%!TEX root = std.tex

\rSec0[basic]{Basic concepts}

%gram: \rSec1[gram.basic]{Basic concepts}

%gram:

\pnum

\enternote This Clause presents the basic concepts of the \Cpp language.

It explains the difference between an \term{object} and a

\term{name} and how they relate to the value categories for expressions.

It introduces the concepts of a

\term{declaration} and a \term{definition} and presents \Cpp's

notion of \term{type}, \term{scope}, \term{linkage}, and

\term{storage} \term{duration}. The mechanisms for starting and

terminating a program are discussed. Finally, this Clause presents the

\term{fundamental} types of the language and lists the ways of constructing

\term{compound} types from these.\exitnote

\pnum

\enternote This Clause does not cover concepts that affect only a single

part of the language. Such concepts are discussed in the relevant

Clauses. \exitnote

\pnum

\indextext{name}%

\indextext{declaration}%

\indextext{type}%

\indextext{object}%

\indextext{storage~class}%

\indextext{scope}%

\indextext{linkage}%

\indextext{region!declarative}%

\indextext{entity}%

An \defn{entity} is a value, object, reference, function, enumerator, type,

class member, template, template specialization, namespace, parameter

pack, or \tcode{this}.

\pnum

A \defn{name} is a use of an \grammarterm{identifier}~(\ref{lex.name}),

\grammarterm{operator-function-id}~(\ref{over.oper}),

\grammarterm{literal-operator-id}~(\ref{over.literal}),

\grammarterm{conversion-function-id}~(\ref{class.conv.fct}), or

\grammarterm{template-id}~(\ref{temp.names}) that denotes an entity or

\grammarterm{label}~(\ref{stmt.goto}, \ref{stmt.label}).

\pnum

Every name that denotes an entity is introduced by a

\term{declaration}. Every name that denotes a label is introduced

either by a \tcode{goto} statement~(\ref{stmt.goto}) or a

\grammarterm{labeled-statement}~(\ref{stmt.label}).

\pnum

A \defn{variable} is introduced by the

declaration of

a reference other than a non-static data member or of

an object. The variable's name, if any, denotes the reference or object.

\pnum

Some names denote types or templates. In general,

whenever a name is encountered it is necessary to determine whether that name denotes

one of these entities before continuing to parse the program that contains it. The

process that determines this is called

\indextext{lookup!name}%

\term{name lookup}~(\ref{basic.lookup}).

\pnum

Two names are \term{the same} if

\begin{itemize}

\item they are \grammarterm{identifier}{s} composed of the same character sequence, or

\item they are \grammarterm{operator-function-id}{s} formed with

the same operator, or

\item they are \grammarterm{conversion-function-id}{s} formed

with the same type, or

\item they are \grammarterm{template-id}{s} that refer to the same class or function~(\ref{temp.type}), or

\item they are the names of literal operators~(\ref{over.literal}) formed with

the same literal suffix identifier.

\end{itemize}

\pnum

\indextext{translation~unit!name~and}%

\indextext{linkage}%

A name used in more than one translation unit can potentially

refer to the same entity in these translation units depending on the

linkage~(\ref{basic.link}) of the name specified in each

translation unit.

\rSec1[basic.def]{Declarations and definitions}

\pnum

\indextext{declaration!definition~versus}%

\indextext{declaration}%

\indextext{declaration!name}%

A declaration (Clause~\ref{dcl.dcl}) may introduce

one or more names into a translation

unit or redeclare names introduced by previous declarations.

If so, the

declaration specifies the interpretation and attributes of these names.

A declaration may also have effects including:

\begin{itemize}

\item a static assertion (Clause~\ref{dcl.dcl}),

\item controlling template instantiation~(\ref{temp.explicit}),

\item use of attributes (Clause~\ref{dcl.dcl}), and

\item nothing (in the case of an \grammarterm{empty-declaration}).

\end{itemize}

\pnum

\indextext{declaration!function}%

\indextext{definition}%

A declaration is a \defn{definition} unless it declares a function

without specifying the function's body~(\ref{dcl.fct.def}), it contains

the

\indextext{declaration!\idxcode{extern}}%

\tcode{extern} specifier~(\ref{dcl.stc}) or a

\grammarterm{linkage-specification}\footnote{Appearing inside the braced-enclosed

\grammarterm{declaration-seq} in a \grammarterm{linkage-specification} does

not affect whether a declaration is a definition.}

(\ref{dcl.link}) and neither an \grammarterm{initializer} nor a

\grammarterm{function-body},

\indextext{declaration!\idxcode{static member}}%

it declares a static data member in a class

definition (\ref{class.mem},~\ref{class.static}),

\indextext{declaration!class~name}%

it is a class name declaration~(\ref{class.name}),

it is an

\indextext{declaration!opaque~enum}%

\grammarterm{opaque-enum-declaration}~(\ref{dcl.enum}),

it is a

\indextext{parameter!template}\indextext{template parameter}%

\grammarterm{template-parameter}~(\ref{temp.param}),

it is a

\indextext{declaration!parameter}\indextext{parameter declaration}%

\grammarterm{parameter-declaration}~(\ref{dcl.fct}) in a function

\indextext{declarator}%

declarator that is not the \grammarterm{declarator} of a

\grammarterm{function-definition},

or it is a

\indextext{declaration!\idxcode{typedef}}%

\tcode{typedef} declaration~(\ref{dcl.typedef}),

an \grammarterm{alias-declaration}~(\ref{dcl.typedef}),

a

\grammarterm{using-declaration}~(\ref{namespace.udecl}),

a \grammarterm{static_assert-declaration} (Clause~\ref{dcl.dcl}), an

\grammarterm{attribute-declaration} (Clause~\ref{dcl.dcl}), an

\grammarterm{empty-declaration} (Clause~\ref{dcl.dcl}),

or a \grammarterm{using-directive}~(\ref{namespace.udir}).

\enterexample all but one of the following are definitions:

\indextext{example!definition}%

\begin{codeblock}

int a; // defines \tcode{a}

extern const int c = 1; // defines \tcode{c}

int f(int x) { return x+a; } // defines \tcode{f} and defines \tcode{x}

struct S { int a; int b; }; // defines \tcode{S}, \tcode{S::a}, and \tcode{S::b}

struct X { // defines \tcode{X}

int x; // defines non-static data member \tcode{x}

static int y; // declares static data member \tcode{y}

X(): x(0) { } // defines a constructor of \tcode{X}

};

int X::y = 1; // defines \tcode{X::y}

enum { up, down }; // defines \tcode{up} and \tcode{down}

namespace N { int d; } // defines \tcode{N} and \tcode{N::d}

namespace N1 = N; // defines \tcode{N1}

X anX; // defines \tcode{anX}

\end{codeblock}

whereas these are just declarations:

\indextext{example!declaration}%

\begin{codeblock}

extern int a; // declares \tcode{a}

extern const int c; // declares \tcode{c}

int f(int); // declares \tcode{f}

struct S; // declares \tcode{S}

typedef int Int; // declares \tcode{Int}

extern X anotherX; // declares \tcode{anotherX}

using N::d; // declares \tcode{d}

\end{codeblock}

\exitexample

\pnum

\enternote

\indextext{implementation-generated}%

In some circumstances, \Cpp implementations implicitly define the

default constructor~(\ref{class.ctor}),

copy constructor~(\ref{class.copy}),

move constructor~(\ref{class.copy}),

copy assignment operator~(\ref{class.copy}),

move assignment operator~(\ref{class.copy}),

or destructor~(\ref{class.dtor}) member functions. \exitnote

\enterexample given

\begin{codeblock}

#include <string>

struct C {

std::string s; // \tcode{std::string} is the standard library class (Clause~\ref{strings})

};

int main() {

C a;

C b = a;

b = a;

}

\end{codeblock}

the implementation will implicitly define functions to make the

definition of \tcode{C} equivalent to

\begin{codeblock}

struct C {

std::string s;

C() : s() { }

C(const C& x): s(x.s) { }

C(C&& x): s(static_cast<std::string&&>(x.s)) { }

// \tcode{: s(std::move(x.s)) \{ \}}

C& operator=(const C& x) { s = x.s; return *this; }

C& operator=(C&& x) { s = static_cast<std::string&&>(x.s); return *this; }

// \tcode{\{ s = std::move(x.s); return *this; \}}

~C() { }

};

\end{codeblock}

\exitexample

\pnum

\enternote A class name can also be implicitly declared by an

\grammarterm{elaborated-type-specifier}~(\ref{dcl.type.elab}).

\exitnote

\pnum

\indextext{type!incomplete}%

A program is ill-formed if the definition of any object gives the object

an incomplete type~(\ref{basic.types}).

\indextext{object!definition}%

\indextext{function!definition}%

\indextext{class!definition}%

\indextext{enumerator!definition}%

\indextext{one-definition~rule|(}%

\rSec1[basic.def.odr]{One definition rule}

\pnum

No translation unit shall contain more than one definition of any

variable, function, class type, enumeration type, or template.

\pnum

An expression is \defn{potentially evaluated} unless it is an

unevaluated operand (Clause~\ref{expr}) or a subexpression thereof.

The set of \defn{potential results} of an expression \tcode{e} is

defined as follows:

\begin{itemize}

\item If \tcode{e} is an

\grammarterm{id-expression}~(\ref{expr.prim.general}), the set

contains only \tcode{e}.

\item If \tcode{e} is a class member access

expression~(\ref{expr.ref}), the set contains the potential results of

the object expression.

\item If \tcode{e} is a pointer-to-member

expression~(\ref{expr.mptr.oper}) whose second operand is a constant

expression, the set contains the potential results of the object

expression.

\item If \tcode{e} has the form \tcode{(e1)}, the set contains the

potential results of \tcode{e1}.

\item If \tcode{e} is a glvalue conditional

expression~(\ref{expr.cond}), the set is the union of the sets of

potential results of the second and third operands.

\item If \tcode{e} is a comma expression~(\ref{expr.comma}), the set

contains the potential results of the right operand.

\item Otherwise, the set is empty.

\end{itemize}

\enternote

This set is a (possibly-empty) set of \grammarterm{id-expression}{s},

each of which is either \tcode{e} or a subexpression of \tcode{e}.

\enterexample

In the following example, the set of potential results of the initializer

of \tcode{n} contains the first \tcode{S::x} subexpression, but not the second

\tcode{S::x} subexpression.

\begin{codeblock}

struct S { static const int x = 0; };

const int &f(const int &r);

int n = b ? (1, S::x) // \tcode{S::x} is not odr-used here

: f(S::x); // \tcode{S::x} is odr-used here, so

// a definition is required

\end{codeblock}

\exitexample

\exitnote

\pnum

A variable \tcode{x} whose name appears as a

potentially-evaluated expression \tcode{ex} is \defn{odr-used} by \tcode{ex} unless

applying the lvalue-to-rvalue conversion (\ref{conv.lval}) to \tcode{x} yields

a constant expression~(\ref{expr.const}) that does not invoke any non-trivial

functions

and, if \tcode{x} is an object, \tcode{ex} is an element of

the set of potential results of an expression \tcode{e}, where either the lvalue-to-rvalue

conversion~(\ref{conv.lval}) is applied to \tcode{e}, or \tcode{e} is

a discarded-value expression~(Clause \ref{expr}).

\tcode{this} is odr-used if it appears as a potentially-evaluated expression

(including as the result of the implicit transformation in the body of a non-static

member function~(\ref{class.mfct.non-static})).

A virtual member

function is odr-used if it is not pure.

A function whose name appears as a potentially-evaluated

expression is odr-used if it is the unique lookup result or the selected

member of a set of overloaded functions~(\ref{basic.lookup}, \ref{over.match}, \ref{over.over}), unless it is a pure virtual

function and its name is not explicitly qualified.

\enternote This covers calls to named

functions~(\ref{expr.call}), operator overloading (Clause~\ref{over}),

user-defined conversions~(\ref{class.conv.fct}), allocation function for

placement new~(\ref{expr.new}), as well as non-default

initialization~(\ref{dcl.init}). A constructor selected to copy or move an

object of class type is odr-used even if the

call is actually elided by the implementation~(\ref{class.copy}). \exitnote An allocation

or deallocation function for a class is odr-used by a new expression

appearing in a potentially-evaluated expression as specified

in~\ref{expr.new} and~\ref{class.free}. A deallocation function for a

class is odr-used by a delete expression appearing in a

potentially-evaluated expression as specified in~\ref{expr.delete}

and~\ref{class.free}. A non-placement allocation or deallocation

function for a class is odr-used by the definition of a constructor of that

class. A non-placement deallocation function for a class is odr-used by the

definition of the destructor of that class, or by being selected by the

lookup at the point of definition of a virtual

destructor~(\ref{class.dtor}).\footnote{An implementation is not required

to call allocation and

deallocation functions from constructors or destructors; however, this

is a permissible implementation technique.}

An assignment operator function in a class is odr-used by an

implicitly-defined

copy-assignment or move-assignment function for another class as specified

in~\ref{class.copy}.

A default constructor for a class is odr-used by

default initialization or value initialization as specified

in~\ref{dcl.init}. A constructor for a class is odr-used as specified

in~\ref{dcl.init}. A destructor for a class is odr-used if it is potentially

invoked~(\ref{class.dtor}).

\pnum

Every program shall contain exactly one definition of every non-inline

function or variable that is odr-used in that program; no diagnostic required.

The definition can appear explicitly in the program, it can be found in

the standard or a user-defined library, or (when appropriate) it is

implicitly defined (see~\ref{class.ctor}, \ref{class.dtor} and

\ref{class.copy}). An inline function shall be defined in every

translation unit in which it is odr-used.

\pnum

\indextext{type!incomplete}%

Exactly one definition of a class is required in a translation unit if

the class is used in a way that requires the class type to be complete.

\enterexample the following complete translation unit is well-formed,

even though it never defines \tcode{X}:

\begin{codeblock}

struct X; // declare \tcode{X} as a struct type

struct X* x1; // use \tcode{X} in pointer formation

X* x2; // use \tcode{X} in pointer formation

\end{codeblock}

\exitexample

\enternote The rules for declarations and expressions

describe in which contexts complete class types are required. A class

type \tcode{T} must be complete if:

\begin{itemize}

\item an object of type \tcode{T} is defined~(\ref{basic.def}), or

\item a non-static class data member of type \tcode{T} is

declared~(\ref{class.mem}), or

\item \tcode{T} is used as the object type or array element type in a

\grammarterm{new-expression}~(\ref{expr.new}), or

\item an lvalue-to-rvalue conversion is applied to

a glvalue referring

to an object of type \tcode{T}~(\ref{conv.lval}), or

\item an expression is converted (either implicitly or explicitly) to

type \tcode{T} (Clause~\ref{conv}, \ref{expr.type.conv},

\ref{expr.dynamic.cast}, \ref{expr.static.cast}, \ref{expr.cast}), or

\item an expression that is not a null pointer constant, and has type

other than \term{cv} \tcode{void*}, is converted to the type pointer to \tcode{T}

or reference to \tcode{T} using a standard conversion

(Clause~\ref{conv}), a \tcode{dynamic_cast}~(\ref{expr.dynamic.cast}) or

a \tcode{static_cast}~(\ref{expr.static.cast}), or

\item a class member access operator is applied to an expression of type

\tcode{T}~(\ref{expr.ref}), or

\item the \tcode{typeid} operator~(\ref{expr.typeid}) or the

\tcode{sizeof} operator~(\ref{expr.sizeof}) is applied to an operand of

type \tcode{T}, or

\item a function with a return type or argument type of type \tcode{T}

is defined~(\ref{basic.def}) or called~(\ref{expr.call}), or

\item a class with a base class of type \tcode{T} is

defined (Clause~\ref{class.derived}), or

\item an lvalue of type \tcode{T} is assigned to~(\ref{expr.ass}), or

\item the type \tcode{T} is the subject of an

\tcode{alignof} expression~(\ref{expr.alignof}), or

\item an \grammarterm{exception-declaration} has type \tcode{T}, reference to

\tcode{T}, or pointer to \tcode{T}~(\ref{except.handle}).

\end{itemize}

\exitnote

\pnum

There can be more than one definition of a class type

(Clause~\ref{class}), enumeration type~(\ref{dcl.enum}), inline function

with external linkage~(\ref{dcl.fct.spec}), class template

(Clause~\ref{temp}), non-static function template~(\ref{temp.fct}),

static data member of a class template~(\ref{temp.static}), member

function of a class template~(\ref{temp.mem.func}), or template

specialization for which some template parameters are not

specified~(\ref{temp.spec}, \ref{temp.class.spec}) in a program provided

that each definition appears in a different translation unit, and

provided the definitions satisfy the following requirements. Given such

an entity named \tcode{D} defined in more than one translation unit,

then

\begin{itemize}

\item each definition of \tcode{D} shall consist of the same sequence of

tokens; and

\item in each definition of \tcode{D}, corresponding names, looked up

according to~\ref{basic.lookup}, shall refer to an entity defined within

the definition of \tcode{D}, or shall refer to the same entity, after

overload resolution~(\ref{over.match}) and after matching of partial

template specialization~(\ref{temp.over}), except that a name can refer

to a non-volatile \tcode{const} object with internal or no linkage if the

object has the same literal type in all definitions of \tcode{D},

and the object is initialized with a constant

expression~(\ref{expr.const}), and the object is not odr-used, and the object

has the same value in all definitions of \tcode{D}; and

\item in each definition of \tcode{D}, corresponding entities shall have the

same language linkage; and

\item in each definition of \tcode{D}, the overloaded operators referred

to, the implicit calls to conversion functions, constructors, operator

new functions and operator delete functions, shall refer to the same

function, or to a function defined within the definition of \tcode{D};

and

\item in each definition of \tcode{D}, a default argument used by an

(implicit or explicit) function call is treated as if its token sequence

were present in the definition of \tcode{D}; that is, the default

argument is subject to the three requirements described above (and, if

the default argument has sub-expressions with default arguments, this

requirement applies recursively).\footnote{\ref{dcl.fct.default}

describes how default argument names are looked up.}

\item if \tcode{D} is a class with an implicitly-declared

constructor~(\ref{class.ctor}), it is as if the constructor was

implicitly defined in every translation unit where it is odr-used, and the

implicit definition in every translation unit shall call the same

constructor for a base class or a class member of \tcode{D}.

\enterexample

\begin{codeblock}

//translation unit 1:

struct X {

X(int);

X(int, int);

};

X::X(int = 0) { }

class D: public X { };

D d2; // \tcode{X(int)} called by \tcode{D()}

//translation unit 2:

struct X {

X(int);

X(int, int);

};

X::X(int = 0, int = 0) { }

class D: public X { }; // \tcode{X(int, int)} called by \tcode{D()};

// \tcode{D()}'s implicit definition

// violates the ODR

\end{codeblock}

\exitexample

\end{itemize}

If \tcode{D} is a template and is defined in more than one

translation unit, then the preceding requirements

shall apply both to names from the template's enclosing scope used in the

template definition~(\ref{temp.nondep}), and also to dependent names at

the point of instantiation~(\ref{temp.dep}). If the definitions of

\tcode{D} satisfy all these requirements, then the program shall behave

as if there were a single definition of \tcode{D}. If the definitions of

\tcode{D} do not satisfy these requirements, then the behavior is

undefined.%

\indextext{one-definition~rule|)}

\rSec1[basic.scope]{Scope}%

\indextext{scope|(}

\rSec2[basic.scope.declarative]{Declarative regions and scopes}%

\indextext{scope!declarations and|(}

\pnum

\indextext{name!scope~of}%

Every name is introduced in some portion of program text called a

\indextext{region!declarative}%

\indextext{scope!potential}%

\defn{declarative region}, which is the largest part of the program

in which that name is \defn{valid}, that is, in which that name may

be used as an unqualified name to refer to the same entity. In general,

each particular name is valid only within some possibly discontiguous

portion of program text called its \defn{scope}. To determine the

scope of a declaration, it is sometimes convenient to refer to the

\defn{potential scope} of a declaration. The scope of a declaration

is the same as its potential scope unless the potential scope contains

another declaration of the same name. In that case, the potential scope

of the declaration in the inner (contained) declarative region is

excluded from the scope of the declaration in the outer (containing)

declarative region.

\pnum

\enterexample

in

\begin{codeblock}

int j = 24;

int main() {

int i = j, j;

j = 42;

}

\end{codeblock}

the identifier \tcode{j} is declared twice as a name (and used twice).

The declarative region of the first \tcode{j} includes the entire

example. The potential scope of the first \tcode{j} begins immediately

after that \tcode{j} and extends to the end of the program, but its

(actual) scope excludes the text between the \tcode{,} and the

\tcode{\}}. The declarative region of the second declaration of

\tcode{j} (the \tcode{j} immediately before the semicolon) includes all

the text between \tcode{\{} and \tcode{\}}, but its potential scope

excludes the declaration of \tcode{i}. The scope of the second

declaration of \tcode{j} is the same as its potential scope.

\exitexample

\pnum

The names declared by a declaration are introduced into the scope in

which the declaration occurs, except that the presence of a

\tcode{friend} specifier~(\ref{class.friend}), certain uses of the

\grammarterm{elaborated-type-specifier}~(\ref{dcl.type.elab}), and

\grammarterm{using-directive}{s}~(\ref{namespace.udir}) alter this general

behavior.

\pnum

Given a set of declarations in a single declarative region, each of

which specifies the same unqualified name,

\begin{itemize}

\item they shall all refer to the same entity, or all refer to functions

and function templates; or

\item exactly one declaration shall declare a class name or enumeration

name that is not a typedef name and the other declarations shall all

refer to the same variable or enumerator, or all refer to functions and

function templates; in this case the class name or enumeration name is

hidden~(\ref{basic.scope.hiding}). \enternote A namespace name or a

class template name must be unique in its declarative

region~(\ref{namespace.alias}, Clause~\ref{temp}). \exitnote

\end{itemize}

\enternote These restrictions apply to the declarative region into which

a name is introduced, which is not necessarily the same as the region in

which the declaration occurs. In particular,

\grammarterm{elaborated-type-specifier}{s}~(\ref{dcl.type.elab}) and

friend declarations~(\ref{class.friend}) may introduce a (possibly not

visible) name into an enclosing namespace; these restrictions apply to

that region. Local extern declarations~(\ref{basic.link}) may introduce

a name into the declarative region where the declaration appears and

also introduce a (possibly not visible) name into an enclosing

namespace; these restrictions apply to both regions. \exitnote

\pnum

\enternote The name lookup rules are summarized in~\ref{basic.lookup}.

\exitnote

\rSec2[basic.scope.pdecl]{Point of declaration}

\pnum

\indextext{name!point~of declaration}%

The \defn{point of declaration} for a name is immediately after its

complete declarator (Clause~\ref{dcl.decl}) and before its

\grammarterm{initializer} (if any), except as noted below. \enterexample

\begin{codeblock}

unsigned char x = 12;

{ unsigned char x = x; }

\end{codeblock}

Here the second \tcode{x} is initialized with its own (indeterminate)

value. \exitexample

\pnum

\enternote

\indextext{name~hiding}%

a name from an outer scope remains visible up

to the point of declaration of the name that hides it.\enterexample

\begin{codeblock}

const int i = 2;

{ int i[i]; }

\end{codeblock}

declares a block-scope array of two integers. \exitexample \exitnote

\pnum

The point of declaration for a class or class template first declared by a

\grammarterm{class-specifier} is immediately after the \grammarterm{identifier} or

\grammarterm{simple-template-id} (if any) in its \grammarterm{class-head}

(Clause~\ref{class}). The point of declaration for an enumeration is

immediately after the \grammarterm{identifier} (if any) in either its

\grammarterm{enum-specifier}~(\ref{dcl.enum}) or its first

\grammarterm{opaque-enum-declaration}~(\ref{dcl.enum}), whichever comes first.

The point of declaration of an alias or alias template immediately

follows the \grammarterm{type-id} to which the

alias refers.

\pnum

The point of declaration of a \grammarterm{using-declaration} that does not name a

constructor is immediately after the \grammarterm{using-declaration}~(\ref{namespace.udecl}).

\pnum

\indextext{declaration!enumerator point~of}%

The point of declaration for an enumerator is immediately after its

\grammarterm{enumerator-definition}.\enterexample

\begin{codeblock}

const int x = 12;

{ enum { x = x }; }

\end{codeblock}

Here, the enumerator \tcode{x} is initialized with the value of the

constant \tcode{x}, namely 12. \exitexample

\pnum

After the point of declaration of a class member, the member name can be

looked up in the scope of its class. \enternote

\indextext{type!incomplete}%

this is true even if the class is an incomplete class. For example,

\begin{codeblock}

struct X {

enum E { z = 16 };

int b[X::z]; // OK

};

\end{codeblock}

\exitnote

\pnum

The point of declaration of a class first declared in an

\grammarterm{elaborated-type-specifier} is as follows:

\begin{itemize}

\item for a declaration of the form

\begin{ncsimplebnf}

class-key attribute-specifier-seq\opt identifier \terminal{;}

\end{ncsimplebnf}

the \grammarterm{identifier} is declared to be a

\grammarterm{class-name} in the scope that contains the declaration,

otherwise

\item for an \grammarterm{elaborated-type-specifier} of the form

\begin{ncsimplebnf}

class-key identifier

\end{ncsimplebnf}

if the

\grammarterm{elaborated-type-specifier} is used in the

\grammarterm{decl-specifier-seq} or \grammarterm{parameter-declaration-clause}

of a function defined in namespace scope, the \grammarterm{identifier} is

declared as a \grammarterm{class-name} in the namespace that contains the

declaration; otherwise, except as a friend declaration, the

\grammarterm{identifier} is declared in the smallest namespace or block

scope that contains the declaration. \enternote

These rules also apply within templates. \exitnote \enternote Other

forms of \grammarterm{elaborated-type-specifier} do not declare a new name,

and therefore must refer to an existing \grammarterm{type-name}.

See~\ref{basic.lookup.elab} and~\ref{dcl.type.elab}. \exitnote

\end{itemize}

\pnum

The point of declaration for an

\grammarterm{injected-class-name} (Clause~\ref{class}) is immediately following

the opening brace of the class definition.

\pnum

The point of declaration for a function-local predefined

variable~(\ref{dcl.fct.def}) is immediately before the

\grammarterm{function-body} of a function definition.

\pnum

The point of declaration for a template parameter is immediately after its complete

\grammarterm{template-parameter}. \enterexample

\begin{codeblock}

typedef unsigned char T;

template<class T

= T // lookup finds the typedef name of \tcode{unsigned char}

, T // lookup finds the template parameter

N = 0> struct A { };

\end{codeblock}

\exitexample

\pnum

\enternote Friend declarations refer to functions or classes that are

members of the nearest enclosing namespace, but they do not introduce

new names into that namespace~(\ref{namespace.memdef}). Function

declarations at block scope and variable declarations with the

\tcode{extern} specifier at block scope refer to declarations that are

members of an enclosing namespace, but they do not introduce new names

into that scope.

\exitnote

\pnum

\enternote For point of instantiation of a template,

see~\ref{temp.point}.\exitnote%

\indextext{scope!declarations and|)}

\rSec2[basic.scope.block]{Block scope}

\pnum

\indextext{scope!block}%

\indextext{local~scope|see{block scope}}%

A name declared in a block~(\ref{stmt.block}) is local to that block; it has

\defn{block scope}.

Its potential scope begins at its point of

declaration~(\ref{basic.scope.pdecl}) and ends at the end of its block.

A variable declared at block scope is a \defn{local variable}.

\pnum

\indextext{parameter!scope~of}%

The potential scope of a function parameter name

(including one appearing in a

\grammarterm{lambda-declarator})

or of a function-local predefined variable

in a function

definition~(\ref{dcl.fct.def}) begins at its point of declaration. If

the function has a \grammarterm{function-try-block} the potential scope of

a parameter

or of a function-local predefined variable

ends at the end of the last associated handler, otherwise it ends

at the end of the outermost block of the function definition. A

parameter name shall not be redeclared in the outermost block of the

function definition nor in the outermost block of any handler associated

with a \grammarterm{function-try-block}.

\pnum

\indextext{scope!exception~declaration}%

The name declared in an \grammarterm{exception-declaration}

is local to the

\grammarterm{handler} and shall not be redeclared in the outermost block of the

\grammarterm{handler}.

\pnum

Names declared in the \grammarterm{for-init-statement}, the \grammarterm{for-range-declaration}, and in the

\grammarterm{condition} of \tcode{if}, \tcode{while}, \tcode{for}, and

\tcode{switch} statements are local to the \tcode{if}, \tcode{while},

\tcode{for}, or \tcode{switch} statement (including the controlled

statement), and shall not be redeclared in a subsequent condition of

that statement nor in the outermost block (or, for the \tcode{if}

statement, any of the outermost blocks) of the controlled statement;

see~\ref{stmt.select}.

\rSec2[basic.scope.proto]{Function prototype scope}

\pnum

\indextext{scope!function~prototype}%

\indextext{function~prototype}%

In a function declaration, or in any function declarator except the

declarator of a function definition~(\ref{dcl.fct.def}), names of

parameters (if supplied) have function prototype scope, which terminates

at the end of the nearest enclosing function declarator.

\rSec2[basic.funscope]{Function scope}

\pnum

\indextext{scope!function}%

\indextext{label!scope~of}%

Labels~(\ref{stmt.label}) have \term{function scope} and

may be used anywhere in the function in which they are declared. Only

labels have function scope.

\rSec2[basic.scope.namespace]{Namespace scope}

\pnum

\indextext{scope!namespace}%

The declarative region of a \grammarterm{namespace-definition} is its

\grammarterm{namespace-body}. The potential scope denoted by an

\grammarterm{original-namespace-name} is the concatenation of the

declarative regions established by each of the

\grammarterm{namespace-definition}{s} in the same declarative region with

that \grammarterm{original-namespace-name}. Entities declared in a

\grammarterm{namespace-body} are said to be \defn{members} of the

namespace, and names introduced by these declarations into the

declarative region of the namespace are said to be \defn{member

names} of the namespace. A namespace member name has namespace scope.

Its potential scope includes its namespace from the name's point of

declaration~(\ref{basic.scope.pdecl}) onwards; and for each

\grammarterm{using-directive}~(\ref{namespace.udir}) that nominates the

member's namespace, the member's potential scope includes that portion

of the potential scope of the \grammarterm{using-directive} that follows

the member's point of declaration. \enterexample

\begin{codeblock}

namespace N {

int i;

int g(int a) { return a; }

int j();

void q();

}

namespace { int l=1; }

// the potential scope of \tcode{l} is from its point of declaration

// to the end of the translation unit

namespace N {

int g(char a) { // overloads \tcode{N::g(int)}

return l+a; // \tcode{l} is from unnamed namespace

}

int i; // error: duplicate definition

int j(); // OK: duplicate function declaration

int j() { // OK: definition of \tcode{N::j()}

return g(i); // calls \tcode{N::g(int)}

}

int q(); // error: different return type

}

\end{codeblock}

\exitexample

\pnum

A namespace member can also be referred to after the \tcode{::} scope

resolution operator~(\ref{expr.prim}) applied to the name of its

namespace or the name of a namespace which nominates the member's

namespace in a \grammarterm{using-directive;} see~\ref{namespace.qual}.

\pnum

\indextext{scope!global namespace}%

\indextext{scope!global}%

The outermost declarative region of a translation unit is also a

namespace, called the \defn{global namespace}. A name declared in

the global namespace has \defn{global namespace scope} (also called

\defn{global scope}). The potential scope of such a name begins at

its point of declaration~(\ref{basic.scope.pdecl}) and ends at the end

of the translation unit that is its declarative region.

\indextext{name!global}%

A name with global namespace scope is said to be a

\defnx{global name}{global}.

\rSec2[basic.scope.class]{Class scope}

\pnum

\indextext{scope!class}%

The following rules describe the scope of names declared in classes.

\begin{enumeraten}

\item The potential scope of a name declared in a class consists not

only of the declarative region following the name's point of

declaration, but also of all function bodies, default arguments,

\grammarterm{exception-specification}{s}, and

\grammarterm{brace-or-equal-initializers} of non-static data members

in that class (including such

things in nested classes).

\item A name \tcode{N} used in a class \tcode{S} shall refer to the same

declaration in its context and when re-evaluated in the completed scope

of \tcode{S}. No diagnostic is required for a violation of this rule.

\item If reordering member declarations in a class yields an alternate

valid program under (1) and (2), the program is ill-formed, no

diagnostic is required.

\item A name declared within a member function hides a declaration of

the same name whose scope extends to or past the end of the member

function's class.

\item The potential scope of a declaration that extends to or past the

end of a class definition also extends to the regions defined by its

member definitions, even if the members are defined lexically outside

the class (this includes static data member definitions, nested class

definitions, and member function definitions, including the member function

body and any portion of the

declarator part of such definitions which follows the \grammarterm{declarator-id},

including a \grammarterm{parameter-declaration-clause} and any default

arguments~(\ref{dcl.fct.default})).\enterexample

\begin{codeblock}

typedef int c;

enum { i = 1 };

class X {

char v[i]; // error: \tcode{i} refers to \tcode{::i}

// but when reevaluated is \tcode{X::i}

int f() { return sizeof(c); } // OK: \tcode{X::c}

char c;

enum { i = 2 };

};

typedef char* T;

struct Y {

T a; // error: \tcode{T} refers to \tcode{::T}

// but when reevaluated is \tcode{Y::T}

typedef long T;

T b;

};

typedef int I;

class D {

typedef I I; // error, even though no reordering involved

};

\end{codeblock}

\exitexample

\end{enumeraten}

\pnum

The name of a class member shall only be used as follows:

\begin{itemize}

\item in the scope of its class (as described above) or a class derived

(Clause~\ref{class.derived}) from its class,

\item after the \tcode{.} operator applied to an expression of the type

of its class~(\ref{expr.ref}) or a class derived from its class,

\item after the \tcode{->} operator applied to a pointer to an object of

its class~(\ref{expr.ref}) or a class derived from its class,

\item after the \tcode{::} scope resolution operator~(\ref{expr.prim})

applied to the name of its class or a class derived from its class.

\end{itemize}

\rSec2[basic.scope.enum]{Enumeration scope}%

\indextext{enumeration scope}%

\indextext{scope!enumeration}

\pnum

The name of a scoped enumerator~(\ref{dcl.enum}) has

\defn{enumeration scope}. Its potential scope begins at

its point of declaration and terminates at the end of the

\grammarterm{enum-specifier}.

\rSec2[basic.scope.temp]{Template parameter scope}%

\indextext{template~parameter~scope}%

\indextext{scope!template~parameter}%

\pnum

The declarative region of the name of a template parameter of a template

\grammarterm{template-parameter} is the smallest \grammarterm{template-parameter-list}

in which the name was introduced.

\pnum

The declarative region of the name of a template parameter of a template is the smallest

\grammarterm{template-declaration} in which the name was introduced. Only template

parameter names belong to this declarative region; any other kind of name introduced by

the \grammarterm{declaration} of a \grammarterm{template-declaration} is instead

introduced into the same declarative region where it would be introduced as a result of

a non-template declaration of the same name. \enterexample

\begin{codeblock}

namespace N {

template<class T> struct A { }; // \#1

template<class U> void f(U) { } // \#2

struct B {

template<class V> friend int g(struct C*); // \#3

};

}

\end{codeblock}

The declarative regions of \tcode{T}, \tcode{U} and \tcode{V} are the

\grammarterm{template-declaration}{s} on lines \tcode{\#1}, \tcode{\#2} and \tcode{\#3},

respectively. But the names \tcode{A}, \tcode{f}, \tcode{g} and \tcode{C} all belong to

the same declarative region --- namely, the \grammarterm{namespace-body} of \tcode{N}.

(\tcode{g} is still considered to belong to this declarative region in spite of its

being hidden during qualified and unqualified name lookup.)

\exitexample

\pnum

The potential scope of a template parameter name begins at its point of

declaration~(\ref{basic.scope.pdecl}) and ends at the end of its declarative region.

\enternote This implies that a \grammarterm{template-parameter} can be used in the

declaration of subsequent \grammarterm{template-parameter}{s} and their default

arguments but cannot be used in preceding \grammarterm{template-parameter}{s} or their

default arguments. For example,

\begin{codeblock}

template<class T, T* p, class U = T> class X { /* ... */ };

template<class T> void f(T* p = new T);

\end{codeblock}

This also implies that a \grammarterm{template-parameter} can be used in the

specification of base classes. For example,

\begin{codeblock}

template<class T> class X : public Array<T> { /* ... */ };

template<class T> class Y : public T { /* ... */ };

\end{codeblock}

The use of a template parameter as a base class implies that a class used as a template

argument must be defined and not just declared when the class template is instantiated.

\exitnote

\pnum

The declarative region of the name of a template parameter is nested within the

immediately-enclosing declarative region. \enternote As a result, a

\grammarterm{template-parameter} hides any entity with the same name in an enclosing

scope~(\ref{basic.scope.hiding}). \enterexample

\begin{codeblock}

typedef int N;

template<N X, typename N, template<N Y> class T> struct A;

\end{codeblock}

Here, \tcode{X} is a non-type template parameter of type \tcode{int} and \tcode{Y} is a

non-type template parameter of the same type as the second template parameter of

\tcode{A}. \exitexample\exitnote