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:
  • 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 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 an 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:
[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
]
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
:
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].
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, non-static data members of anonymous union 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 of the aggregate.
[Example
:
char cv[4] = { 'a', 's', 'd', 'f', 0 };     // error
is ill-formed.
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 ([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 an initializer list, there shall not be more than one explicitly initialized element.
[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
u f = { .b = "asdf" };
u g = { .a = 1, .b = "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.