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Showing content from https://timsong-cpp.github.io/cppwp/n4140/dcl.init below:

[dcl.init]

8.5 Initializers [dcl.init]

Except for objects declared with the constexpr specifier, for which see [dcl.constexpr], an initializer in the definition of a variable can consist of arbitrary expressions involving literals and previously declared variables and functions, regardless of the variable's storage duration. [ Example:

int f(int);
int a = 2;
int b = f(a);
int c(b);

 — end example ]

The order of initialization of variables with static storage duration is described in [basic.start] and [stmt.dcl].  — end note ]

A declaration of a block-scope variable with external or internal linkage that has an initializer is ill-formed.

To zero-initialize an object or reference of type T means:

To default-initialize an object of type T means:

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

To value-initialize an object of type T means:

An object that is value-initialized is deemed to be constructed and thus subject to provisions of this International Standard applying to “constructed” objects, objects “for which the constructor has completed,” etc., even if no constructor is invoked for the object's initialization.

A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed.

[ Note: Every object of static storage duration is zero-initialized at program startup before any other initialization takes place. In some cases, additional initialization is done later.  — end note ]

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

[ Note: Since () is not permitted by the syntax for initializer,

X a();

is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X. The form () is permitted in certain other initialization contexts ([expr.new], [expr.type.conv], [class.base.init]).  — end note ]

If no initializer is specified for an object, the object is default-initialized. When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced ([expr.ass]). [ Note: Objects with static or thread storage duration are zero-initialized, see [basic.start.init].  — end note ] If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:

[ Example:

  int f(bool b) {
    unsigned char c;
    unsigned char d = c;     int e = d;               return b ? d : 0;      }

 — end example ]

An initializer for a static member is in the scope of the member's class. [ Example:

int a;

struct X {
  static int a;
  static int b;
};

int X::a = 1;
int X::b = a;

 — end example ]

The form of initialization (using parentheses or =) is generally insignificant, but does matter when the initializer or the entity being initialized has a class type; see below. If the entity being initialized does not have class type, the expression-list in a parenthesized initializer shall be a single expression.

The semantics of initializers are as follows. The destination type is the type of the object or reference being initialized and the source type is the type of the initializer expression. If the initializer is not a single (possibly parenthesized) expression, the source type is not defined.

8.5.1 Aggregates [dcl.init.aggr]

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 members of the aggregate, in increasing subscript or member order. Each member is copy-initialized from the corresponding initializer-clause. If the initializer-clause is an expression and a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed. [ Note: If an initializer-clause is itself an initializer list, the member is list-initialized, which will result in a recursive application of the rules in this section if the member 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.  — 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 size 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.106

Static data members and anonymous bit-fields are not considered members of the class for purposes of aggregate initialization. [ 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 anonymous bit-field before it.  — end example ]

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

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

is ill-formed.  — end example ]

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from its brace-or-equal-initializer or, if there is no brace-or-equal-initializer, from an empty initializer list ([dcl.init.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 an aggregate class C contains a subaggregate member m that has no members for purposes of aggregate initialization, the initializer-clause for m shall not be omitted from an initializer-list for an object of type C unless the initializer-clauses for all members of C following m are also omitted. [ Example:

struct S { } s;
struct A {
  S s1;
  int i1;
  S s2;
  int i2;
  S s3;
  int i3;
} a = {
  { },        0,
  s,          0
};

 — end example ]

If an incomplete or empty initializer-list leaves a member of reference type uninitialized, the program is ill-formed.

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 members of a subaggregate; it is erroneous for there to be more initializer-clauses than members. 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 members of the subaggregate; any remaining initializer-clauses are left to initialize the next member of the aggregate of which the current subaggregate is a member. [ 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 aggregate member with an assignment-expression. If the assignment-expression can initialize a member, the member is initialized. Otherwise, if the member is itself a subaggregate, brace elision is assumed and the assignment-expression is considered for the initialization of the first member of the subaggregate. [ Note: As specified above, brace elision cannot apply to subaggregates with no members for purposes of aggregate initialization; 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 members 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.init] 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;                        u d = { 0, "asdf" };            u e = { "asdf" };

 — 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 ]

8.5.2 Character arrays [dcl.init.string]

An array of narrow character type ([basic.fundamental]), char16_t array, char32_t array, or wchar_t array can be initialized by a narrow string literal, char16_t string literal, char32_t string literal, or wide string literal, respectively, or by an appropriately-typed string literal enclosed in braces ([lex.string]). Successive characters of the value of the string literal initialize the elements of the array. [ Example:

char msg[] = "Syntax error on line %s\n";

shows a character array whose members are initialized with a string-literal. Note that because '\n' is a single character and because a trailing '\0' is appended, sizeof(msg) is 25.  — end example ]

There shall not be more initializers than there are array elements. [ Example:

char cv[4] = "asdf";

is ill-formed since there is no space for the implied trailing '\0'.  — end example ]

If there are fewer initializers than there are array elements, each element not explicitly initialized shall be zero-initialized ([dcl.init]).

8.5.3 References [dcl.init.ref]

A variable declared to be a T& or T&&, that is, “reference to type T” ([dcl.ref]), shall be initialized by an object, or function, of type T or by an object that can be converted into a T. [ Example:

int g(int);
void f() {
  int i;
  int& r = i;                     r = 1;                          int* p = &r;                    int& rr = r;                    int (&rg)(int) = g;             rg(i);                          int a[3];
  int (&ra)[3] = a;               ra[1] = i;                    }

 — end example ]

A reference cannot be changed to refer to another object after initialization. Note that initialization of a reference is treated very differently from assignment to it. Argument passing ([expr.call]) and function value return ([stmt.return]) are initializations.

The initializer can be omitted for a reference only in a parameter declaration ([dcl.fct]), in the declaration of a function return type, in the declaration of a class member within its class definition ([class.mem]), and where the extern specifier is explicitly used. [ Example:

int& r1;                        extern int& r2;

 — end example ]

Given types “ cv1 T1” and “ cv2 T2,” “ cv1 T1” is reference-related to “ cv2 T2” if T1 is the same type as T2, or T1 is a base class of T2. “ cv1 T1” is reference-compatible with “ cv2 T2” if T1 is reference-related to T2 and cv1 is the same cv-qualification as, or greater cv-qualification than, cv2. In all cases where the reference-related or reference-compatible relationship of two types is used to establish the validity of a reference binding, and T1 is a base class of T2, a program that necessitates such a binding is ill-formed if T1 is an inaccessible (Clause [class.access]) or ambiguous ([class.member.lookup]) base class of T2.

A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:

In all cases except the last (i.e., creating and initializing a temporary from the initializer expression), the reference is said to bind directly to the initializer expression.

[ Note: [class.temporary] describes the lifetime of temporaries bound to references.  — end note ]

8.5.4 List-initialization [dcl.init.list]

List-initialization is initialization of an object or reference from a braced-init-list. Such an initializer is called an initializer list, and the comma-separated initializer-clauses of the list are called the elements of the initializer list. An initializer list may be empty. List-initialization can occur in direct-initialization or copy-initialization contexts; list-initialization in a direct-initialization context is called direct-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization. [ Note: List-initialization can be used

[ Example:

int a = {1};
std::complex<double> z{1,2};
new std::vector<std::string>{"once", "upon", "a", "time"};  f( {"Nicholas","Annemarie"} );  return { "Norah" };             int* e {};                      x = double{1};                  std::map<std::string,int> anim = { {"bear",4}, {"cassowary",2}, {"tiger",7} };

 — end example ]  — end note ]

A constructor is an initializer-list constructor if its first parameter is of type std::initializer_list<E> or reference to possibly cv-qualified std::initializer_list<E> for some type E, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]). [ Note: Initializer-list constructors are favored over other constructors in list-initialization ([over.match.list]). Passing an initializer list as the argument to the constructor template template<class T> C(T) of a class C does not create an initializer-list constructor, because an initializer list argument causes the corresponding parameter to be a non-deduced context ([temp.deduct.call]).  — end note ] The template std::initializer_list is not predefined; if the header <initializer_list> is not included prior to a use of std::initializer_list — even an implicit use in which the type is not named ([dcl.spec.auto]) — the program is ill-formed.

List-initialization of an object or reference of type T is defined as follows:

Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions ([temp.variadic]), are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list. [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call.  — end note ]

An object of type std::initializer_list<E> is constructed from an initializer list as if the implementation allocated a temporary array of N elements of type const E, where N is the number of elements in the initializer list. Each element of that array is copy-initialized with the corresponding element of the initializer list, and the std::initializer_list<E> object is constructed to refer to that array. [ Note: A constructor or conversion function selected for the copy shall be accessible (Clause [class.access]) in the context of the initializer list.  — end note ] If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.[ Example:

struct X {
  X(std::initializer_list<double> v);
};
X x{ 1,2,3 };

The initialization will be implemented in a way roughly equivalent to this:

const double __a[3] = {double{1}, double{2}, double{3}};
X x(std::initializer_list<double>(__a, __a+3));

assuming that the implementation can construct an initializer_list object with a pair of pointers.  — end example ]

The array has the same lifetime as any other temporary object ([class.temporary]), except that initializing an initializer_list object from the array extends the lifetime of the array exactly like binding a reference to a temporary. [ Example:

typedef std::complex<double> cmplx;
std::vector<cmplx> v1 = { 1, 2, 3 };

void f() {
  std::vector<cmplx> v2{ 1, 2, 3 };
  std::initializer_list<int> i3 = { 1, 2, 3 };
}

struct A {
  std::initializer_list<int> i4;
  A() : i4{ 1, 2, 3 } {}  };

For v1 and v2, the initializer_list object is a parameter in a function call, so the array created for { 1, 2, 3 } has full-expression lifetime. For i3, the initializer_list object is a variable, so the array persists for the lifetime of the variable. For i4, the initializer_list object is initialized in a constructor's ctor-initializer, so the array persists only until the constructor exits, and so any use of the elements of i4 after the constructor exits produces undefined behavior.  — end example ] [ Note: The implementation is free to allocate the array in read-only memory if an explicit array with the same initializer could be so allocated.  — end note ]

A narrowing conversion is an implicit conversion

[ Note: As indicated above, such conversions are not allowed at the top level in list-initializations. — end note ] [ Example:

int x = 999;              const int y = 999;
const int z = 99;
char c1 = x;              char c2{x};               char c3{y};               char c4{z};               unsigned char uc1 = {5};  unsigned char uc2 = {-1}; unsigned int ui1 = {-1};  signed int si1 =
  { (unsigned int)-1 };   int ii = {2.0};           float f1 { x };           float f2 { 7 };           int f(int);
int a[] =
  { 2, f(2), f(2.0) };

 — end example ]

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