9 Declarations [dcl.dcl]

9.4 Initializers [dcl.init]

9.4.2 Aggregates [dcl.init.aggr]

An aggregate is an array or a class ([class]) with
[Note 1: 
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.
The explicitly initialized elements of the aggregate are determined as follows:
  • If the initializer list is a brace-enclosed designated-initializer-list, the aggregate shall be of class type, the identifier in each designator shall name a direct non-static data member of the class, and the explicitly initialized elements of the aggregate are the elements that are, or contain, those members.
  • If the initializer list is a brace-enclosed initializer-list, the explicitly initialized elements of the aggregate are the first n elements of the aggregate, where n is the number of elements in the initializer list.
  • Otherwise, the initializer list must be {}, and there are no explicitly initialized elements.
For each explicitly initialized element:
  • If the element is an anonymous union member and the initializer list is a brace-enclosed designated-initializer-list, the element is initialized by the braced-init-list { D }, where D is the designated-initializer-clause naming a member of the anonymous union member.
    There shall be only one such designated-initializer-clause.
    [Example 1: 
    struct C { union { int a; const char* p; }; int x; } c = { .a = 1, .x = 3 }; initializes c.a with 1 and c.x with 3.
    — end example]
  • Otherwise, the element is copy-initialized from the corresponding initializer-clause or is initialized with the brace-or-equal-initializer of the corresponding designated-initializer-clause.
    If that initializer is of the form assignment-expression or = assignment-expression and a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed.
    [Note 2: 
    If the initialization is by designated-initializer-clause, its form determines whether copy-initialization or direct-initialization is performed.
    — end note]
    [Note 3: 
    If an initializer is itself an initializer list, the element is list-initialized, which will result in a recursive application of the rules in this subclause if the element is an aggregate.
    — end note]
    [Example 2: 
    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]
For a non-union aggregate, each element that is not an explicitly initialized element 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 3: 
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
struct A { string a; int b = 42; int c = -1; };
A{.c=21} has the following steps:
  • Initialize a with {}
  • Initialize b with = 42
  • Initialize c with = 21
— end example]
The initializations of the elements of the aggregate are evaluated in the element order.
That is, all value computations and side effects associated with a given element are sequenced before those of any element that follows it in order.
An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in [dcl.init].
The destructor for each element of class type is potentially invoked ([class.dtor]) from the context where the aggregate initialization occurs.
[Note 4: 
This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).
— end note]
An array of unknown bound initialized with a brace-enclosed initializer-list containing n initializer-clauses is defined as having n elements ([dcl.array]).
[Example 4: 
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 array of unknown bound shall not be initialized with an empty braced-init-list {}.83
[Note 5: 
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 5: struct S { int y[] = { 0 }; // error: non-static data member of incomplete type }; — end example]
— end note]
[Note 6: 
Static data members, non-static data members of anonymous union members, and unnamed bit-fields are not considered elements of the aggregate.
[Example 6: 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 of the aggregate.
[Example 7: 
char cv[4] = { 'a', 's', 'd', 'f', 0 }; // error is ill-formed.
— end example]
If a member has a 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 8: struct A; extern A a; struct A { const A& a1 { A{a,a} }; // OK const A& a2 { A{} }; // error }; A a{a,a}; // OK struct B { int n = B{}.n; // error }; — 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 9: 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 multidimensional array, the initializer-clauses initialize the elements with the last (rightmost) index of the array varying the fastest ([dcl.array]).
[Example 10: 
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 11: 
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 ([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 7: 
As specified above, brace elision cannot apply to subaggregates with no elements; an initializer-clause for the entire subobject is required.
— end note]
[Example 12: 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 8: 
An aggregate array or an aggregate class can contain elements of a class type with a user-declared constructor ([class.ctor]).
Initialization of these aggregate objects is described in [class.expl.init].
— end note]
[Note 9: 
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 an initializer list, there shall not be more than one explicitly initialized element.
[Example 13: 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 u f = { .b = "asdf" }; u g = { .a = 1, .b = "asdf" }; // error — end example]
[Note 10: 
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 braced-init-lists, but nonetheless C++ does not have zero length arrays.