11 Classes [class]

11.5 Unions [class.union]

11.5.1 General [class.union.general]

1
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A union is a class defined with the class-key union.
2
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In a union, a non-static data member is active if its name refers to an object whose lifetime has begun and has not ended ([basic.life]).
At most one of the non-static data members of an object of union type can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.
[Note 1: 
One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence ([class.mem]), and if a non-static data member of an object of this standard-layout union type is active and is one of the standard-layout structs, the common initial sequence of any of the standard-layout struct members can be inspected; see [class.mem].
— end note]
3
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The size of a union is sufficient to contain the largest of its non-static data members.
Each non-static data member is allocated as if it were the sole member of a non-union class.
[Note 2: 
A union object and its non-static data members are pointer-interconvertible ([basic.compound], [expr.static.cast]).
As a consequence, all non-static data members of a union object have the same address.
— end note]
4
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A union can have member functions (including constructors and destructors), but it shall not have virtual ([class.virtual]) functions.
A union shall not have base classes.
A union shall not be used as a base class.
If a union contains a non-static data member of reference type, the program is ill-formed.
[Note 3: 
Absent default member initializers ([class.mem]), if any non-static data member of a union has a non-trivial default constructor ([class.default.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), or destructor ([class.dtor]), the corresponding member function of the union must be user-provided or it will be implicitly deleted ([dcl.fct.def.delete]) for the union.
[Example 1: 
Consider the following union: union U { int i; float f; std::string s; };
Since std​::​string ([string.classes]) declares non-trivial versions of all of the special member functions, U will have an implicitly deleted default constructor, copy/move constructor, copy/move assignment operator, and destructor.
To use U, some or all of these member functions must be user-provided.
— end example]
— end note]
5
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When the left operand of an assignment operator involves a member access expression ([expr.ref]) that nominates a union member, it may begin the lifetime of that union member, as described below.
For an expression E, define the set S(E) of subexpressions of E as follows:
  • (5.1)
    If E is of the form A.B, S(E) contains the elements of S(A), and also contains A.B if B names a union member of a non-class, non-array type, or of a class type with a trivial default constructor that is not deleted, or an array of such types.
  • (5.2)
    If E is of the form A[B] and is interpreted as a built-in array subscripting operator, S(E) is S(A) if A is of array type, S(B) if B is of array type, and empty otherwise.
  • (5.3)
    Otherwise, S(E) is empty.
In an assignment expression of the form E1 = E2 that uses either the built-in assignment operator ([expr.ass]) or a trivial assignment operator ([class.copy.assign]), for each element X of S(E1) and each anonymous union member X ([class.union.anon]) that is a member of a union and has such an element as an immediate subobject (recursively), if modification of X would have undefined behavior under [basic.life], an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment.
[Note 4: 
This ends the lifetime of the previously-active member of the union, if any ([basic.life]).
— end note]
[Example 2: union A { int x; int y[4]; }; struct B { A a; }; union C { B b; int k; }; int f() { C c; // does not start lifetime of any union member c.b.a.y[3] = 4; // OK, S(c.b.a.y[3]) contains c.b and c.b.a.y; // creates objects to hold union members c.b and c.b.a.y return c.b.a.y[3]; // OK, c.b.a.y refers to newly created object (see [basic.life]) } struct X { const int a; int b; }; union Y { X x; int k; }; void g() { Y y = { { 1, 2 } }; // OK, y.x is active union member ([class.mem]) int n = y.x.a; y.k = 4; // OK, ends lifetime of y.x, y.k is active member of union y.x.b = n; // undefined behavior: y.x.b modified outside its lifetime, // S(y.x.b) is empty because X's default constructor is deleted, // so union member y.x's lifetime does not implicitly start } — end example]
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[Note 5: 
In cases where the above rule does not apply, the active member of a union can only be changed by the use of a placement new-expression.
— end note]
[Example 3: 
Consider an object u of a union type U having non-static data members m of type M and n of type N.
If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of u can be safely switched from m to n using the destructor and placement new-expression as follows: u.m.~M(); new (&u.n) N;
— end example]

11.5.2 Anonymous unions [class.union.anon]

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