members of the anonymous union are considered to have
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+ been defined in the scope in which the anonymous union is declared.
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ void f() {
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+ union { int a; const char* p; };
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+ a = 1;
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+ p = "Jennifer";
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+ }
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+ ```
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+
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+ Here `a` and `p` are used like ordinary (non-member) variables, but
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+ since they are union members they have the same address.
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+
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+ — *end example*]
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+
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+ Anonymous unions declared in a named namespace or in the global
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+ namespace shall be declared `static`. Anonymous unions declared at block
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+ scope shall be declared with any storage class allowed for a block-scope
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+ variable, or with no storage class. A storage class is not allowed in a
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+ declaration of an anonymous union in a class scope. An anonymous union
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+ shall not have `private` or `protected` members (Clause 
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+ [[class.access]]). An anonymous union shall not have member functions.
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+
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+ A union for which objects, pointers, or references are declared is not
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+ an anonymous union.
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+
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+ [*Example 2*:
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+
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+ ``` cpp
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+ void f() {
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+ union { int aa; char* p; } obj, *ptr = &obj;
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+ aa = 1; // error
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+ ptr->aa = 1; // OK
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+ }
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+ ```
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+
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+ The assignment to plain `aa` is ill-formed since the member name is not
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+ visible outside the union, and even if it were visible, it is not
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+ associated with any particular object.
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+
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+ — *end example*]
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+
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+ [*Note 2*: Initialization of unions with no user-declared constructors
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+ is described in  [[dcl.init.aggr]]. — *end note*]
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+
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+ A *union-like class* is a union or a class that has an anonymous union
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+ as a direct member. A union-like class `X` has a set of *variant
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+ members*. If `X` is a union, a non-static data member of `X` that is not
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+ an anonymous union is a variant member of `X`. In addition, a non-static
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+ data member of an anonymous union that is a member of `X` is also a
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+ variant member of `X`. At most one variant member of a union may have a
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+ default member initializer.
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+
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+ [*Example 3*:
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+
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+ ``` cpp
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+ union U {
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+ int x = 0;
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+ union {
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+ int k;
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+ };
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+ union {
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+ int z;
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+ int y = 1; // error: initialization for second variant member of U
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+ };
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+ };
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+ ```
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+
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+ — *end example*]
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+