11 Declarators [dcl.decl]

11.6 Initializers [dcl.init]

11.6.1 Aggregates [dcl.init.aggr]

An aggregate is an array or a class with

[Note: Aggregate initialization does not allow accessing protected and private base class' members or constructors. end note]

The elements of an aggregate are:

When an aggregate is initialized by an initializer list as specified in [dcl.init.list], the elements of the initializer list are taken as initializers for the elements of the aggregate, in order. Each element is copy-initialized from the corresponding initializer-clause. If the initializer-clause is an expression and a narrowing conversion is required to convert the expression, the program is ill-formed. [Note: If an initializer-clause is itself an initializer list, the element is list-initialized, which will result in a recursive application of the rules in this section if the element is an aggregate. end note] [Example:

struct A {
  int x;
  struct B {
    int i;
    int j;
  } b;
} a = { 1, { 2, 3 } };

initializes a.x with 1, a.b.i with 2, a.b.j with 3.

struct base1 { int b1, b2 = 42; };
struct base2 {
  base2() {
    b3 = 42;
  int b3;
struct derived : base1, base2 {
  int d;

derived d1{{1, 2}, {}, 4};
derived d2{{}, {}, 4};

initializes d1.b1 with 1, d1.b2 with 2, d1.b3 with 42, d1.d with 4, and d2.b1 with 0, d2.b2 with 42, d2.b3 with 42, d2.d with 4. end example]

An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in [dcl.init].

An array of unknown bound initialized with a brace-enclosed initializer-list containing n initializer-clauses, where n shall be greater than zero, is defined as having n elements ([dcl.array]). [Example:

int x[] = { 1, 3, 5 };

declares and initializes x as a one-dimensional array that has three elements since no size was specified and there are three initializers. end example] An empty initializer list {} shall not be used as the initializer-clause for an array of unknown bound.105 [Note: A default member initializer does not determine the bound for a member array of unknown bound. Since the default member initializer is ignored if a suitable mem-initializer is present ([class.base.init]), the default member initializer is not considered to initialize the array of unknown bound. [Example:

struct S {
  int y[] = { 0 };          // error: non-static data member of incomplete type

end example] end note]

[Note: Static data members and unnamed bit-fields are not considered elements of the aggregate. [Example:

struct A {
  int i;
  static int s;
  int j;
  int :17;
  int k;
} a = { 1, 2, 3 };

Here, the second initializer 2 initializes a.j and not the static data member A​::​s, and the third initializer 3 initializes a.k and not the unnamed bit-field before it. end example] end note]

An initializer-list is ill-formed if the number of initializer-clauses exceeds the number of elements to initialize. [Example:

char cv[4] = { 'a', 's', 'd', 'f', 0 };     // error

is ill-formed. end example]

If there are fewer initializer-clauses in the list than there are elements in a non-union aggregate, then each element not explicitly initialized is initialized as follows:

If the aggregate is a union and the initializer list is empty, then


struct S { int a; const char* b; int c; int d = b[a]; };
S ss = { 1, "asdf" };

initializes ss.a with 1, ss.b with "asdf", ss.c with the value of an expression of the form int{} (that is, 0), and ss.d with the value of ss.b[ss.a] (that is, 's'), and in

struct X { int i, j, k = 42; };
X a[] = { 1, 2, 3, 4, 5, 6 };
X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };

a and b have the same value end example]

If a reference member is initialized from its default member initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that default member initializer, the program is ill-formed. [Example:

  struct A;
  extern A a;
  struct A {
    const A& a1 { A{a,a} };     // OK
    const A& a2 { A{} };        // error
  A a{a,a};                     // OK

end example]

If an aggregate class C contains a subaggregate element e with no elements, the initializer-clause for e shall not be omitted from an initializer-list for an object of type C unless the initializer-clauses for all elements of C following e are also omitted. [Example:

struct S { } s;
struct A {
  S s1;
  int i1;
  S s2;
  int i2;
  S s3;
  int i3;
} a = {
  { },              // Required initialization
  s,                // Required initialization
};                  // Initialization not required for A​::​s3 because A​::​i3 is also not initialized

end example]

When initializing a multi-dimensional array, the initializer-clauses initialize the elements with the last (rightmost) index of the array varying the fastest ([dcl.array]). [Example:

int x[2][2] = { 3, 1, 4, 2 };

initializes x[0][0] to 3, x[0][1] to 1, x[1][0] to 4, and x[1][1] to 2. On the other hand,

float y[4][3] = {
  { 1 }, { 2 }, { 3 }, { 4 }

initializes the first column of y (regarded as a two-dimensional array) and leaves the rest zero. end example]

Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the elements of a subaggregate; it is erroneous for there to be more initializer-clauses than elements. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clauses from the list are taken to initialize the elements of the subaggregate; any remaining initializer-clauses are left to initialize the next element of the aggregate of which the current subaggregate is an element. [Example:

float y[4][3] = {
  { 1, 3, 5 },
  { 2, 4, 6 },
  { 3, 5, 7 },

is a completely-braced initialization: 1, 3, and 5 initialize the first row of the array y[0], namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early and therefore y[3]s elements are initialized as if explicitly initialized with an expression of the form float(), that is, are initialized with 0.0. In the following example, braces in the initializer-list are elided; however the initializer-list has the same effect as the completely-braced initializer-list of the above example,

float y[4][3] = {
  1, 3, 5, 2, 4, 6, 3, 5, 7

The initializer for y begins with a left brace, but the one for y[0] does not, therefore three elements from the list are used. Likewise the next three are taken successively for y[1] and y[2]. end example]

All implicit type conversions (Clause [conv]) are considered when initializing the element with an assignment-expression. If the assignment-expression can initialize an element, the element is initialized. Otherwise, if the element is itself a subaggregate, brace elision is assumed and the assignment-expression is considered for the initialization of the first element of the subaggregate. [Note: As specified above, brace elision cannot apply to subaggregates with no elements; an initializer-clause for the entire subobject is required.end note]


struct A {
  int i;
  operator int();
struct B {
  A a1, a2;
  int z;
A a;
B b = { 4, a, a };

Braces are elided around the initializer-clause for b.a1.i. b.a1.i is initialized with 4, b.a2 is initialized with a, b.z is initialized with whatever a.operator int() returns. end example]

[Note: An aggregate array or an aggregate class may contain elements of a class type with a user-provided constructor ([class.ctor]). Initialization of these aggregate objects is described in [class.expl.init]. end note]

[Note: Whether the initialization of aggregates with static storage duration is static or dynamic is specified in [basic.start.static], [basic.start.dynamic], and [stmt.dcl]. end note]

When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer-clause for the first non-static data member of the union. [Example:

union u { int a; const char* b; };
u a = { 1 };
u b = a;
u c = 1;                        // error
u d = { 0, "asdf" };            // error
u e = { "asdf" };               // error

end example]

[Note: As described above, the braces around the initializer-clause for a union member can be omitted if the union is a member of another aggregate. end note]

The syntax provides for empty initializer-lists, but nonetheless C++ does not have zero length arrays.