An aggregate is an array or a class with
no user-provided, explicit, or inherited constructors ([class.ctor]),
no private or protected non-static data members (Clause [class.access]),
no virtual functions, and
no virtual, private, or protected base classes ([class.mi]).
[ Note: Aggregate initialization does not allow accessing protected and private base class' members or constructors. — end note ]
The elements of an aggregate are:
for an array, the array elements in increasing subscript order, or
for a class, the direct base classes in declaration order, followed by the direct non-static data members ([class.mem]) that are not members of an anonymous union, in declaration order.
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 element has a default member initializer ([class.mem]), the element is initialized from that initializer.
Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
Otherwise, the program is ill-formed.
If the aggregate is a union and the initializer list is empty, then
if any variant member has a default member initializer, that member is initialized from its default member initializer;
otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list.
[ Example:
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
0,
s, // Required initialization
0
}; // 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 ]
[ Example:
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.