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cppdraft/dcl/array.md
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cppdraft/dcl/array.md
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[dcl.array]
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# 9 Declarations [[dcl]](./#dcl)
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## 9.3 Declarators [[dcl.decl]](dcl.decl#dcl.array)
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### 9.3.4 Meaning of declarators [[dcl.meaning]](dcl.meaning#dcl.array)
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#### 9.3.4.5 Arrays [dcl.array]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3465)
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In a declaration T D where D has the form
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D1 [ [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]")opt ] [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt
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and the type of the contained [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") in the declaration T D1 is â*derived-declarator-type-list* Tâ,
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the type of the [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") in D is
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â*derived-declarator-type-list* array of N Tâ[.](#1.sentence-1)
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The [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") shall be a converted constant expression of type std::size_t ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#1.sentence-2)
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Its value N specifies the [*array bound*](#def:array,bound "9.3.4.5 Arrays [dcl.array]"),
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i.e., the number of elements in the array;N shall be greater than zero[.](#1.sentence-3)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3482)
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In a declaration T D where D has the form
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D1 [ ] [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt
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and the type of the contained [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") in the declaration T D1 is â*derived-declarator-type-list* Tâ,
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the type of the [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") in D is
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â*derived-declarator-type-list* array of unknown bound of Tâ, except as specified below[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3493)
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A type of the form âarray of N Uâ or
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âarray of unknown bound of Uâ is an [*array type*](#def:array_type "9.3.4.5 Arrays [dcl.array]")[.](#3.sentence-1)
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The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") appertains to the array type[.](#3.sentence-2)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3501)
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U is called the array [*element type*](#def:element_type "9.3.4.5 Arrays [dcl.array]");
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this type shall not be
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a reference type,
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a function type,
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an array of unknown bound, orcv void[.](#4.sentence-1)
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[*Note [1](#note-1)*:
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An array can be constructed
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from one of the fundamental types (except void),
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from a pointer,
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from a pointer to member,
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from a class,
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from an enumeration type,
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or from an array of known bound[.](#4.sentence-2)
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â *end note*]
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[*Example [1](#example-1)*:
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float fa[17], *afp[17]; declares an array of float numbers and
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an array of pointers to float numbers[.](#4.sentence-3)
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â *end example*]
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3525)
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Any type of the form
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â[*cv-qualifier-seq*](dcl.decl.general#nt:cv-qualifier-seq "9.3.1 General [dcl.decl.general]") array of N Uâ
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is adjusted to
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âarray of N [*cv-qualifier-seq*](dcl.decl.general#nt:cv-qualifier-seq "9.3.1 General [dcl.decl.general]") Uâ,
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and similarly for âarray of unknown bound of Uâ[.](#5.sentence-1)
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[*Example [2](#example-2)*: typedef int A[5], AA[2][3];typedef const A CA; // type is âarray of 5 const int''typedef const AA CAA; // type is âarray of 2 array of 3 const int'' â *end example*]
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[*Note [2](#note-2)*:
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An âarray of N [*cv-qualifier-seq*](dcl.decl.general#nt:cv-qualifier-seq "9.3.1 General [dcl.decl.general]") Uâ
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has cv-qualified type; see [[basic.type.qualifier]](basic.type.qualifier "6.9.5 CV-qualifiers")[.](#5.sentence-2)
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â *end note*]
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3543)
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An object of type âarray of N Uâ consists of
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a contiguously allocated non-empty set
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of N subobjects of type U,
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known as the [*elements*](#def:array,element "9.3.4.5 Arrays [dcl.array]") of the array,
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and numbered 0 to N-1[.](#6.sentence-1)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3550)
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In addition to declarations in which an incomplete object type is allowed,
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an array bound may be omitted in some cases in the declaration of a function
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parameter ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"))[.](#7.sentence-1)
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An array bound may also be omitted
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when an object (but not a non-static data member) of array type is initialized
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and the declarator is followed by
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an initializer ([[dcl.init]](dcl.init "9.5 Initializers"), [[class.mem]](class.mem "11.4 Class members"), [[expr.type.conv]](expr.type.conv "7.6.1.4 Explicit type conversion (functional notation)"), [[expr.new]](expr.new "7.6.2.8 New"))[.](#7.sentence-2)
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In these cases, the array bound is calculated
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from the number of initial elements (say, N)
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supplied ([[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates")),
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and the type of the array is âarray of N Uâ[.](#7.sentence-3)
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3564)
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Furthermore, if there is a reachable declaration of the entity
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that specifies a bound and
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has the same host scope ([[basic.scope.scope]](basic.scope.scope "6.4.1 General")),
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an omitted array bound is taken to
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be the same as in that earlier declaration, and similarly for the definition
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of a static data member of a class[.](#8.sentence-1)
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[*Example [3](#example-3)*: extern int x[10];struct S {static int y[10];};
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int x[]; // OK, bound is 10int S::y[]; // OK, bound is 10void f() {extern int x[]; int i = sizeof(x); // error: incomplete object type}namespace A { extern int z[3]; }int A::z[] = {}; // OK, defines an array of 3 elements â *end example*]
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3591)
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[*Note [3](#note-3)*:
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When several âarray ofâ specifications are adjacent,
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a multidimensional array type is created;
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only the first of the constant expressions
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that specify the bounds of the arrays can be omitted[.](#9.sentence-1)
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[*Example [4](#example-4)*:
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int x3d[3][5][7]; declares an array of three elements,
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each of which is an array of five elements,
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each of which is an array of seven integers[.](#9.sentence-2)
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The overall array can be viewed as a
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three-dimensional array of integers,
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with rank 3 ×5 ×7[.](#9.sentence-3)
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Any of the expressionsx3d,x3d[i],x3d[i][j],x3d[i][j][k] can reasonably appear in an expression[.](#9.sentence-4)
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The expressionx3d[i] is equivalent to*(x3d + i);
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in that expression,x3d is subject to the array-to-pointer conversion ([[conv.array]](conv.array "7.3.3 Array-to-pointer conversion"))
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and is first converted to
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a pointer to a 2-dimensional
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array with rank5 ×7 that points to the first element of x3d[.](#9.sentence-5)
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Then i is added,
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which on typical implementations involves multiplyingi by the
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length of the object to which the pointer points,
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which is sizeof(int)×5 ×7[.](#9.sentence-6)
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The result of the addition and indirection is
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an lvalue denoting
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the ith array element ofx3d (an array of five arrays of seven integers)[.](#9.sentence-7)
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If there is another subscript,
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the same argument applies again, sox3d[i][j] is
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an lvalue denoting
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the jth array element of
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the ith array element ofx3d (an array of seven integers), andx3d[i][j][k] is
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an lvalue denoting
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the kth array element of
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the jth array element of
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the ith array element ofx3d (an integer)[.](#9.sentence-8)
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â *end example*]
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The first subscript in the declaration helps determine
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the amount of storage consumed by an array
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but plays no other part in subscript calculations[.](#9.sentence-9)
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â *end note*]
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3657)
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[*Note [4](#note-4)*:
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Conversions affecting expressions of array type are described in [[conv.array]](conv.array "7.3.3 Array-to-pointer conversion")[.](#10.sentence-1)
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â *end note*]
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[11](#11)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3662)
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[*Note [5](#note-5)*:
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The subscript operator can be overloaded for a class ([[over.sub]](over.sub "12.4.5 Subscripting"))[.](#11.sentence-1)
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For the operator's built-in meaning, see [[expr.sub]](expr.sub "7.6.1.2 Subscripting")[.](#11.sentence-2)
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â *end note*]
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