\rSec0[dcl]{Declarations}%

\indextext{declaration|(}

\gramSec[gram.dcl]{Declarations}

\indextext{linkage specification|see{specification, linkage}}

\rSec1[dcl.pre]{Preamble}

Declarations generally specify how names are to be interpreted. Declarations have

the form

\begin{bnf}

\nontermdef{declaration-seq}\br

declaration \opt{declaration-seq}

\end{bnf}

\begin{bnf}

\nontermdef{declaration}\br

name-declaration\br

special-declaration

\end{bnf}

\begin{bnf}

\nontermdef{name-declaration}\br

block-declaration\br

nodeclspec-function-declaration\br

function-definition\br

friend-type-declaration\br

template-declaration\br

deduction-guide\br

linkage-specification\br

namespace-definition\br

empty-declaration\br

attribute-declaration\br

module-import-declaration

\end{bnf}

\begin{bnf}

\nontermdef{special-declaration}\br

explicit-instantiation\br

explicit-specialization\br

export-declaration

\end{bnf}

\begin{bnf}

\nontermdef{block-declaration}\br

simple-declaration\br

asm-declaration\br

namespace-alias-definition\br

using-declaration\br

using-enum-declaration\br

using-directive\br

static_assert-declaration\br

consteval-block-declaration\br

alias-declaration\br

opaque-enum-declaration

\end{bnf}

\begin{bnf}

\nontermdef{nodeclspec-function-declaration}\br

\opt{attribute-specifier-seq} declarator \terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{alias-declaration}\br

\keyword{using} identifier \opt{attribute-specifier-seq} \terminal{=} defining-type-id \terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{sb-identifier}\br

\opt{\terminal{...}} identifier \opt{attribute-specifier-seq}

\end{bnf}

\begin{bnf}

\nontermdef{sb-identifier-list}\br

sb-identifier\br

sb-identifier-list \terminal{,} sb-identifier

\end{bnf}

\begin{bnf}

\nontermdef{structured-binding-declaration}\br

\opt{attribute-specifier-seq} decl-specifier-seq \opt{ref-qualifier} \terminal{[} sb-identifier-list \terminal{]}

\end{bnf}

\begin{bnf}

\nontermdef{simple-declaration}\br

decl-specifier-seq \opt{init-declarator-list} \terminal{;}\br

attribute-specifier-seq decl-specifier-seq init-declarator-list \terminal{;}\br

structured-binding-declaration initializer \terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{static_assert-message}\br

unevaluated-string\br

constant-expression

\end{bnf}

\begin{bnf}

\nontermdef{static_assert-declaration}\br

\keyword{static_assert} \terminal{(} constant-expression \terminal{)} \terminal{;}\br

\keyword{static_assert} \terminal{(} constant-expression \terminal{,} static_assert-message \terminal{)} \terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{consteval-block-declaration}\br

\keyword{consteval} compound-statement

\end{bnf}

\begin{bnf}

\nontermdef{empty-declaration}\br

\terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{attribute-declaration}\br

attribute-specifier-seq \terminal{;}

\end{bnf}

\begin{note}

\grammarterm{asm-declaration}{s} are described in~\ref{dcl.asm}, and

\grammarterm{linkage-specification}{s} are described in~\ref{dcl.link};

\grammarterm{function-definition}{s} are described in~\ref{dcl.fct.def} and

\grammarterm{template-declaration}{s} and

\grammarterm{deduction-guide}{s} are described in \ref{temp.deduct.guide};

\grammarterm{namespace-definition}{s} are described in~\ref{namespace.def},

\grammarterm{using-declaration}{s} are described in~\ref{namespace.udecl} and

\grammarterm{using-directive}{s} are described in~\ref{namespace.udir}.

\end{note}

\indextext{declaration}%

\indextext{scope}%

Certain declarations contain one or more scopes\iref{basic.scope.scope}.

Unless otherwise stated, utterances in

\ref{dcl} about components in, of, or contained by a

declaration or subcomponent thereof refer only to those components of

the declaration that are \emph{not} nested within scopes nested within

the declaration.

If a \grammarterm{name-declaration} matches

the syntactic requirements of \grammarterm{friend-type-declaration},

it is a \grammarterm{friend-type-declaration}.

A

\grammarterm{simple-declaration} or

\grammarterm{nodeclspec-function-declaration} of the form

\begin{ncsimplebnf}

\opt{attribute-specifier-seq} \opt{decl-specifier-seq} \opt{init-declarator-list} \terminal{;}

\end{ncsimplebnf}

is divided into three parts.

Attributes are described in~\ref{dcl.attr}.

\grammarterm{decl-specifier}{s}, the principal components of

a \grammarterm{decl-specifier-seq}, are described in~\ref{dcl.spec}.

\grammarterm{declarator}{s}, the components of an

\grammarterm{init-declarator-list}, are described in \ref{dcl.decl}.

The \grammarterm{attribute-specifier-seq}

appertains to each of the entities declared by

the \grammarterm{declarator}{s}

of the \grammarterm{init-declarator-list}.

\begin{note}

In the declaration for an entity, attributes appertaining to that

entity can appear at the start of the declaration and after the

\grammarterm{declarator-id} for that declaration.

\end{note}

\begin{example}

\begin{codeblock}

[[noreturn]] void f [[noreturn]] (); // OK

\end{codeblock}

\end{example}

If a \grammarterm{declarator-id} is a name, the

\grammarterm{init-declarator} and (hence) the declaration introduce that name.

\begin{note}

Otherwise, the \grammarterm{declarator-id} is

a \grammarterm{qualified-id} or

names a destructor or

its \grammarterm{unqualified-id} is a \grammarterm{template-id} and

no name is introduced.

\end{note}

The \grammarterm{defining-type-specifier}{s}\iref{dcl.type} in

the \grammarterm{decl-specifier-seq} and

the recursive \grammarterm{declarator} structure

describe a type\iref{dcl.meaning},

which is then associated with the \grammarterm{declarator-id}.

\indextext{identifier}%

\indextext{declarator}%

In a \grammarterm{simple-declaration}, the optional

\grammarterm{init-declarator-list} can be omitted only when declaring a

class\iref{class.pre} or enumeration\iref{dcl.enum}, that is,

when the \grammarterm{decl-specifier-seq} contains either a

\grammarterm{class-specifier}, an \grammarterm{elaborated-type-specifier} with

a \grammarterm{class-key}\iref{class.name}, or an

\grammarterm{enum-specifier}. In these cases and whenever a

\grammarterm{class-specifier} or \grammarterm{enum-specifier} is present in

the \grammarterm{decl-specifier-seq}, the identifiers in these specifiers

are also declared (as

\grammarterm{class-name}{s}, \grammarterm{enum-name}{s}, or

\grammarterm{enumerator}{s}, depending on the syntax). In such cases,

the \grammarterm{decl-specifier-seq} shall (re)introduce one or more names into

the program.

\begin{example}

\begin{codeblock}

enum { }; // error

typedef class { }; // error

\end{codeblock}

\end{example}

A \grammarterm{simple-declaration} or a \grammarterm{condition}

with a \grammarterm{structured-binding-declaration} is called

a \defn{structured binding declaration}\iref{dcl.struct.bind}.

Each \grammarterm{decl-specifier} in the \grammarterm{decl-specifier-seq}

shall be

\tcode{constexpr},

\tcode{constinit},

\tcode{static},

\tcode{thread_local},

\tcode{auto}\iref{dcl.spec.auto}, or

a \grammarterm{cv-qualifier}.

The declaration shall contain at most one \grammarterm{sb-identifier}

whose \grammarterm{identifier} is preceded by an ellipsis.

If the declaration contains any such \grammarterm{sb-identifier},

it shall declare a templated entity\iref{temp.pre}.

\begin{example}

\begin{codeblock}

template<class T> concept C = true;

C auto [x, y] = std::pair{1, 2}; // error: constrained \grammarterm{placeholder-type-specifier}

// not permitted for structured bindings

\end{codeblock}

\end{example}

The \grammarterm{initializer} shall be

of the form ``\tcode{=} \grammarterm{assignment-expression}'',

of the form ``\tcode{\{} \grammarterm{assignment-expression} \tcode{\}}'',

or

of the form ``\tcode{(} \grammarterm{assignment-expression} \tcode{)}''.

If the \grammarterm{structured-binding-declaration} appears as

a \grammarterm{condition},

the \grammarterm{assignment-expression} shall be of non-union class type.

Otherwise,

the \grammarterm{assignment-expression} shall be of

array or non-union class type.

If the \grammarterm{decl-specifier-seq} contains the \keyword{typedef}

specifier, the declaration is a \defnx{typedef declaration}{declaration!typedef}

and each \grammarterm{declarator-id}

is declared to be a \grammarterm{typedef-name}\iref{dcl.typedef}.

\begin{note}

Such a \grammarterm{declarator-id} is

an \grammarterm{identifier}\iref{class.conv.fct}.

\end{note}

Otherwise, if the type associated with a \grammarterm{declarator-id}

is a function type\iref{dcl.fct},

the declaration is a \defnx{function declaration}{declaration!function}.

Otherwise, if the type associated with a \grammarterm{declarator-id}

is an object or reference type, the declaration is

an \defnx{object declaration}{declaration!object}.

Otherwise, the program is ill-formed.

\begin{example}

\begin{codeblock}

int f(), x; // OK, function declaration for \tcode{f} and object declaration for \tcode{x}

extern void g(), // OK, function declaration for \tcode{g}

y; // error: \tcode{void} is not an object type

\end{codeblock}

\end{example}

\indextext{initialization!definition and}%

An object definition causes

storage of appropriate size and alignment to be reserved and

any appropriate initialization\iref{dcl.init} to be done.

\indextext{definition!declaration as}%

Syntactic components beyond those found in the general form of

\grammarterm{simple-declaration} are added to a function declaration to make a

\grammarterm{function-definition}.

A token sequence starting with \tcode{\{} or \tcode{=}

is treated as a \grammarterm{function-body}\iref{dcl.fct.def.general}

if the type of the \grammarterm{declarator-id}\iref{dcl.meaning.general}

is a function type, and

is otherwise

treated as a \grammarterm{brace-or-equal-initializer}\iref{dcl.init.general}.

\begin{note}

If the declaration acquires a function type through template instantiation,

the program is ill-formed; see \ref{temp.spec.general}.

The function type of a function definition

cannot be specified with a \grammarterm{typedef-name}\iref{dcl.fct}.

\end{note}

A \grammarterm{nodeclspec-function-declaration} shall declare a

constructor, destructor, or conversion function.

\begin{note}

Because a member function cannot be subject to a non-defining declaration

outside of a class definition\iref{class.mfct}, a \grammarterm{nodeclspec-function-declaration}

can only be used in a \grammarterm{template-declaration}\iref{temp.pre},

\grammarterm{explicit-instantiation}\iref{temp.explicit}, or

\grammarterm{explicit-specialization}\iref{temp.expl.spec}.

\end{note}

If a \grammarterm{static_assert-message}

matches the syntactic requirements of \grammarterm{unevaluated-string},

it is an \grammarterm{unevaluated-string} and

the text of the \grammarterm{static_assert-message} is

the text of the \grammarterm{unevaluated-string}.

Otherwise, a \grammarterm{static_assert-message} shall be an expression $M$

such that

\begin{itemize}

the expression \tcode{$M$.size()} is

implicitly convertible to the type \tcode{std::size_t}, and

the expression \tcode{$M$.data()} is

implicitly convertible to the type ``pointer to \tcode{\keyword{const} \keyword{char}}''.

\end{itemize}

\indextext{\idxcode{static_assert}}%

In a \grammarterm{static_assert-declaration},

the \grammarterm{constant-expression} $E$

is contextually converted to \keyword{bool} and

the converted expression shall be a constant expression\iref{expr.const.const}.

If the value of the expression $E$ when so converted is \tcode{true} or

the expression is evaluated in the context of a template definition,

the declaration has no effect and

the \grammarterm{static_assert-message} is

an unevaluated operand\iref{term.unevaluated.operand}.

Otherwise,

the \grammarterm{static_assert-declaration} \defnx{fails}{\idxcode{static_assert}!failed} and

\begin{itemize}

the program is ill-formed, and

if the \grammarterm{static_assert-message} is

a \grammarterm{constant-expression} $M$,

\begin{itemize}

\tcode{$M$.size()} shall be a converted constant expression of

type \tcode{std::size_t} and

let $N$ denote the value of that expression,

\tcode{$M$.data()}, implicitly converted to

the type ``pointer to \tcode{\keyword{const} \keyword{char}}'',

shall be a core constant expression and let $D$ denote the converted expression,

for each $i$ where $0 \le i < N$,

\tcode{$D$[$i$]} shall be an integral constant expression, and

the text of the \grammarterm{static_assert-message} is formed by

the sequence of $N$ code units, starting at $D$, of

the ordinary literal encoding\iref{lex.charset}.

\end{itemize}

\end{itemize}

When a \grammarterm{static_assert-declaration} fails,

the resulting diagnostic message should include the text of

the \grammarterm{static_assert-message}, if one is supplied.

\begin{example}

\begin{codeblock}

static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");

static_assert(sizeof(int[2])); // OK, narrowing allowed

template <class T>

void f(T t) {

if constexpr (sizeof(T) == sizeof(int)) {

use(t);

} else {

static_assert(false, "must be int-sized");

}

}

void g(char c) {

f(0); // OK

f(c); // error on implementations where \tcode{sizeof(int) > 1}: must be \tcode{int}-sized

}

\end{codeblock}

\end{example}

For a \grammarterm{consteval-block-declaration} $D$,

the expression $E$ corresponding to $D$ is:

\begin{codeblock}

[] static consteval -> void @\grammarterm{compound-statement}@ ()

\end{codeblock}

$E$ shall be a constant expression\iref{expr.const.const}.

\begin{note}

The evaluation of the expression

corresponding to a \grammarterm{consteval-block-declaration}\iref{lex.phases}

can produce injected declarations\iref{expr.const.reflect} as side effects.

\end{note}

\begin{example}

\begin{codeblock}

struct S;

consteval {

std::meta::define_aggregate(^^S, {}); // OK

template<class T>

struct X { }; // error: local templates are not allowed

template<class T>

concept C = true; // error: local concepts are not allowed

return; // OK

}

\end{codeblock}

\end{example}

An \grammarterm{empty-declaration} has no effect.

Except where otherwise specified, the meaning of an \grammarterm{attribute-declaration}

is \impldef{meaning of attribute declaration}.

\rSec1[dcl.spec]{Specifiers}%

\rSec2[dcl.spec.general]{General}%

\indextext{specifier|(}

\indextext{specifier!declaration}%

The specifiers that can be used in a declaration are

\begin{bnf}

\nontermdef{decl-specifier}\br

storage-class-specifier\br

defining-type-specifier\br

function-specifier\br

\keyword{friend}\br

\keyword{typedef}\br

\keyword{constexpr}\br

\keyword{consteval}\br

\keyword{constinit}\br

\keyword{inline}

\end{bnf}

\begin{bnf}

\nontermdef{decl-specifier-seq}\br

decl-specifier \opt{attribute-specifier-seq}\br

decl-specifier decl-specifier-seq

\end{bnf}

The optional \grammarterm{attribute-specifier-seq} in a \grammarterm{decl-specifier-seq}

appertains to the type determined by the preceding

\grammarterm{decl-specifier}{s}\iref{dcl.meaning}. The \grammarterm{attribute-specifier-seq}

affects the type only for the declaration it appears in, not other declarations involving the

same type.

At most one of each of the \grammarterm{decl-specifier}s

\keyword{friend}, \keyword{typedef}, or \keyword{inline}

shall appear in a \grammarterm{decl-specifier-seq}.

At most one of

the \keyword{constexpr}, \keyword{consteval}, and \keyword{constinit} keywords

shall appear in a \grammarterm{decl-specifier-seq}.

\indextext{ambiguity!declaration type}%

If a \grammarterm{type-name} is encountered while parsing a \grammarterm{decl-specifier-seq},

it is interpreted as part of the \grammarterm{decl-specifier-seq} if and only if there is no

previous \grammarterm{defining-type-specifier} other than a \grammarterm{cv-qualifier} in the

\grammarterm{decl-specifier-seq}.

The sequence shall be self-consistent as

described below.

\begin{example}

\begin{codeblock}

typedef char* Pc;

static Pc; // error: name missing

\end{codeblock}

Here, the declaration \keyword{static} \tcode{Pc} is ill-formed because no

name was specified for the static variable of type \tcode{Pc}. To get a

variable called \tcode{Pc}, a \grammarterm{type-specifier} (other than

\keyword{const} or \tcode{volatile}) has to be present to indicate that

the \grammarterm{typedef-name} \tcode{Pc} is the name being (re)declared,

rather than being part of the \grammarterm{decl-specifier} sequence. For

another example,

\begin{codeblock}

void f(const Pc); // \tcode{void f(char* const)} (not \tcode{const char*})

void g(const int Pc); // \tcode{void g(const int)}

\end{codeblock}

\end{example}

\indextext{\idxcode{signed}!typedef@\tcode{typedef} and}%

\indextext{\idxcode{unsigned}!typedef@\tcode{typedef} and}%

\indextext{\idxcode{long}!typedef@\tcode{typedef} and}%

\indextext{\idxcode{short}!typedef@\tcode{typedef} and}%

\begin{note}

Since \tcode{signed}, \tcode{unsigned}, \tcode{long}, and \tcode{short}

by default imply \tcode{int}, a \grammarterm{type-name} appearing after one

of those specifiers is treated as the name being (re)declared.

\begin{example}

\begin{codeblock}

void h(unsigned Pc); // \tcode{void h(unsigned int)}

void k(unsigned int Pc); // \tcode{void k(unsigned int)}

\end{codeblock}

\end{example}

\end{note}

\rSec2[dcl.stc]{Storage class specifiers}%

\indextext{specifier!storage class}%

\indextext{declaration!storage class}%

\indextext{\idxcode{static}}%

\indextext{\idxcode{thread_local}}%

\indextext{\idxcode{extern}}%

\indextext{\idxcode{mutable}}

The storage class specifiers are

\begin{bnf}

\nontermdef{storage-class-specifier}\br

\keyword{static}\br

\keyword{thread_local}\br

\keyword{extern}\br

\keyword{mutable}

\end{bnf}

At most one \grammarterm{storage-class-specifier} shall appear in a given

\grammarterm{decl-specifier-seq}, except that \keyword{thread_local} may appear with \keyword{static} or

\keyword{extern}. If \keyword{thread_local} appears in any declaration of

a variable it shall be present in all declarations of that entity. If a

\grammarterm{storage-class-specifier}

appears in a \grammarterm{decl-specifier-seq}, there can be no

\tcode{typedef} specifier in the same \grammarterm{decl-specifier-seq} and

the \grammarterm{init-declarator-list} of the \grammarterm{simple-declaration} or

the \grammarterm{member-declarator-list} of the \grammarterm{member-declaration}

shall not be empty (except for an anonymous union declared in a namespace scope\iref{class.union.anon}).

The \grammarterm{storage-class-specifier} applies to the name declared by each

\grammarterm{init-declarator} in the list and not to any names declared by

other specifiers.

\begin{note}

See \ref{temp.expl.spec} and \ref{temp.explicit} for restrictions

in explicit specializations and explicit instantiations, respectively.

\end{note}

\begin{note}

A variable declared without a \grammarterm{storage-class-specifier}

at block scope or declared as a function parameter

has \deflinkx{automatic storage duration}{storage duration!automatic}{basic.stc.auto} by default.

\end{note}

The \keyword{thread_local} specifier

indicates that the named entity has thread storage duration\iref{basic.stc.thread}. It

shall be applied only

to the declaration of a variable of namespace or block scope,

to a structured binding declaration\iref{dcl.struct.bind}, or

to the declaration of a static data member.

When \keyword{thread_local} is applied to a variable of block scope the

\grammarterm{storage-class-specifier} \keyword{static} is implied if no other

\grammarterm{storage-class-specifier} appears in the

\grammarterm{decl-specifier-seq}.

\indextext{restriction!\idxcode{static}}%

The \keyword{static} specifier shall be applied only

to the declaration of a variable or function,

to a structured binding declaration\iref{dcl.struct.bind}, or

to the declaration of an anonymous union\iref{class.union.anon}.

There can be no

\keyword{static} function declarations within a block, nor any

\keyword{static} function parameters. A \tcode{static} specifier used in

the declaration of a variable declares the variable to have static storage

duration\iref{basic.stc.static}, unless accompanied by the

\keyword{thread_local} specifier, which declares the variable to have thread

storage duration\iref{basic.stc.thread}. A \keyword{static} specifier can be

used in declarations of class members;~\ref{class.static} describes its

effect.

\indextext{\idxcode{static}!linkage of}%

For the linkage of a name declared with a \keyword{static} specifier,

see~\ref{basic.link}.

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

The \keyword{extern} specifier shall be applied only to the declaration of a variable

or function. The \keyword{extern} specifier shall not be used in the

declaration of a class member or function parameter.

\indextext{\idxcode{extern}!linkage of}%

\indextext{consistency!linkage}%

For the linkage of a name declared with an \keyword{extern} specifier,

see~\ref{basic.link}.

\begin{note}

The \keyword{extern} keyword can also be used in

\grammarterm{explicit-instantiation}{s} and

\grammarterm{linkage-specification}{s}, but it is not a

\grammarterm{storage-class-specifier} in such contexts.

\end{note}

All declarations for a given entity shall give its name the same linkage.

\begin{note}

The linkage given by some declarations is affected by previous declarations.

Overloads are distinct entities.

\end{note}

\begin{example}

\begin{codeblock}

static char* f(); // \tcode{f()} has internal linkage

char* f() // \tcode{f()} still has internal linkage

{ @\commentellip@ }

char* g(); // \tcode{g()} has external linkage

static char* g() // error: inconsistent linkage

{ @\commentellip@ }

void h();

inline void h(); // external linkage

inline void l();

void l(); // external linkage

inline void m();

extern void m(); // external linkage

static void n();

inline void n(); // internal linkage

static int a; // \tcode{a} has internal linkage

int a; // error: two definitions

static int b; // \tcode{b} has internal linkage

extern int b; // \tcode{b} still has internal linkage

int c; // \tcode{c} has external linkage

static int c; // error: inconsistent linkage

extern int d; // \tcode{d} has external linkage

static int d; // error: inconsistent linkage

\end{codeblock}

\end{example}

\indextext{declaration!forward}%

The name of a declared but undefined class can be used in an

\keyword{extern} declaration. Such a declaration can only be used in ways

that do not require a complete class type.

\begin{example}

\begin{codeblock}

struct S;

extern S a;

extern S f();

extern void g(S);

void h() {

g(a); // error: \tcode{S} is incomplete

f(); // error: \tcode{S} is incomplete

}

\end{codeblock}

\end{example}

The \keyword{mutable} specifier shall appear only in the declaration of

a non-static data member\iref{class.mem}

whose type is neither const-qualified nor a reference type.

\begin{example}

\begin{codeblock}

class X {

mutable const int* p; // OK

mutable int* const q; // error

};

\end{codeblock}

\end{example}

\begin{note}

The \keyword{mutable} specifier on a class data member nullifies a

\keyword{const} specifier applied to the containing class object and

permits modification of the mutable class member even though the rest of

the object is const\iref{basic.type.qualifier,dcl.type.cv}.

\end{note}

\rSec2[dcl.fct.spec]{Function specifiers}%

\indextext{specifier!function}%

\indextext{function|seealso{friend function}}

\indextext{function|seealso{member function}}

\indextext{function|seealso{inline function}}

\indextext{function|seealso{virtual function}}

A

\grammarterm{function-specifier}

can be used only in a function declaration.

At most one \grammarterm{explicit-specifier} and

at most one \keyword{virtual} keyword shall appear in

a \grammarterm{decl-specifier-seq}.

\begin{bnf}

\nontermdef{function-specifier}\br

\keyword{virtual}\br

explicit-specifier

\end{bnf}

\begin{bnf}

\nontermdef{explicit-specifier}\br

\keyword{explicit} \terminal{(} constant-expression \terminal{)}\br

\keyword{explicit}

\end{bnf}

\indextext{specifier!\idxcode{virtual}}%

The \keyword{virtual} specifier shall be used only in the initial

declaration of a non-static member function; see~\ref{class.virtual}.

\indextext{specifier!\idxcode{explicit}}%

An \grammarterm{explicit-specifier} shall be used only in the declaration of

a constructor or conversion function within its class definition;

see~\ref{class.conv.ctor} and~\ref{class.conv.fct}.

In an \grammarterm{explicit-specifier},

the \grammarterm{constant-expression}, if supplied, shall be a

contextually converted constant expression of type \tcode{bool}\iref{expr.const.const}.

The \grammarterm{explicit-specifier} \keyword{explicit}

without a \grammarterm{constant-expression} is equivalent to

the \grammarterm{explicit-specifier} \tcode{explicit(true)}.

If the constant expression evaluates to \tcode{true},

the function is explicit. Otherwise, the function is not explicit.

A \tcode{(} token that follows \keyword{explicit} is parsed as

part of the \grammarterm{explicit-specifier}.

\begin{example}

\begin{codeblock}

struct S {

explicit(sizeof(char[2])) S(char); // error: narrowing conversion of value 2 to type \keyword{bool}

explicit(sizeof(char)) S(bool); // OK, conversion of value 1 to type \keyword{bool} is non-narrowing

};

\end{codeblock}

\end{example}

\rSec2[dcl.typedef]{The \keyword{typedef} specifier}%

\indextext{specifier!\idxcode{typedef}}

Declarations containing the \grammarterm{decl-specifier} \keyword{typedef}

declare \defnadjx{type}{aliases}{alias}.

The \keyword{typedef} specifier shall not be

combined in a \grammarterm{decl-specifier-seq} with any other kind of

specifier except a \grammarterm{defining-type-specifier}, and it shall not be used in the

\grammarterm{decl-specifier-seq} of a

\grammarterm{parameter-declaration}\iref{dcl.fct} nor in the

\grammarterm{decl-specifier-seq} of a

\grammarterm{function-definition}\iref{dcl.fct.def}.

If a \keyword{typedef} specifier appears in a declaration without a \grammarterm{declarator},

the program is ill-formed.

\begin{bnf}

\nontermdef{typedef-name}\br

identifier\br

simple-template-id

\end{bnf}

A name declared with the \keyword{typedef} specifier becomes a

\grammarterm{typedef-name}.

The underlying entity of the type alias is

the type associated with the \grammarterm{identifier}\iref{dcl.decl}

or \grammarterm{simple-template-id}\iref{temp.pre}.

\indextext{declaration!typedef@\tcode{typedef} as type}%

\indextext{equivalence!type}%

A \grammarterm{typedef-name} does not introduce a new type the way a class

declaration\iref{class.name} or enum declaration\iref{dcl.enum} does.

\begin{example}

After

\begin{codeblock}

typedef int MILES, *KLICKSP;

\end{codeblock}

the constructions

\begin{codeblock}

MILES distance;

extern KLICKSP metricp;

\end{codeblock}

are all correct declarations; the type of \tcode{distance} is

\tcode{int} and that of \tcode{metricp} is ``pointer to \tcode{int}''.

\end{example}

A type alias can also be declared by an

\grammarterm{alias-declaration}. The \grammarterm{identifier} following the

\tcode{using} keyword is not looked up;

it becomes the \grammarterm{typedef-name} of a type alias

and the optional \grammarterm{attribute-specifier-seq} following the

\grammarterm{identifier} appertains to that type alias.

Such a type alias has the same

semantics as if it were introduced by the \keyword{typedef} specifier.

\begin{example}

\begin{codeblock}

using handler_t = void (*)(int);

extern handler_t ignore;

extern void (*ignore)(int); // redeclare \tcode{ignore}

template<class T> struct P { };

using cell = P<cell*>; // error: \tcode{cell} not found\iref{basic.scope.pdecl}

\end{codeblock}

\end{example}

The \grammarterm{defining-type-specifier-seq}

of the \grammarterm{defining-type-id} shall not define

a class or enumeration if the \grammarterm{alias-declaration}

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

\indextext{class name!\idxcode{typedef}}%

A \grammarterm{simple-template-id} is only a \grammarterm{typedef-name}

if its \grammarterm{template-name} names

an alias template or a type template template parameter.

\begin{note}

A \grammarterm{simple-template-id} that names a class template specialization

is a \grammarterm{class-name}\iref{class.name}.

If a \grammarterm{typedef-name} is used to identify the subject of an

\grammarterm{elaborated-type-specifier}\iref{dcl.type.elab}, a \link{class

definition}{class}, a \link{constructor

declaration}{class.ctor}, or a \link{destructor

declaration}{class.dtor}, the program is ill-formed.

\end{note}

\begin{example}

\begin{codeblock}

struct S {

S();

~S();

};

typedef struct S T;

S a = T(); // OK

struct T * p; // error

\end{codeblock}

\end{example}

\indextext{class name!\idxcode{typedef}}%

\indextext{enum name!\idxcode{typedef}}%

\indextext{class!unnamed}%

An unnamed class or enumeration $C$ defined in a typedef declaration has

the first \grammarterm{typedef-name}

declared by the declaration to be of type $C$

as its \defn{typedef name for linkage purposes}\iref{basic.link}.

\begin{note}

A typedef declaration involving a \grammarterm{lambda-expression}

does not itself define the associated closure type,

and so the closure type is not given a typedef name for linkage purposes.

\end{note}

\begin{example}

\begin{codeblock}

typedef struct { } *ps, S; // \tcode{S} is the typedef name for linkage purposes

typedef decltype([]{}) C; // the closure type has no typedef name for linkage purposes

\end{codeblock}

\end{example}

An unnamed class with a typedef name for linkage purposes shall not

\begin{itemize}

declare any members

other than non-static data members, member enumerations, or member classes,

have any base classes or default member initializers, or

contain a \grammarterm{lambda-expression},

\end{itemize}

and all member classes shall also satisfy these requirements (recursively).

\begin{example}

\begin{codeblock}

typedef struct {

int f() {}

} X; // error: struct with typedef name for linkage has member functions

\end{codeblock}

\end{example}

\rSec2[dcl.friend]{The \keyword{friend} specifier}%

\indextext{specifier!\idxcode{friend}}

The \keyword{friend} specifier is used to specify access to class members;

see~\ref{class.friend}.

\rSec2[dcl.constexpr]{The \keyword{constexpr} and \keyword{consteval} specifiers}%

\indextext{specifier!\idxcode{constexpr}}

\indextext{specifier!\idxcode{consteval}}

The \keyword{constexpr} specifier shall be applied only to

the definition of a variable or variable template,

a structured binding declaration, or

the declaration of a function or function template.

The \keyword{consteval} specifier shall be applied only to

the declaration of a function or function template.

A function or static data member

declared with the \keyword{constexpr} or \keyword{consteval} specifier

on its first declaration

is implicitly an inline function or variable\iref{dcl.inline}.

If any declaration of a function or function template has

a \keyword{constexpr} or \keyword{consteval} specifier,

then all its declarations shall contain the same specifier.

\begin{note}

An explicit specialization can differ from the template declaration

with respect to the \keyword{constexpr} or \keyword{consteval} specifier.

\end{note}

\begin{note}

Function parameters cannot be declared \keyword{constexpr}.

\end{note}

\begin{example}

\begin{codeblock}

constexpr void square(int &x); // OK, declaration

constexpr int bufsz = 1024; // OK, definition

constexpr struct pixel { // error: \tcode{pixel} is a type

int x;

int y;

constexpr pixel(int); // OK, declaration

};

constexpr pixel::pixel(int a)

: x(a), y(x) // OK, definition

{ square(x); }

constexpr pixel small(2); // error: \tcode{square} not defined, so \tcode{small(2)}

// not constant\iref{expr.const.core} so \keyword{constexpr} not satisfied

constexpr void square(int &x) { // OK, definition

x *= x;

}

constexpr pixel large(4); // OK, \tcode{square} defined

int next(constexpr int x) { // error: not for parameters

return x + 1;

}

extern constexpr int memsz; // error: not a definition

\end{codeblock}

\end{example}

A \keyword{constexpr} or \keyword{consteval} specifier

used in the declaration of a function

declares that function to be

a \defnx{constexpr function}{specifier!\idxcode{constexpr}!function}.

\begin{note}

A function declared with the \keyword{consteval} specifier

is an immediate function\iref{expr.const.imm}.

\end{note}

A destructor, an allocation function, or a deallocation function

shall not be declared with the \keyword{consteval} specifier.

\indextext{specifier!\idxcode{constexpr}!function}%

\indextext{constexpr function}%

A function is \defn{constexpr-suitable} if

it is not a coroutine\iref{dcl.fct.def.coroutine}.

Except for instantiated constexpr functions,

non-templated constexpr functions shall be constexpr-suitable.

\begin{example}

\begin{codeblock}

constexpr int square(int x)

{ return x * x; } // OK

constexpr long long_max()

{ return 2147483647; } // OK

constexpr int abs(int x) {

if (x < 0)

x = -x;

return x; // OK

}

constexpr int constant_non_42(int n) { // OK

if (n == 42) {

static int value = n;

return value;

}

return n;

}

constexpr int uninit() {

struct { int a; } s;

return s.a; // error: uninitialized read of \tcode{s.a}

}

constexpr int prev(int x)

{ return --x; } // OK

constexpr int g(int x, int n) { // OK

int r = 1;

while (--n > 0) r *= x;

return r;

}

\end{codeblock}

\end{example}

An invocation of a constexpr function in a given context

produces the same result as

an invocation of an equivalent non-constexpr function in the same context

in all respects except that

\begin{itemize}

an invocation of a constexpr function

can appear in a constant expression\iref{expr.const.core} and

copy elision is not performed in a constant expression\iref{class.copy.elision}.

\end{itemize}

\begin{note}

Declaring a function constexpr can change whether an expression