%!TEX root = std.tex

\rSec0[special]{Special member functions}

%gram: \rSec1[gram.special]{Special member functions}

%gram:

\indextext{special~member~function|see{constructor, destructor, inline function,

user-defined conversion, virtual function}}%

\indextext{\idxcode{X(X\&)}|see{copy~constructor}}%

\indextext{\~@\tcode{\tilde}|see{destructor}}%

\indextext{assignment!copy|see{assignment operator, copy}}%

\indextext{assignment!move|see{assignment operator, move}}%

\indextext{implicitly-declared~default~constructor|see{constructor, default}}

\pnum

\indextext{constructor!default}%

\indextext{constructor!copy}%

\indextext{constructor!move}%

\indextext{assignment operator!copy}%

\indextext{assignment operator!move}%

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

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

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

and destructor~(\ref{class.dtor}) are

\term{special member functions}.

\enternote The implementation will implicitly declare these member functions for some class

types when the program does not explicitly declare them.

The implementation will implicitly define them if they are odr-used~(\ref{basic.def.odr}).

See~\ref{class.ctor}, \ref{class.dtor} and~\ref{class.copy}. \exitnote

An implicitly-declared special member function is declared at the closing

\tcode{\}} of the \grammarterm{class-specifier}.

Programs shall not define implicitly-declared special member functions.

\pnum

Programs may explicitly refer to implicitly-declared special member functions.

\enterexample

a program may explicitly call, take the address of or form a pointer to member

to an implicitly-declared special member function.

\begin{codeblock}

struct A { }; // implicitly declared \tcode{A::operator=}

struct B : A {

B& operator=(const B &);

};

B& B::operator=(const B& s) {

this->A::operator=(s); // well formed

return *this;

}

\end{codeblock}

\exitexample

\pnum

\enternote

The special member functions affect the way objects of class type are created,

copied, moved, and destroyed, and how values can be converted to values of other types.

Often such special member functions are called implicitly.

\exitnote

\pnum

\indextext{access~control!member~function~and}%

Special member functions obey the usual access rules (Clause~\ref{class.access}).

\enterexample

declaring a constructor

\tcode{protected}

ensures that only derived classes and friends can create objects using it.

\exitexample

\pnum

For a class, its non-static data members, its non-virtual direct base classes,

and, if the class is not abstract~(\ref{class.abstract}), its virtual base

classes are called its \term{potentially constructed subobjects}.

\rSec1[class.ctor]{Constructors}%

\indextext{constructor}

\pnum

Constructors do not have names.

In a declaration of a constructor, the \grammarterm{declarator} is a

function declarator~(\ref{dcl.fct}) of the form

\begin{ncbnf}

ptr-declarator \terminal{(} parameter-declaration-clause \terminal{)} exception-specification\opt attribute-specifier-seq\opt

\end{ncbnf}

where the \grammarterm{ptr-declarator} consists solely of an

\grammarterm{id-expression}, an optional \grammarterm{attribute-specifier-seq},

and optional surrounding parentheses, and the \grammarterm{id-expression} has

one of the following forms:

\begin{itemize}

\item

in a \grammarterm{member-declaration} that belongs to the

\grammarterm{member-specification} of a class but is not a friend

declaration~(\ref{class.friend}), the \grammarterm{id-expression} is the

injected-class-name (Clause~\ref{class}) of the immediately-enclosing class;

\item

in a \grammarterm{member-declaration} that belongs to the

\grammarterm{member-specification} of a class template but is not a friend

declaration, the \grammarterm{id-expression} is a \grammarterm{class-name} that

names the current instantiation~(\ref{temp.dep.type}) of the

immediately-enclosing class template; or

\item

in a declaration at namespace scope or in a friend declaration, the

\grammarterm{id-expression} is a \grammarterm{qualified-id} that names a

constructor~(\ref{class.qual}).

\end{itemize}

The \grammarterm{class-name} shall not be a \grammarterm{typedef-name}. In a constructor

declaration, each \grammarterm{decl-specifier} in the optional

\grammarterm{decl-specifier-seq} shall be \tcode{friend}, \tcode{inline},

\tcode{explicit}, or \tcode{constexpr}.

\enterexample

\begin{codeblock}

struct S {

S(); // declares the constructor

};

S::S() { } // defines the constructor

\end{codeblock}

\exitexample

\pnum

A constructor is used to initialize objects of its class type.

Because constructors do not have names, they are never found during

name lookup; however an explicit type conversion using the functional

notation~(\ref{expr.type.conv}) will cause a constructor to be called to

initialize an object.

\enternote

For initialization of objects of class type see~\ref{class.init}.

\exitnote

\pnum

\indextext{\idxcode{const}!constructor~and}%

\indextext{\idxcode{volatile}!constructor~and}%

A constructor can be invoked for a

\tcode{const},

\tcode{volatile}

or

\tcode{const}

\tcode{volatile}

object.

\indextext{restriction!constructor}%

\tcode{const}

and

\tcode{volatile}

semantics~(\ref{dcl.type.cv}) are not applied on an object under construction.

They come into effect when the constructor for the

most derived object~(\ref{intro.object}) ends.

\pnum

\indextext{constructor!inheritance~of}%

\indextext{constructor!default}%

\indextext{constructor!non-trivial}%

A

\term{default}

constructor for a class

\tcode{X}

is a constructor of class

\tcode{X}

that either has no parameters or else each parameter that is not a

function parameter pack has a default argument.

\indextext{implicitly-declared~default~constructor}%

If there is no user-declared constructor for class

\tcode{X},

a constructor having no parameters is implicitly declared

as defaulted~(\ref{dcl.fct.def}).

An implicitly-declared default constructor is an

\tcode{inline}

\tcode{public}

member of its class.

A defaulted default constructor for class \tcode{X} is defined as deleted if:

\begin{itemize}

\item \tcode{X} is a union-like class that has a variant

member with a non-trivial default constructor,

\item any non-static data member with no \grammarterm{brace-or-equal-initializer} is of reference type,

\item any non-variant non-static data member of const-qualified type (or array

thereof) with no \grammarterm{brace-or-equal-initializer} does not have a user-provided default constructor,

\item \tcode{X} is a union and all of its variant members are of const-qualified

type (or array thereof),

\item \tcode{X} is a non-union class and all members of any anonymous union member are

of const-qualified type (or array thereof),

\item any potentially constructed subobject, except for a non-static data member

with a \grammarterm{brace-or-equal-initializer}, has

class type \tcode{M} (or array thereof) and either \tcode{M}

has no default constructor or overload resolution

(\ref{over.match}) as applied to \tcode{M}'s default

constructor results in an ambiguity or in a function that is deleted or

inaccessible from the defaulted default constructor, or

\item any potentially constructed subobject has a type

with a destructor that is deleted or inaccessible from the defaulted default

constructor.

\end{itemize}

A default constructor is

trivial

if it is not user-provided and if:

\begin{itemize}

\item

its class has no virtual functions~(\ref{class.virtual}) and no virtual base

classes~(\ref{class.mi}), and

\item no non-static data member of its class has a \grammarterm{brace-or-equal-initializer}, and

\item

all the direct base classes of its class have trivial default constructors, and

\item

for all the non-static data members of its class that are of class

type (or array thereof), each such class has a trivial default constructor.

\end{itemize}

Otherwise, the default constructor is

\grammarterm{non-trivial}.

\pnum

\indextext{constructor!implicitly defined}%

A default constructor

that is defaulted and not defined as deleted

is

\term{implicitly defined}

when it is odr-used~(\ref{basic.def.odr})

to create an object of its class type~(\ref{intro.object})

or when it is explicitly defaulted after its first declaration.

The implicitly-defined default constructor performs the set of

initializations of the class that would be performed by a user-written default

constructor for that class with no

\grammarterm{ctor-initializer}~(\ref{class.base.init}) and an empty

\grammarterm{compound-statement}.

If that user-written default constructor would be ill-formed,

the program is ill-formed.

If that user-written default constructor would satisfy the requirements

of a \tcode{constexpr} constructor~(\ref{dcl.constexpr}), the implicitly-defined

default constructor is \tcode{constexpr}.

Before the defaulted default constructor for a class is

implicitly defined,

all the non-user-provided default constructors for its base classes and

its non-static data members shall have been implicitly defined.

\enternote

An implicitly-declared default constructor has an

exception specification~(\ref{except.spec}).

An explicitly-defaulted definition might have an

implicit exception specification, see~\ref{dcl.fct.def}.

\exitnote

\pnum

\indextext{constructor!implicitly called}%

Default constructors are called implicitly to create class objects of static, thread,

or automatic storage duration~(\ref{basic.stc.static}, \ref{basic.stc.thread}, \ref{basic.stc.auto}) defined

without an initializer~(\ref{dcl.init}),

are called to create class objects of dynamic storage duration~(\ref{basic.stc.dynamic}) created by a

\grammarterm{new-expression}

in which the

\grammarterm{new-initializer}

is omitted~(\ref{expr.new}), or

are called when the explicit type conversion syntax~(\ref{expr.type.conv}) is

used.

A program is ill-formed if the default constructor for an object

is implicitly used and the constructor is not accessible (Clause~\ref{class.access}).

\pnum

\enternote

\indextext{order~of~execution!base~class constructor}%

\indextext{order~of~execution!member constructor}%

\ref{class.base.init} describes the order in which constructors for base

classes and non-static data members are called and

describes how arguments can be specified for the calls to these constructors.

\exitnote

\pnum

\indextext{restriction!constructor}%

A

\tcode{return}

statement in the body of a constructor shall not specify a return value.

\indextext{constructor!address~of}%

The address of a constructor shall not be taken.

\pnum

\indextext{object!unnamed}%

\indextext{constructor!explicit call}%

A functional notation type conversion~(\ref{expr.type.conv}) can be used

to create new objects of its type.

\enternote

The syntax looks like an explicit call of the constructor.

\exitnote

\enterexample

\indextext{example!constructor}%

\begin{codeblock}

complex zz = complex(1,2.3);

cprint( complex(7.8,1.2) );

\end{codeblock}

\exitexample

\pnum

An object created in this way is unnamed.

\enternote

\ref{class.temporary} describes the lifetime of temporary objects.

\exitnote

\enternote

Explicit constructor calls do not yield lvalues, see~\ref{basic.lval}.

\exitnote

\pnum

\enternote

\indextext{member function!constructor~and}%

some language constructs have special semantics when used during construction;

see~\ref{class.base.init} and~\ref{class.cdtor}.

\exitnote

\pnum

During the construction of a

\tcode{const}

object, if the value of the object or any of its subobjects is

accessed through a glvalue that is not obtained, directly or indirectly, from

the constructor's

\tcode{this}

pointer, the value of the object or subobject thus obtained is unspecified.

\enterexample

\begin{codeblock}

struct C;

void no_opt(C*);

struct C {

int c;

C() : c(0) { no_opt(this); }

};

const C cobj;

void no_opt(C* cptr) {

int i = cobj.c * 100; // value of \tcode{cobj.c} is unspecified

cptr->c = 1;

cout << cobj.c * 100 // value of \tcode{cobj.c} is unspecified

<< '\n';

}

\end{codeblock}

\exitexample

\rSec1[class.temporary]{Temporary objects}

\pnum

\indextext{object~temporary|see{temporary}}%

\indextext{temporary}%

\indextext{optimization~of~temporary|see{elimination~of~temporary}}%

\indextext{temporary!elimination~of}%

\indextext{temporary!implementation-defined~generation~of}%

Temporaries of class type are created in various contexts:

binding a reference to a prvalue~(\ref{dcl.init.ref}),

returning a prvalue~(\ref{stmt.return}),

a conversion that creates a prvalue~(\ref{conv.lval}, \ref{expr.static.cast},

\ref{expr.const.cast}, \ref{expr.cast}),

throwing an exception~(\ref{except.throw}), and in some initializations~(\ref{dcl.init}).

\enternote

The lifetime of exception objects is described in~\ref{except.throw}.

\exitnote

Even when the creation of the temporary object is

unevaluated (Clause~\ref{expr}) or otherwise

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

all the semantic restrictions shall be respected as if the temporary object

had been created and later destroyed.

\enternote

This includes accessibility~(\ref{class.access}) and whether it is deleted,

for the constructor selected and for the destructor. However, in the special

case of a function call used as the operand of a

\grammarterm{decltype-specifier}~(\ref{expr.call}), no temporary is introduced,

so the foregoing does not apply to the prvalue of any such function call.

\exitnote

\pnum

\enterexample Consider the following code:

\begin{codeblock}

class X {

public:

X(int);

X(const X&);

X& operator=(const X&);

~X();

};

class Y {

public:

Y(int);

Y(Y&&);

~Y();

};

X f(X);

Y g(Y);

void h() {

X a(1);

X b = f(X(2));

Y c = g(Y(3));

a = f(a);

}

\end{codeblock}

\indextext{class~object~copy|see{copy~constructor}}%

\indextext{constructor!copy}%

An implementation might use a temporary in which to construct

\tcode{X(2)}

before passing it to

\tcode{f()}

using

\tcode{X}'s

copy constructor; alternatively,

\tcode{X(2)}

might be constructed in the space used to hold the argument.

Likewise, an implementation might use a temporary in which to construct

\tcode{Y(3)} before passing it to \tcode{g()} using \tcode{Y}'s move constructor;

alternatively, \tcode{Y(3)} might be constructed in the space used to hold the argument.

Also, a temporary might be used to hold the result of

\tcode{f(X(2))}

before copying it to

\tcode{b}

using

\tcode{X}'s

copy constructor; alternatively,

\tcode{f()}'s

result might be constructed in

\tcode{b}.

Likewise, a temporary might be used to hold the result of \tcode{g(Y(3))} before

moving it to \tcode{c} using \tcode{Y}'s move constructor; alternatively, \tcode{g()}'s

result might be constructed in \tcode{c}.

On the other hand, the expression

\tcode{a=f(a)}

requires a temporary for

the result of \tcode{f(a)}, which is then assigned to \tcode{a}.

\exitexample

\pnum

\indextext{temporary!constructor~for}%

\indextext{temporary!destructor~for}%

\indextext{temporary!destruction~of}%

When an implementation introduces a temporary object of a class that has a

non-trivial constructor~(\ref{class.ctor}, \ref{class.copy}), it shall ensure that

a constructor is called for the temporary object.

Similarly, the destructor shall be called for a temporary with a non-trivial

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

Temporary objects are destroyed as the last step

in evaluating

the full-expression~(\ref{intro.execution})

that (lexically) contains the point where

they were created.

This is true even if that evaluation ends in throwing an exception.

The

\indextext{value computation}%

value computations and

\indextext{side effects}%

side effects of destroying a temporary object

are associated only with the full-expression, not with any specific

subexpression.

\pnum

\indextext{initializer!temporary~and declarator}%

\indextext{temporary!order~of destruction~of}%

There are two contexts in which temporaries are destroyed at a different

point than the end of the full-expression.

The first context is when a default constructor is called to initialize an

element of an array. If the constructor has one or more default arguments,

the destruction of every temporary created in

a default argument is sequenced before the construction of the next array element, if any.

\pnum

The second context is when a reference is bound to a

temporary.\footnote{The same rules apply to initialization of an

\tcode{initializer_list} object~(\ref{dcl.init.list}) with its

underlying temporary array}

The temporary to which the reference is bound or the temporary

that is the complete object of a subobject to which the reference is bound

persists for the lifetime of the reference except:

\begin{itemize}

\item A temporary object bound to a reference parameter in a function call~(\ref{expr.call})

persists until the completion of the full-expression containing the call.

\item The lifetime of a temporary bound to the returned value in a function return statement~(\ref{stmt.return}) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.

\item A temporary bound to a reference in a \grammarterm{new-initializer}~(\ref{expr.new}) persists until the completion of the full-expression containing the \grammarterm{new-initializer}. \enterexample

\begin{codeblock}

struct S { int mi; const std::pair<int,int>& mp; };

S a { 1, {2,3} };

S* p = new S{ 1, {2,3} }; // Creates dangling reference

\end{codeblock}

\exitexample \enternote This may introduce a dangling reference, and implementations are encouraged to issue a warning in such a case. \exitnote

\end{itemize}

The destruction of a temporary whose lifetime is not extended by being

bound to a reference is sequenced before the destruction of every

temporary which is constructed earlier in the same full-expression.

If the lifetime of two or more temporaries to which references are bound ends

at the same point,

these temporaries are destroyed at that point in the reverse order of the

completion of their construction.

In addition, the destruction of temporaries bound to references shall

take into account the ordering of destruction of objects with static, thread, or

automatic storage duration~(\ref{basic.stc.static}, \ref{basic.stc.thread}, \ref{basic.stc.auto});

that is, if

\tcode{obj1}

is an object with the same storage duration as the temporary and

created before the temporary is created

the temporary shall be destroyed before

\tcode{obj1}

is destroyed;

if

\tcode{obj2}

is an object with the same storage duration as the temporary and

created after the temporary is created

the temporary shall be destroyed after

\tcode{obj2}

is destroyed.

\enterexample

\begin{codeblock}

struct S {

S();

S(int);

friend S operator+(const S&, const S&);

~S();

};

S obj1;

const S& cr = S(16)+S(23);

S obj2;

\end{codeblock}

the expression

\tcode{S(16) + S(23)}

creates three temporaries:

a first temporary

\tcode{T1}

to hold the result of the expression

\tcode{S(16)},

a second temporary

\tcode{T2}

to hold the result of the expression

\tcode{S(23)},

and a third temporary

\tcode{T3}

to hold the result of the addition of these two expressions.

The temporary

\tcode{T3}

is then bound to the reference

\tcode{cr}.

It is unspecified whether

\tcode{T1}

or

\tcode{T2}

is created first.

On an implementation where

\tcode{T1}

is created before

\tcode{T2},

\tcode{T2}

shall be destroyed before

\tcode{T1}.

The temporaries

\tcode{T1}

and

\tcode{T2}

are bound to the reference parameters of

\tcode{operator+};

these temporaries are destroyed at the end of the full-expression

containing the call to

\tcode{operator+}.

The temporary

\tcode{T3}

bound to the reference

\tcode{cr}

is destroyed at the end of

\tcode{cr}'s

lifetime, that is, at the end of the program.

In addition, the order in which

\tcode{T3}

is destroyed takes into account the destruction order of other objects with

static storage duration.

That is, because

\tcode{obj1}

is constructed before

\tcode{T3},

and

\tcode{T3}

is constructed before

\tcode{obj2},

\tcode{obj2}

shall be destroyed before

\tcode{T3},

and

\tcode{T3}

shall be destroyed before

\tcode{obj1}.

\exitexample

\rSec1[class.conv]{Conversions}

\pnum

\indextext{conversion!class}%

\indextext{conversion!user-defined}%

\indextext{constructor, conversion by|see{conversion, user-defined}}%

\indextext{conversion~function|see{conversion, user-defined}}%

\indextext{conversion!implicit}%

Type conversions of class objects can be specified by constructors and

by conversion functions.

These conversions are called

\term{user-defined conversions}

and are used for implicit type conversions (Clause~\ref{conv}),

for initialization~(\ref{dcl.init}),

and for explicit type conversions~(\ref{expr.cast}, \ref{expr.static.cast}).

\pnum

User-defined conversions are applied only where they are unambiguous~(\ref{class.member.lookup}, \ref{class.conv.fct}).

Conversions obey the access control rules (Clause~\ref{class.access}).

Access control is applied after ambiguity resolution~(\ref{basic.lookup}).

\pnum

\enternote

See~\ref{over.match} for a discussion of the use of conversions in function calls

as well as examples below.

\exitnote

\pnum

\indextext{conversion!implicit user-defined}%

At most one user-defined conversion (constructor or conversion function)

is implicitly applied to a single value.

\enterexample

\begin{codeblock}

struct X {

operator int();

};

struct Y {

operator X();

};

Y a;

int b = a; // error

// \tcode{a.operator X().operator int()} not tried

int c = X(a); // OK: \tcode{a.operator X().operator int()}

\end{codeblock}

\exitexample

\pnum

User-defined conversions are used implicitly only if they are unambiguous.

\indextext{name~hiding!user-defined conversion~and}%

A conversion function in a derived class does not hide a conversion function

in a base class unless the two functions convert to the same type.

Function overload resolution~(\ref{over.match.best}) selects the best

conversion function to perform the conversion.

\enterexample

\begin{codeblock}

struct X {

operator int();

};

struct Y : X {

operator char();

};

void f(Y& a) {

if (a) { // ill-formed:

// \tcode{X::operator int()} or \tcode{Y::operator char()}

}

}

\end{codeblock}

\exitexample

\rSec2[class.conv.ctor]{Conversion by constructor}%

\indextext{conversion!user-defined}

\pnum

A constructor declared without the

\grammarterm{function-specifier}

\tcode{explicit}

specifies a conversion from

the types of its parameters

to the type of its class.

Such a constructor is called a

\indexdefn{constructor!converting}%

\term{converting constructor}.

\enterexample

\indextext{Jessie}%

\begin{codeblock}

struct X {

X(int);

X(const char*, int =0);

X(int, int);

};

void f(X arg) {

X a = 1; // \tcode{a = X(1)}

X b = "Jessie"; // \tcode{b = X("Jessie",0)}

a = 2; // \tcode{a = X(2)}

f(3); // \tcode{f(X(3))}

f({1, 2}); // \tcode{f(X(1,2))}

}

\end{codeblock}

\exitexample

\pnum

An explicit constructor constructs objects just like non-explicit

constructors, but does so only where the direct-initialization syntax~(\ref{dcl.init}) or where casts~(\ref{expr.static.cast}, \ref{expr.cast}) are explicitly

used.

A default constructor may be an explicit constructor; such a constructor

will be used to perform default-initialization

or value-initialization~(\ref{dcl.init}).

\enterexample

\begin{codeblock}

struct Z {

explicit Z();

explicit Z(int);

explicit Z(int, int);

};

Z a; // OK: default-initialization performed

Z a1 = 1; // error: no implicit conversion

Z a3 = Z(1); // OK: direct initialization syntax used

Z a2(1); // OK: direct initialization syntax used

Z* p = new Z(1); // OK: direct initialization syntax used

Z a4 = (Z)1; // OK: explicit cast used

Z a5 = static_cast<Z>(1); // OK: explicit cast used

Z a6 = { 3, 4 }; // error: no implicit conversion

\end{codeblock}

\exitexample

\pnum

A

non-explicit

copy/move constructor~(\ref{class.copy}) is a converting constructor.

An implicitly-declared copy/move constructor is not an explicit constructor;

it may be called for implicit type conversions.

\rSec2[class.conv.fct]{Conversion functions}%

\indextext{function!conversion}%

\indextext{fundamental~type~conversion|see{conversion, user-defined}}%

\indextext{conversion!user-defined}%

\indextext{conversion operator|see{conversion, user defined}}

\pnum

A member function of a class \tcode{X} having no parameters with a name of the form

\begin{bnf}

\nontermdef{conversion-function-id}\br

\terminal{operator} conversion-type-id

\end{bnf}

\begin{bnf}

\nontermdef{conversion-type-id}\br

type-specifier-seq conversion-declarator\opt

\end{bnf}

\begin{bnf}

\nontermdef{conversion-declarator}\br

ptr-operator conversion-declarator\opt

\end{bnf}

specifies a conversion from

\tcode{X}

to the type specified by the

\grammarterm{conversion-type-id}.

Such functions are called conversion functions.

No return type can be specified.

\indextext{conversion!type~of}%

If a conversion function is a member function, the type of the conversion function~(\ref{dcl.fct}) is

``function taking no parameter returning

\grammarterm{conversion-type-id}''.

A conversion function is never used to convert a (possibly cv-qualified) object

to the (possibly cv-qualified) same object type (or a reference to it),

to a (possibly cv-qualified) base class of that type (or a reference to it),

or to (possibly cv-qualified) void.\footnote{These conversions are considered

as standard conversions for the purposes of overload resolution~(\ref{over.best.ics}, \ref{over.ics.ref}) and therefore initialization~(\ref{dcl.init}) and explicit casts~(\ref{expr.static.cast}). A conversion to \tcode{void} does not invoke any conversion function~(\ref{expr.static.cast}).

Even though never directly called to perform a conversion,

such conversion functions can be declared and can potentially

be reached through a call to a virtual conversion function in a base class.}

\enterexample

\begin{codeblock}

struct X {

operator int();

};

void f(X a) {

int i = int(a);

i = (int)a;

i = a;

}

\end{codeblock}

In all three cases the value assigned will be converted by

\tcode{X::operator int()}.

\exitexample

\pnum

A conversion function may be explicit~(\ref{dcl.fct.spec}), in which case it is only considered as a user-defined conversion for direct-initialization~(\ref{dcl.init}). Otherwise, user-defined conversions are not restricted to use in assignments and initializations.

\enterexample

\begin{codeblock}

class Y { };

struct Z {

explicit operator Y() const;

};

void h(Z z) {

Y y1(z); // OK: direct-initialization

Y y2 = z; // ill-formed: copy-initialization

Y y3 = (Y)z; // OK: cast notation

}

void g(X a, X b) {

int i = (a) ? 1+a : 0;

int j = (a&&b) ? a+b : i;

if (a) {

}

}

\end{codeblock}

\exitexample

\pnum

The

\grammarterm{conversion-type-id}

shall not represent a function type nor an array type.

The

\grammarterm{conversion-type-id}

in a

\grammarterm{conversion-function-id}

is the longest possible sequence of

\grammarterm{conversion-declarator}{s}.

\enternote

This prevents ambiguities between the declarator operator * and its expression

counterparts.

\enterexample

\begin{codeblock}

&ac.operator int*i; // syntax error:

// parsed as: \tcode{\&(ac.operator int *)i}

// not as: \tcode{\&(ac.operator int)*i}

\end{codeblock}

The \tcode{*} is the pointer declarator and not the multiplication operator.

\exitexample

\exitnote

\pnum

\indextext{conversion!inheritance~of user-defined}%

Conversion functions are inherited.

\pnum

\indextext{conversion!virtual user-defined}%

Conversion functions can be virtual.

\pnum

\indextext{conversion!static user-defined}%

Conversion functions cannot be declared

\tcode{static}.

\pnum

\indextext{conversion!deduced~return~type~of user-defined}%

A conversion function template shall not have a

deduced return type~(\ref{dcl.spec.auto}).

\rSec1[class.dtor]{Destructors}%

\indextext{destructor}

\pnum

In a declaration of a destructor, the \grammarterm{declarator} is a

function declarator~(\ref{dcl.fct}) of the form

\begin{ncbnf}

ptr-declarator \terminal{(} parameter-declaration-clause \terminal{)} exception-specification\opt attribute-specifier-seq\opt

\end{ncbnf}

where the \grammarterm{ptr-declarator} consists solely of an

\grammarterm{id-expression}, an optional \grammarterm{attribute-specifier-seq},

and optional surrounding parentheses, and the \grammarterm{id-expression} has

one of the following forms:

\begin{itemize}

\item

in a \grammarterm{member-declaration} that belongs to the

\grammarterm{member-specification} of a class but is not a friend

declaration~(\ref{class.friend}), the \grammarterm{id-expression} is

\tcode{\~}\grammarterm{class-name} and the \grammarterm{class-name} is the

injected-class-name (Clause~\ref{class}) of the immediately-enclosing class;

\item

in a \grammarterm{member-declaration} that belongs to the

\grammarterm{member-specification} of a class template but is not a friend

declaration, the \grammarterm{id-expression} is

\tcode{\~}\grammarterm{class-name} and the \grammarterm{class-name} names the

current instantiation~(\ref{temp.dep.type}) of the immediately-enclosing class template; or

\item

in a declaration at namespace scope or in a friend declaration, the

\grammarterm{id-expression} is \grammarterm{nested-name-specifier}

\tcode{\~}\grammarterm{class-name} and the \grammarterm{class-name} names the

same class as the \grammarterm{nested-name-specifier}.

\end{itemize}

The \grammarterm{class-name} shall not be a \grammarterm{typedef-name}. A

destructor shall take no arguments (\ref{dcl.fct}). In a destructor

declaration, each \grammarterm{decl-specifier} of the optional

\grammarterm{decl-specifier-seq} shall be \tcode{friend}, \tcode{inline}, or

\tcode{virtual}.

\pnum

A destructor is used to destroy objects of its class type.

\indextext{restriction!destructor}%

The address of a destructor shall not be taken.

\indextext{\idxcode{const}!destructor~and}%

\indextext{\idxcode{volatile}!destructor~and}%

A destructor can be invoked for a

\tcode{const},

\tcode{volatile}

or

\tcode{const}

\tcode{volatile}

object.

\tcode{const}

and

\tcode{volatile}

semantics~(\ref{dcl.type.cv}) are not applied on an object under destruction.

They stop being in effect when the destructor for the

most derived object~(\ref{intro.object}) starts.

\pnum

A declaration of a destructor that does not have an \grammarterm{exception-specification}

has the same exception specification as if had been implicitly declared~(\ref{except.spec}).

\pnum

\indextext{generated~destructor|see{destructor, default}}%

\indextext{destructor!default}%

\indextext{destructor!non-trivial}%

If a class has no user-declared

destructor, a destructor is implicitly

declared as defaulted~(\ref{dcl.fct.def}).

An implicitly-declared destructor is an

\tcode{inline}

\tcode{public}

member of its class.

\pnum

A defaulted destructor for a class

\tcode{X} is defined as deleted if:

\begin{itemize}

\item \tcode{X} is a union-like class that has a variant

member with a non-trivial destructor,

\item any potentially constructed subobject has class type

\tcode{M} (or array thereof) and

\tcode{M} has a deleted destructor or a destructor

that is inaccessible from the defaulted destructor,

\item or, for a virtual destructor, lookup of the non-array deallocation

function results in an ambiguity or in a function that is deleted or

inaccessible from the defaulted destructor.

\end{itemize}

A destructor is trivial if it is not user-provided and if:

\begin{itemize}

\item the destructor is not \tcode{virtual},

\item all of the direct base classes of its class have trivial destructors, and

\item for all of the non-static data members of its class that are of class

type (or array thereof), each such class has a trivial destructor.

\end{itemize}

Otherwise, the destructor is

\grammarterm{non-trivial}.

\pnum

\indextext{destructor!implicitly defined}%

A destructor

that is defaulted and not defined as deleted

is

\term{implicitly defined}

when it is odr-used~(\ref{basic.def.odr}) to destroy an object of its class type~(\ref{basic.stc})

or when it is explicitly defaulted after its first declaration.

\pnum

Before the

defaulted destructor for a class is implicitly defined, all the non-user-provided

destructors for its base classes and its non-static data members shall have been