ISO/IEC JTC1 SC22 WG21 N2346 = 07-0206 - 2007-07-19
Lawrence Crowl, Lawrence@Crowl.org, crowl@google.com
This document is a revision of N2326 = 07-0186 - 2007-06-22.
We propose a syntax and semantics for explicitly using the default definition of a function. We also propose a syntax and semantics for deleting the definition of an otherwise visible function.
By default, C++ provides four default special member functions:
operator =
Also by default, C++ applies several global operators to classes:
operator ,
operator &
operator *
operator ->
operator ->*
operator new
operator delete
The management of defaults has several problems:
The most common encounter with these problems
is when disabling copying of a class.
The accepted technique is to
declare a private copy constructor and a private copy assignment operator,
and then fail to define either.
Unfortunately, the type becomes non-POD
and its default constructor must be explicitly written.
Further,
while diagnostics for non-friend uses are relatively straightforward,
the diagnostics for friend uses are deferred to link time
and a cryptic a missing symbol
message.
The specific problem of concisely ensuring that a class is non-copyable has received attention from Boost. See the documentation and header for the boost::noncopyable mixin. This mixin works, though not with entirely satisfactory results. First, the mechanism uses a base class, which under the current language makes the class non-POD. Second, the error messages can be somewhat misleading.
Any successful solution to these problems must be relatively easy to learn and implement.
The problems of defaults has been addressed via four primary approaches, as described below. These approaches have significant overlap, but none provide all of the capabilities of any of the others.
In N1602 Class Scope Using Declarations & private Members, Francis Glassborow notes that some programmers make problematic overloads private members to prevent their use. This approach makes using valid overloads in derived classes an error, thus inhibiting the technique. He proposes defering the error diagnostic to the point where a private overload is selected in a call.
In N1717 explicit class and default definitions, Francis Glassborow and Lois Goldthwaite define syntax to indicate that a class has none of the four default special member functions and then to explicitly resurrect the desired defaults. The paper proposed no change to the default application of global operators.
The explicit class approach has a significant syntactic burden for removing a single special member function -- remove them all and rewrite the declarations for the others. This approach encourages a view that explicit classes are inherently different, rather than a restriction of regular classes.
The paper states that
Special member functions which have been explicitly declared and default-defined are never trivial. Therefore an explicit class can never be a POD, even if its special member functions are default-defined. (The justification for this restriction is that the semantics of a class should not change if its inline default-defined functions are moved out of line.)However, this is too much of a restriction. An inline default-defined function should be trivial if and only if the implicit definition would be trivial. Of course, non-inline definitions should be non-trivial. In essence, we agree with the justification, but not the restriction. Fortunately, subsequent work on the definition of PODs by Beman Dawes in N2294 POD's Revisited; Resolving Core Issue 568 (Revision 4)) provides tools to revisit this issue.
The syntax suggested in N1717 for explicit defaults permits simultaneous specification of a member initializer list, which is clearly not right. Rather than add additional restrictive text, a restrictive syntax is preferable.
The paper left incomplete the specification that an explicit class without a copy constructor is deemed to have an inaccessible copy constructor. This specification was intended to avoid reverse slicing, where only a sub-object is initialized. The situation is similar for copy assignment operators.
In N1890 Initialization and Initializers section 7, as elaborated in the draft Control of class defaults, Bjarne Stroustrup describes a syntax for controlling defaults, with more goals than N1717. These goals include not only controlling member defaults, but also controlling the application of global operators, other possible default implementations, and control of inheritance.
The paper's proposed syntax is very terse, and requires a mental mapping from the short codes of the default control list to the actual functions. For the most part, this mapping is straightforward, but still must be memorized. Further, its different syntactic style will slow adoption by users and compiler vendors.
In contrast to N1717, the paper provides no mechanism to define a non-inline member using the default definition.
The paper also includes mechanisms for controlling
class defaults
,
which essentially involve inheritance.
In
N2123 Adding the prohibited access specifier to C++09
Alisdair Meredith
describes a prohibited
access specifier
that disables undesired operations within classes.
This notion was first introduced by
Daveed Vandevoorde in
N1964 Modules in C++ (revision 3).
The proposal also enables defining prohibited conversions via declaration of a prohibited overload. This aspect of the proposal is probably the most significant, as it extends into user-defined operations and addresses a major weakness in C++, agressive conversion.
The technique works because the prohibited functions are still visible. Syntactically, the specifier also applies to other data members, for which it does not make sense and requires additional rules. A more precise mechanism is desirable.
Furthermore, the access specifier does not work for free functions. This missing feature seems like such an obvious next step that its absence will be felt.
The prohibited
keyword is new,
which will break some programs.
A Google code search yields about 500 hits.
Sensibly reusing an existing keyword will not affect existing programs.
Unlike N1717, the paper does not propose a means to specify non-inline default implementations. Unlike N1890, the paper does not propose a means to specify default implementations for functions that have no implicit default.
We propose to define two new mechanisms to explicitly use the default definition of a function and to delete a function definition. This solution
Syntactically, the proposal uses
and =default;
in alternate rules for function-definition.
We considered the forms
=delete;
, as suggested by
N1717, and
default;
, as adopted by
N1717,
but chose not to use them.
The reasons are that the former introduces
more choice points into the parse of declarations,
while the second introduces either
more choice points in the parse of function blocks
or substantial lookahead to determine that the syntax
{default}
is not a function block.
{default}
The definition form
indicates that the function's default definition should be used.
As expected, this works when the definition is inside the class body,
but it also works when the definition is outside the class body,
but as with the current language,
you must declare the function within the class body.
=default;
struct type { type() = default; // commonly specified redundancy is now efficent virtual ~type(); // virtual requires a declaration type( const type & ); // a simple declaration, but }; inline type::type( const type & ) = default; // now efficient type::~type() = default; // a non-inline default definition
Critical to exploiting the benefits of N2294, a explicitly-defaulted definition within its class is trivial if and only if the implicit definition would have been trivial.
struct type { type() = default; // trivial virtual ~type() = default; // non-trivial because virtual type & operator =( const type & ); // non-trivial // because not defaulted here }; inline type & type::operator =( const type & ) = default;
Non-inline definitions may also be defaulted.
struct type { type( const type & ); // not defaulted here }; type::type( const type & ) = default;
This rule enables efficient execution and concise definition while enabling a stable binary interface to an evolving implementation. That is, the machine-level calling sequence remains unchanged even when the definition changes to a non-default. For example, consider the header
type.h : struct type { int x; type(); };
and two (mutually exclusive) definitions as the software evolves,
type1.cc : type::type() = default; type2.cc : type::type() { x = 3; }
In some cases, the class body can change without requiring a change in member function definition because the default changes with declaration of additional members.
Unlike the implicit default,
the explicit default has normal exception specification semantics.
This inconsistency seems at the surface to be undesireable.
The reason for the change in semantics is twofold.
First, if the explicit default used the implicit exception specification,
there would be no syntax to get back to a throws anything
state.
Second, the users of a function
cannot tell if it is defaulted when it is non-inline,
as in the example above.
It would be undesirable to have semantics of use
change due to a change in implementation.
So, the explicit declaration needs to have the the normal
exception semantics.
Users desiring precisely the same exception specification
as the implicit declaration
must specify it.
N1717
reached the same conclusion.
So far, this proposal has shown how to make explicit the implicit default definition. There is another opportunity, which is to use a default implementation that is not implicit. For example, consider the equality operator. There is no default equality operator, but an explicit default definition would be able to use a standard-defined, if not implicit, default definition.
struct type { bool operator ==( const & type ) = default; bool operator !=( const & type ) = default; };
Indeed, there are several potential operations that could have non-implicit default implementations. However, this paper proposes none. We believe that at this point in the standards process, such defaults are best left to a technical report. However, should such a technical report be forthcoming, the syntax is ready.
The definition form
indicates that the function may not be used.
However, all lookup and overload resolution
occurs before the deleted definition is noted.
That is, it is the definition that is deleted, not the symbol;
overloads that resolve to that definition are ill-formed.
=delete;
The primary power of this approach is twofold. First, use of default language facilities can be made an error by deleting the definition of functions that they require. Second, problematic conversions can be made an error by deleting the definition for the offending conversion (or overloaded function).
This approach of checking for a delete definition late has two benefits. First, it achieves the goal of making a bad overload visible. Second, it is relativley easy to implement, requiring no change to the already complicated lookup and overload rules.
The deleted definition of a function must be its first declaration. This rule prevents inconsistency in interpretation.
The deleted definition is an inline definition, thus requiring consistent definition throughout the program via the one-definition rule.
The deleted definition mechanism is orthogonal to access specifiers, though accessibility is somewhat moot if the function has been deleted.
Deleted functions are trivial. This rule is necessary to obtain maximum benefit from N2294.
One can define a template with a deleted definition. Specialization and argument deduction occur as they would with a regular template function, but explicit specialization is not permitted.
One cannot use a deleted function in a sizeof
expression.
We believe the existing language rules will prevent this.
A deleted virtual function may not override a non-deleted virtual function and vice-versa.
The canonical example of disabling language features is disabling copying. Simply declare the copy assignment operator and the copy constructor with deleted definitions. Because a declaration of any constructor disables the default constructor, a programmer may choose to add it back with its default definition.
struct type { type & operator =( const type & ) = delete; type( const type & ) = delete; type() = default; };
We avoid the problem of reverse slicing identified in N1717 because the declaration for a deleted definition still hides the base-class copy constructor.
A more subtle example involves
indirectly controlling the allocation of a type.
Deleting the definition of a class-specific operator new
will prevent allocation in free store
because new
expressions
involving type
will be ill-formed.
struct type { void * operator new( std::size_t ) = delete; };
In contrast, deleting the definition of a destructor will require allocation in free store because static and automatic variables implicitly call the destructor.
Unfortunately, the approach also prevents deleting a free-store object, thus either limiting its use to singletons or requiring the employment of a garbage collector.struct type { ~type() = delete; // disable destructor };
Removing dangerous conversions is as important as removing undesired language defaults.
struct type { type( long long ); // can initialize with an long long type( long ) = delete; // but not anything less }; extern void bar( type, long long ); // and the same for bad overloads void bar( type, long ) = delete; // of free functions
Of considerable note
is the interaction of explicit constructors and explicit conversion operators
with deleted definitions.
(Lois Goldthwaite, Michael Wong, and Jens Maurer
describe explicit conversion operators in
N2333 Explicit Conversion Operator
Draft Working Paper Revision 1.)
The short answer is that the two facilities are orthogonal,
explicit
controls the set of functions considered
and delete
comments on the final choice.
The more helpful answer is a bit more subtle.
For example, consider
struct type { type( long long ); explicit type( long ) = delete; }; extern void function( type );
Under this proposal, lookup and the semantics of explicit
are unchanged,
so in the overload resoultion for
,
42 promotes to type(42)
long
in the overload resolution,
and a deleted function is selected, which is an error.
function( type( 42 ) ); // error 42 promotes to long function( 42 ); // okay type(long long); type(long) not considered
In practice, explicit
and delete
will probably not be used together.
Programmers desiring more restrictive diagnostics
will use delete
alone
rather than explicit
.
Our solution provides all known strengths of
N1602,
N1717 and
N2123
while avoiding many of the identified weakness.
In particular,
the explicit
class qualifier and
the prohibited
access specifier
would no longer be necessary.
The solution addresses most of the goals of
N1890(7),
specifically those related to function definitions,
but not those related to class defaults
of inheritance.
We have chosen not to address class defaults for the following reasons.
Paper N2145 C++ Atomic Types and Operations described a failure to achieve desired semantics for atomic types with with C++ 2003. With this proposal and others, an effective and correct definition is defined in N2324 C++ Atomic Types and Operations and thus is a motivating use case for the proposal.
The following code, as defined in this proposal and in conjunction with N2215 Initializer lists (Rev.3), N2294 POD's Revisited; Resolving Core Issue 568 (Revison 4), and N2235 Generalized Constant Expressions — Revision 5, appears to satisfy the requirements of N2145.
typedef struct atomic_int { #ifdef __cplusplus // destructor implicitly declared and defined atomic_int() = default; // otherwise suppressed by other constructors constexpr atomic_int( int v ) : f ( v ) { } // construct from value int operator =( int v ) volatile; // assign from value atomic_int( const atomic_int & ) = delete; // too dangerous atomic_int & operator =( const atomic_int & ) = delete; // also operator int(); // load the value private: #endif int f; } atomic_int;
The destructor is implicitly-defaulted and trivial. The default constructor is explicitly-defaulted and trivial. The copy constructor and copy assignment operators are deleted and therefore trivial.
N2235 and the constexpr
value constructor
together permit static initialization.
N2215 permits aggregate initialization syntax.
The erroneous defaults for copying are deleted, preventing their use. One can still copy from one atomic to another, but only going through a non-atomic value, which makes clear that a copy between two atomics is not itself atomic.
atomic_int w = { 3 }; // static initialization atomic_int x; // default zero initialization void function( atomic_int * p ) { atomic_int y( *p ); // error copy constructor is deleted atomic_int z( int(*p) ); // okay copy construct through value *p = x; // error copy assignment is deleted *p = int(x); // okay copy assignment through value }
We propose the following changes to the standard. The base document is N2315 Working Draft, Standard for Programming Language C++.
Within paragraph 16, edit
A new-expression that creates an object of typeT
initializes that object as follows:
- If the new-initializer is omitted:
- If
T
is a (possibly cv-qualified) non-POD class type (or array thereof), the object is default-initialized (8.5). IfT
is a const-qualified type, the underlying class type shall have auser-declareduser-provided default constructor.- Otherwise, the object created has indeterminate value. If
T
is a const-qualified type, or a (possibly cv-qualified) POD class type (or array thereof) containing (directly or indirectly) a member of const-qualified type, the program is ill-formed;
In paragraph 1, edit
Function definitions have the form
- function-definition:
- decl-specifier-seqopt declarator ctor-initializeropt function-body
- decl-specifier-seqopt declarator function-try-block
- decl-specifier-seqopt declarator
= default ;
- decl-specifier-seqopt declarator
= delete ;
Add a new paragraph 7.
A function definition of the form:decl-specifier-seqopt declaratoris called an explicitly-defaulted definition. Only special member functions may be explicitly defaulted, and the implementation will define them as if they had implicit definitions (12.1, 12.4, 12.8). A special member function is user-provided if it is user-declared and not explicitly defaulted on its first declaration. A user-provided explicitly-defaulted function is defined at the point where it is explicitly defaulted. [ Note: While an implicitly-declared special member function is inline (clause 12), an explicitly-defaulted definition may be non-inline. Non-inline definitions are user-provided, and hence non-trivial (12.1, 12.4, 12.8). This rule enables efficient execution and concise definition while enabling a stable binary interface to an evolving codebase. — end note ] [ Example:= default ;
— end example ]struct trivial { trivial() = default; trivial( const trivial & ) = default; trivial & operator =( const trivial & ) = default; ~trivial() = default; }; struct nontrivial1 { nontrivial1(); }; nontrivial1::nontrivial1() = default; // not inline struct nontrivial2 { nontrivial2(); }; inline nontrivial2::nontrivial2() = default; // not first declaration struct nontrivial3 { virtual ~nontrivial3() = 0; // virtual }; inline nontrivial3::~nontrivial3() = default; // not first declaration
Add a new paragraph 8.
A function definition of of the form:decl-specifier-seqopt declaratoris called a deleted definition. A function with a deleted definition is also called a deleted function. A deleted definition of a function shall be the first declaration of the function. [ Example:= delete ;
— end example ] A deleted function is implicitly inline. [ Note: The one-definition rule (3.2 [basic.def.odr]) applies to deleted definitions. — end note ] A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed. [ Note: This includes calling the function explicitly or implicitly and forming a pointer or pointer-to-member to the function. It applies even for references in expressions that are not potentially-evaluated. If a function is overloaded, it is referenced only if the function is selected by overload resolution. — end note ] [ Example: One can enforce non-default initialization and non-integral initialization withstruct sometype { sometype(); }; sometype::sometype() = delete; // ill-formed; not first declaration
— end example ] [ Example: One can prevent use of a class in certainstruct sometype { sometype() = delete ; // redundant, but legal sometype( std::intmax_t ) = delete; sometype( double ); };
new
expressions by using deleted definitions of a user-declaredoperator new
for that class.— end example ]struct sometype { void * operator new( std::size_t ) = delete; void * operator new[]( std::size_t ) = delete; }; sometype * p = new sometype; // error, deleted class operator new sometype * p = new sometype[3]; // error, deleted class operator new[]
Within paragraph 5, edit
To value-initialize an object of typeT
means:
- if
T
is a class type (clause 9) with auser-declareduser-provided constructor (12.1), then the default constructor forT
is called (and the initialization is ill-formed ifT
has no accessible default constructor);- if
T
is a non-union class type without auser-declareduser-provided constructor, then every non-static data member and base-class component ofT
is value-initialized;
In paragraph 9, edit
If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have auser-declareduser-provided default constructor. Otherwise, if no initializer is specified for a non-static object, the object and its subobjects, if any, have an indeterminate initial value92); if the object or any of its subobjects are of const-qualified type, the program is ill-formed.
In paragraph 1, edit
An aggregate is an array or a class (clause 9) with nouser-declareduser-provided constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).
In paragraph 13, edit
[ Note: An aggregate array or an aggregate class may contain members of a class type with auser-declareduser-provided constructor (12.1). Initialization of these aggregate objects is described in 12.6.1. — end note ]
In paragraph 4, edit
A union is a class defined with the class-keyunion
; it holds only one data member at a time (9.5). [ Note: aggregates of class type are described in 8.5.1. — end note ] A POD class is an aggregate class that has no non-static data members of non-POD type (or array of such a type) or reference, and has nouser-declareduser-provided copy assignment operator and nouser-declareduser-provided destructor. A POD-struct is a POD class defined with the class-keystruct
or the class-keyclass
. A POD-union is a POD class defined with the class-keyunion
.
Paragraph 14, unchanged.
In addition, if classT
has a user-declared constructor (12.1), every non-static data member of classT
shall have a name different fromT
.
At the end of the section, add a new paragraph
A function with a deleted definition ([dcl.fct.def]) shall not override a function that does not have a deleted definition. Likewise, a function that does not have a deleted definition shall not override a function with a deleted definition.
Within paragraph 5, edit
A default constructor is trivial if it isimplicitly-declarednot user-provided (8.4 [dcl.fct.def]) and if:
In paragraph 7, edit
Animplicitly-declarednon-user-provided default constructor for a class is implicitly defined when it is used (3.2) to create an object of its class type (1.8). The implicitly-defined or explicitly-defaulted default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with an empty mem-initializer-list (12.6.2) and an empty function body. If that user-written default constructor would be ill-formed, the program is ill-formed. Before theimplicitly-declarednon-user-provided default constructor for a class is implicitly defined, all theimplicitly-declarednon-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined. [ Note: an implicitly-declared default constructor has an exception-specification (15.4). An explicitly-defaulted definition has no implicit exception-specification. — end note ]
Within paragraph 3, edit
If a class has no user-declared destructor, a destructor is declared implicitly. An implicitly-declared destructor is an inline public member of its class. A destructor is trivial if it isimplicitly-declarednot user-provided (8.4 [dcl.fct.def]) and if:
In paragraph 5, edit
Animplicitly-declarednon-user-provided destructor is implicitly defined when it is used to destroy an object of its class type (3.7). A program is ill-formed if the class for which a destructor is implicitly defined or explicitly defaulted has:Before the
- a non-static data member of class type (or array thereof) with an inaccessible destructor, or
- a base class with an inaccessible destructor.
implicitly-declarednon-user-provided destructor for a class is implicitly defined, all theimplicitly-declarednon-user-provided destructors for its base classes and its non-static data members shall have been implicitly defined. [ Note: an implicitly-declared destructor has an exception-specification (15.4). An explicitly-defaulted definition has no implicit exception-specification. — end note ]
Within paragraph 4, edit
If a given non-static data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then
- If the entity is a non-static data member of (possibly cv-qualified) class type (or array thereof) or a base class, and the entity class is a non-POD class, the entity is default-initialized (8.5). If the entity is a non-static data member of a const-qualified type, the entity class shall have a
user-declareduser-provided (8.4 [dcl.fct.def]) default constructor.
In paragraph 4, no change.
If the class definition does not explicitly declare a copy constructor, one is declared implicitly. Thus, for the class definitiona copy constructor is implicitly-declared. If the user-declared constructor is later defined asstruct X { X(const X&, int); };
then any use of X’s copy constructor is ill-formed because of the ambiguity; no diagnostic is required.X::X(const X& x, int i =0) { /* ... */ }
Within paragraph 6, edit
A copy constructor for classX
is trivial if it isimplicitly declarednot user-provided (8.4 [dcl.fct.def]) and if
In paragraph 7, edit
Animplicitly-declarednon-user-provided copy constructor is implicitly defined if it is used to initialize an object of its class type from a copy of an object of its class type or of a class type derived from its class type108). [ Note: the copy constructor is implicitly defined even if the implementation elided its use (12.2). — end note ] A program is ill-formed if the class for which a copy constructor is implicitly defined, or explictly defaulted, has:Before the
- a non-static data member of class type (or array thereof) with an inaccessible or ambiguous copy constructor, or
- a base class with an inaccessible or ambiguous copy constructor.
implicitly-declarednon-user-provided copy constructor for a class is implicitly defined, allimplicitly-declarednon-user-provided copy constructors for its direct and virtual base classes and its non-static data members shall have been implicitly defined. [ Note: an implicitly-declared copy constructor has an exception-specification (15.4). An explicitly-defaulted definition has no implicit exception-specification. — end note ]
Within paragraph 8, edit
The implicitly-defined or explicitly-defaulted copy constructor
for class X
performs a memberwise copy of its subobjects.
Within paragraph 9, unchanged.
A user-declared copy assignment operatorX::operator=
is a non-static non-template member function of classX
with exactly one parameter of typeX
,X&
,const X&
,volatile X&
orconst volatile X&
.109)
Within paragraph 11, edit
A copy assignment operator for classX
is trivial if it isimplicitly declarednot user-provided and if
In paragraph 12, edit
Animplicitly-declarednon-user-provided copy assignment operator is implicitly defined when an object of its class type is assigned a value of its class type or a value of a class type derived from its class type. A program is ill-formed if the class for which a copy assignment operator is implicitly defined or explicitly defaulted, has:Before the
- a non-static data member of const type, or
- a non-static data member of reference type, or
- a non-static data member of class type (or array thereof) with an inaccessible copy assignment operator, or
- a base class with an inaccessible copy assignment operator.
implicitly-declarednon-user-provided copy assignment operator for a class is implicitly defined, allimplicitly-declarednon-user-provided copy assignment operators for its direct base classes and its non-static data members shall have been implicitly defined. [ Note: an implicitly-declared copy assignment operator has an exception-specification (15.4). An explicitly-defaulted definition has no implicit exception-specification. — end note ]
Within paragraph 13, edit
The implicitly-defined or explicitly-defaulted copy assignment operator for class X performs memberwise assignment of its subobjects. ....itIt is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined or explicitly-defaulted copy assignment operator.
In paragraph 1, edit
An explicit specialization of any of the following:can be declared by a declaration introduced by template<>;
- non-deleted function template
- class template
- non-deleted member function of a class template
- static data member of a class template
- member class of a class template
- member class template of a class template
- non-deleted member function template of a class template
In paragraph 1, edit
12.8 (copying class objects)
- Change:
- The implicitly-declared copy constructor and implicitly-declared copy assignment operator cannot make a copy of a volatile lvalue. For example, the following is valid in ISO C:
struct X { int i; }; struct X x1, x2; volatile struct X x3 = {0}; x1 = x3; // invalid C++ x2 = x3; // also invalid C++
- Rationale:
- Several alternatives were debated at length. Changing the parameter to
volatile const X&
would greatly complicate the generation of efficient code for class objects. Discussion of providing two alternative signatures for these implicitly-defined operations raised unanswered concerns about creating ambiguities and complicating the rules that specify the formation of these operators according to the bases and members.- Effect on original feature:
- Deletion of semantically well-defined feature.
- Difficulty of converting:
- Semantic transformation. If volatile semantics are required for the copy, a user-declared constructor or assignment must be provided. [ Note: This user-declared constructor may be explicitly defaulted. — end note ] If non-volatile semantics are required, an explicit const_cast can be used.
- How widely used:
- Seldom.