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cppdraft/dcl/ref.md

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+[dcl.ref]
+
+# 9 Declarations [[dcl]](./#dcl)
+
+## 9.3 Declarators [[dcl.decl]](dcl.decl#dcl.ref)
+
+### 9.3.4 Meaning of declarators [[dcl.meaning]](dcl.meaning#dcl.ref)
+
+#### 9.3.4.3 References [dcl.ref]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3137)
+
+In a declarationTD whereD has either of the forms
+
+& [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt D1  
+&& [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt D1
+
+and the type of the contained [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") in the declarationTD1 is “*derived-declarator-type-list*T”,
+the type of the [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") inD is “*derived-declarator-type-list* reference toT”[.](#1.sentence-1)
+
+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 reference type[.](#1.sentence-2)
+
+Cv-qualified references are ill-formed except when the cv-qualifiers
+are introduced through the use of a[*typedef-name*](dcl.typedef#nt:typedef-name "9.2.4 The typedef specifier [dcl.typedef]") ([[dcl.typedef]](dcl.typedef "9.2.4 The typedef specifier"), [[temp.param]](temp.param "13.2 Template parameters")) or[*decltype-specifier*](dcl.type.decltype#nt:decltype-specifier "9.2.9.6 Decltype specifiers [dcl.type.decltype]") ([[dcl.type.decltype]](dcl.type.decltype "9.2.9.6 Decltype specifiers")),
+in which case the cv-qualifiers are ignored[.](#1.sentence-3)
+
+[*Example [1](#example-1)*: typedef int& A;const A aref = 3; // error: lvalue reference to non-const initialized with rvalue
+
+The type ofaref is “lvalue reference to int”,
+not “lvalue reference to const int”[.](#1.sentence-4)
+
+— *end example*]
+
+[*Note [1](#note-1)*:
+
+A reference can be thought of as a name of an object[.](#1.sentence-5)
+
+— *end note*]
+
+Forming the type
+“reference to cv void”
+is ill-formed[.](#1.sentence-6)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3183)
+
+A reference type that is declared using & is called an[*lvalue reference*](#def:lvalue_reference "9.3.4.3 References [dcl.ref]"), and a reference type that
+is declared using && is called an[*rvalue reference*](#def:rvalue_reference "9.3.4.3 References [dcl.ref]")[.](#2.sentence-1)
+
+Lvalue references and
+rvalue references are distinct types[.](#2.sentence-2)
+
+Except where explicitly noted, they are
+semantically equivalent and commonly referred to as references[.](#2.sentence-3)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3191)
+
+[*Example [2](#example-2)*:
+
+void f(double& a) { a += 3.14; }// ...double d = 0;
+f(d); declaresa to be a reference parameter off so the callf(d) will add3.14 tod[.](#3.sentence-1)
+
+int v[20];// ...int& g(int i) { return v[i]; }// ... g(3) = 7; declares the functiong() to return a reference to an integer sog(3)=7 will assign7 to the fourth element of the arrayv[.](#3.sentence-2)
+
+For another example,struct link { link* next;};
+
+link* first;
+
+void h(link*& p) { // p is a reference to pointer p->next = first;
+ first = p;
+ p = 0;}void k() { link* q = new link;
+ h(q);} declaresp to be a reference to a pointer tolink soh(q) will leaveq with the value zero[.](#3.sentence-3)
+
+See also [[dcl.init.ref]](dcl.init.ref "9.5.4 References")[.](#3.sentence-4)
+
+— *end example*]
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3258)
+
+It is unspecified whether or not
+a reference requires storage ([[basic.stc]](basic.stc "6.8.6 Storage duration"))[.](#4.sentence-1)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3262)
+
+There shall be no references to references,
+no arrays of references, and no pointers to references[.](#5.sentence-1)
+
+The declaration of a reference shall contain an[*initializer*](dcl.init.general#nt:initializer "9.5.1 General [dcl.init.general]") ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))
+except when the declaration contains an explicitextern specifier ([[dcl.stc]](dcl.stc "9.2.2 Storage class specifiers")),
+is a class member ([[class.mem]](class.mem "11.4 Class members")) declaration within a class definition,
+or is the declaration of a parameter or a return type ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")); see [[basic.def]](basic.def "6.2 Declarations and definitions")[.](#5.sentence-2)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3275)
+
+Attempting to bind a reference to a function where
+the converted initializer is a glvalue whose
+type is not call-compatible ([[expr.call]](expr.call "7.6.1.3 Function call"))
+with the type of the function's definition
+results in undefined behavior[.](#6.sentence-1)
+
+Attempting to bind a reference to an object where
+the converted initializer is a glvalue through which
+the object is not type-accessible ([[basic.lval]](basic.lval "7.2.1 Value category"))
+results in undefined behavior[.](#6.sentence-2)
+
+[*Note [2](#note-2)*:
+
+The object designated by such a glvalue can be
+outside its lifetime ([[basic.life]](basic.life "6.8.4 Lifetime"))[.](#6.sentence-3)
+
+Because a null pointer value or a pointer past the end of an object
+does not point to an object,
+a reference in a well-defined program cannot refer to such things;
+see [[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators")[.](#6.sentence-4)
+
+As described in [[class.bit]](class.bit "11.4.10 Bit-fields"), a reference cannot be bound directly
+to a bit-field[.](#6.sentence-5)
+
+— *end note*]
+
+The behavior of an evaluation of a reference ([[expr.prim.id]](expr.prim.id "7.5.5 Names"), [[expr.ref]](expr.ref "7.6.1.5 Class member access")) that
+does not happen after ([[intro.races]](intro.races "6.10.2.2 Data races")) the initialization of the reference
+is undefined[.](#6.sentence-6)
+
+[*Example [3](#example-3)*: int &f(int&);int &g();extern int &ir3;int *ip = 0;int &ir1 = *ip; // undefined behavior: null pointerint &ir2 = f(ir3); // undefined behavior: ir3 not yet initializedint &ir3 = g();int &ir4 = f(ir4); // undefined behavior: ir4 used in its own initializerchar x alignas(int);int &ir5 = *reinterpret_cast<int *>(&x); // undefined behavior: initializer refers to char object — *end example*]
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3315)
+
+If a [*typedef-name*](dcl.typedef#nt:typedef-name "9.2.4 The typedef specifier [dcl.typedef]") ([[dcl.typedef]](dcl.typedef "9.2.4 The typedef specifier"), [[temp.param]](temp.param "13.2 Template parameters"))
+or a [*decltype-specifier*](dcl.type.decltype#nt:decltype-specifier "9.2.9.6 Decltype specifiers [dcl.type.decltype]") ([[dcl.type.decltype]](dcl.type.decltype "9.2.9.6 Decltype specifiers")) denotes a type TR that
+is a reference to a type T, an attempt to create the type “lvalue reference to cv TR”
+creates the type “lvalue reference to T”, while an attempt to create
+the type “rvalue reference to cv TR” creates the type TR[.](#7.sentence-1)
+
+[*Note [3](#note-3)*:
+
+This rule is known as reference collapsing[.](#7.sentence-2)
+
+— *end note*]
+
+[*Example [4](#example-4)*: int i;typedef int& LRI;typedef int&& RRI;
+
+LRI& r1 = i; // r1 has the type int&const LRI& r2 = i; // r2 has the type int&const LRI&& r3 = i; // r3 has the type int& RRI& r4 = i; // r4 has the type int& RRI&& r5 = 5; // r5 has the type int&&decltype(r2)& r6 = i; // r6 has the type int&decltype(r2)&& r7 = i; // r7 has the type int& — *end example*]
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L3343)
+
+[*Note [4](#note-4)*:
+
+Forming a reference to function type is ill-formed if the function
+type has [*cv-qualifier*](dcl.decl.general#nt:cv-qualifier "9.3.1 General [dcl.decl.general]")*s* or a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]");
+see [[dcl.fct]](dcl.fct "9.3.4.6 Functions")[.](#8.sentence-1)
+
+— *end note*]