12 Classes [class]

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. 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: 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, 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, it is permitted to inspect the common initial sequence of any of the standard-layout struct members; see [class.mem].  — end note ]

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 struct. [ Note: A union object and its non-static data members are pointer-interconvertible ([expr.static.cast]). As a consequence, all non-static data members of a union object have the same address.  — end note ]

A union can have member functions (including constructors and destructors), but it shall not have 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: Absent default member initializers, if any non-static data member of a union has a non-trivial default constructor ([class.ctor]), copy constructor, move constructor ([class.copy]), copy assignment operator, move assignment operator, or destructor, the corresponding member function of the union must be user-provided or it will be implicitly deleted for the union.  — end note ]

[ Example: Consider the following union:

union U {
  int i;
  float f;
  std::string s;
};

Since std​::​string 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 ]

When the left operand of an assignment operator involves a member access expression 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 or a trivial assignment operator, for each element X of S(E1), 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: This ends the lifetime of the previously-active member of the union, if any.  — end note ] [ Example:

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 ]

[ Note: In general, one must use explicit destructor calls and placement new-expression to change the active member of a union.  — end note ] [ Example: 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 ]