P1152R1
Deprecating volatile

1. Abstract

We propose deprecating most of volatile. See §3 Wording for the details.

The proposed deprecation preserves the useful parts of volatile, and removes the dubious / already broken ones. This paper aims at breaking at compile-time code which is today subtly broken at runtime or through a compiler update. The paper might also break another type of code: that which doesn’t exist. This removes a significant foot-gun and removes unintuitive corner cases from the languages.

The first version of this paper, [P1152R0], has extensive background information which is not repeated here:

• Overview

• How we got here

• Why the proposed changes?

• Examples

See [P1382R0] for the follow-up paper on volatile_load<T> / volatile_store<T> requested by SG1.

2. Edit History

2.1. r0 → r1

[P1152R0] was seen by SG1 and EWG in San Diego. This update does the following:

• Remove background information from the paper.

• Follow the guidance from SG1 and EWG, based on the polls below.

3. Wording

3.1. Program execution [intro.execution]

No changes.

Accesses through volatile glvalues are evaluated strictly according to the rules of the abstract machine.

Reading an object designated by a volatile glvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment. Evaluation of an expression (or a subexpression) in general includes both value computations (including determining the identity of an object for glvalue evaluation and fetching a value previously assigned to an object for prvalue evaluation) and initiation of side effects. When a call to a library I/O function returns or an access through a volatile glvalue is evaluated the side effect is considered complete, even though some external actions implied by the call (such as the I/O itself) or by the volatile access may not have completed yet.

3.2. Data races [intro.races]

No changes.

Two accesses to the same object of type volatile std::sig_atomic_t do not result in a data race if both occur in the same thread, even if one or more occurs in a signal handler. For each signal handler invocation, evaluations performed by the thread invoking a signal handler can be divided into two groups A and B, such that no evaluations in B happen before evaluations in A, and the evaluations of such volatile std::sig_atomic_t objects take values as though all evaluations in A happened before the execution of the signal handler and the execution of the signal handler happened before all evaluations in B.

3.3. Forward progress [intro.progress]

No changes.

The implementation may assume that any thread will eventually do one of the following:

• terminate,

• make a call to a library I/O function,

• perform an access through a volatile glvalue, or

• perform a synchronization operation or an atomic operation

During the execution of a thread of execution, each of the following is termed an execution step:

• termination of the thread of execution,

• performing an access through a volatile glvalue, or

• completion of a call to a library I/O function, a synchronization operation, or an atomic operation.

3.4. Class member access [expr.ref]

No changes.

Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression. If E2 is a bit-field, E1.E2 is a bit-field. The type and value category of E1.E2 are determined as follows. In the remainder of [expr.ref], cq represents either const or the absence of const and vq represents either volatile or the absence of volatile. cv represents an arbitrary set of cv-qualifiers.

• If E2 is a non-static data member and the type of E1 is “cq1 vq1 X”, and the type of E2 is “cq2 vq2 T”, the expression designates the named member of the object designated by the first expression. If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue. Let the notation vq12 stand for the “union” of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is volatile. Similarly, let the notation cq12 stand for the “union” of cq1 and cq2; that is, if cq1 or cq2 is const, then cq12 is const. If E2 is declared to be a mutable member, then the type of E1.E2 is “vq12 T”. If E2 is not declared to be a mutable member, then the type of E1.E2 is “cq12 vq12 T”.

3.5. The cv-qualifiers [dcl.type.cv]

No changes.

The semantics of an access through a volatile glvalue are implementation-defined. If an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue, the behavior is undefined.

[ Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object. See [intro.execution] for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C. —end note ]

3.6. Functions [dcl.fct]

Modify as follows.

The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [ Note: The parameter-declaration-clause is used to convert the arguments specified on the function call; see [expr.call] —end note ] If the parameter-declaration-clause is empty, the function takes no arguments. A parameter list consisting of a single unnamed parameter of non-dependent type void is equivalent to an empty parameter list. Except for this special case, a parameter shall not have type cv void. A parameter’s declarator shall only allow const as its parameters-and-qualifiers's cv-qualifier-seq. If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack, the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where "..." is not part of an abstract-declarator, ", ..." is synonymous with "...".

[...]

The type of a function is determined using the following rules. The type of each parameter (including function parameter packs) is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type "array of T" or of function type T is adjusted to be "pointer to T". After producing the list of parameter types, any top-level cv-qualifiers modifying a parameter type are deleted when forming the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function’s parameter-type-list.

[...]

Functions shall not have a return type of type array or function, although they may have a return type of type pointer or reference to such things. There shall be no arrays of functions, although there can be arrays of pointers to functions.

Functions shall not have a volatile qualified return type.

3.7. Non-static member functions [class.mfct.non-static]

No changes.

A non-static member function may be declared const, volatile, or const volatile. These cv-qualifiers affect the type of the this pointer. They also affect the function type of the member function; a member function declared const is a const member function, a member function declared volatile is a volatile member function and a member function declared const volatile is a const volatile member function.

3.8. The this pointer [class.this]

No changes.

In the body of a non-static member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.

volatile semantics apply in volatile member functions when accessing the object and its non-static data members.

3.9. Constructors [class.ctor]

No changes.

A constructor can be invoked for a const, volatile or const volatile object. const and volatile semantics are not applied on an object under construction. They come into effect when the constructor for the most derived object ends.

3.10. Destructors [class.dtor]

No changes.

A destructor is used to destroy objects of its class type. The address of a destructor shall not be taken. A destructor can be invoked for a const, volatile or const volatile object. const and volatile semantics are not applied on an object under destruction. They stop being in effect when the destructor for the most derived object starts.

3.11. Overloadable declarations [over.load]

Modify as follows.

Parameter declarations that differ only in the presence or absence of const and/or volatile are equivalent. That is, the const and volatile type-specifiers for each parameter type are ignored when determining which function is being declared, defined, or called.

3.12. Built-in operators [over.built]

Modify as follows.

In the remainder of this section, vq represents either volatile or no cv-qualifier.

For every pair (T, vq), where T is an arithmetic type T other than bool, there exist candidate operator functions of the form

vq T & operator++(vq T &);
T operator++(vq T &, int);

For every pair (T, vq), where T is an arithmetic type T other than bool, there exist candidate operator functions of the form

vq T & operator--(vq T &);
T operator--(vq T &, int);

For every pair (T, vq) T , where T is a cv-qualified or cv-unqualified object type, there exist candidate operator functions of the form

T*vq& operator++(T*vq&);
T*vq& operator--(T*vq&);
T* operator++(T*vq&, int);
T* operator--(T*vq&, int);

For every quintuple (C1, C2, T, cv1, cv2), where C2 is a class type, C1 is the same type as C2 or is a derived class of C2, and T is an object type or a function type, there exist candidate operator functions of the form

cv12 T& operator->*(cv1 C1*, cv2 T C2::*);

For every triple pair (L, vq, R), where L is an arithmetic type, and R is a promoted arithmetic type, there exist candidate operator functions of the form

void operator=(volatile L&, R);
vq L& operator=(vq L&, R);
vq L& operator*=(vq L&, R);
vq L& operator/=(vq L&, R);
vq L& operator+=(vq L&, R);
vq L& operator-=(vq L&, R);

For every pair (T, vq), where T is any type all types T , there exist candidate operator functions of the form

void operator=(T*volatile&, T*);
T*vq& operator=(T*vq&, T*);

For every pair (T, vq), where T is an enumeration or pointer to member type T , there exist candidate operator functions of the form

void operator=(volatile T&, T );
vq T& operator=(vq T&, T );

For every pair (T, vq) T , where T is a cv-qualified or cv-unqualified object type, there exist candidate operator functions of the form

T*vq& operator+=(T*vq&, std::ptrdiff_t);
T*vq& operator-=(T*vq&, std::ptrdiff_t);

For every triple pair (L, vq, R), where L is an integral type, and R is a promoted integral type, there exist candidate operator functions of the form

vq L& operator%=(vq L&, R);
vq L& operator<<=(vq L&, R);
vq L& operator>>=(vq L&, R);
vq L& operator&=(vq L&, R);
vq L& operator^=(vq L&, R);
vq L& operator|=(vq L&, R);

3.13. Tuples [tuple]

Modify as follows.

Header <tuple> synopsis [tuple.syn]:

namespace std {
    
[...]
    
// [tuple.helper, tuple helper classes
template<class T> class tuple_size;                  // not defined
template<class T> class tuple_size<const T>;
template<class T> class tuple_size<volatile T>;
template<class T> class tuple_size<const volatile T>;
    
template<class... Types> class tuple_size<tuple<Types...>>;
    
template<size_t I, class T> class tuple_element;     // not defined
template<size_t I, class T> class tuple_element<I, const T>;
template<size_t I, class T> class tuple_element<I, volatile T>;
template<size_t I, class T> class tuple_element<I, const volatile T>;

[...]
    
}
    

[...]

Tuple helper classes [tuple.helper]

template<class T> class tuple_size<const T>;

emplate<class T> class tuple_size<volatile T>;

emplate<class T> class tuple_size<const volatile T>;

Let TS denote tuple_size<T> of the cv-unqualified type T. If the expression TS::value is well-formed when treated as an unevaluated operand, then each of the three templates shall satisfy the TransformationTrait requirements with a base characteristic of

integral_constant<size_t, TS::value>

Otherwise, they shall have no member value.

Access checking is performed as if in a context unrelated to TS and T. Only the validity of the immediate context of the expression is considered. [ Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. —end note ]

In addition to being available via inclusion of the <tuple> header, the three templates are template is available when any of the headers <array>, <ranges>, or <utility> are included.

  
template<size_t I, class T> class tuple_element<I, const T>;
template<size_t I, class T> class tuple_element<I, volatile T>;
template<size_t I, class T> class tuple_element<I, const volatile T>;

Let TE denote tuple_element_t<I, T> of the cv-unqualified type T. Then each of the three templates the template shall satisfy the TransformationTrait requirements with a member typedef type that names the following type: add_const_t<TE>.

• for the first specialization, add_const_t<TE>,
• for the second specialization, add_volatile_t<TE>, and
• for the third specialization, add_cv_t<TE>.

In addition to being available via inclusion of the <tuple> header, the three templates are template is available when any of the headers <array>, <ranges>, or <utility> are included.

3.14. Variants [variant]

Modify as follows.

<variant> synopsis [variant.syn]

  
namespace std {
// [variant.variant], class template variant
template<class... Types>
  class variant;

// [variant.helper], variant helper classes
template<class T> struct variant_size;                   // not defined
template<class T> struct variant_size<const T>;
template<class T> struct variant_size<volatile T>;
template<class T> struct variant_size<const volatile T>;
template<class T>
  inline constexpr size_t variant_size_v = variant_size<T>::value;

template<class... Types>
  struct variant_size<variant<Types...>>;

template<size_t I, class T> struct variant_alternative;  // not defined
template<size_t I, class T> struct variant_alternative<I, const T>;
template<size_t I, class T> struct variant_alternative<I, volatile T>;
template<size_t I, class T> struct variant_alternative<I, const volatile T>;

[...]
  
}
  

variant helper classes [variant.helper]

template<class T> struct variant_size;

Remark: All specializations of variant_size shall satisfy the UnaryTypeTrait requirements with a base characteristic of integral_constant<size_t, N> for some N.

template<class T> class variant_size<const T>;
template<class T> class variant_size<volatile T>;
template<class T> class variant_size<const volatile T>;

Let VS denote variant_size<T> of the cv-unqualified type T. Then each of the three templates the template shall satisfy the UnaryTypeTrait requirements with a base characteristic of integral_constant<size_t, VS::value>.

template<class... Types>
  struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { };

template<size_t I, class T> class variant_alternative<I, const T>;
template<size_t I, class T> class variant_alternative<I, volatile T>;
template<size_t I, class T> class variant_alternative<I, const volatile T>;

Let VA denote variant_alternative<I, T> of the cv-unqualified type T. Then each of the three templates the template shall meet the TransformationTrait requirements with a member typedef type that names the following type: add_const_t<VA::type>.

• for the first specialization, add_const_t<VA::type>,
• for the second specialization, add_volatile_t<VA::type>, and
• for the third specialization, add_cv_t<VA::type>.

3.15. Atomic operations library [atomics]

Modify as follows.

Operations on atomic types [atomics.types.operations]

[ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. —end note ]

[...]

bool is_lock_free() const volatile noexcept;
bool is_lock_free() const noexcept;

Returns: true if the object’s operations are lock-free, false otherwise.

[ Note: The return value of the is_lock_free member function is consistent with the value of is_always_lock_free for the same type. —end note ]

void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;
void store(T desired, memory_order order = memory_order::seq_cst) noexcept;

Requires: The order argument shall not be memory_order::consume, memory_order::acquire, nor memory_order::acq_rel.

Effects: Atomically replaces the value pointed to by this with the value of desired. Memory is affected according to the value of order.

Remarks: The volatile overload shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

T operator=(T desired) volatile noexcept;
T operator=(T desired) noexcept;

Effects: Equivalent to store(desired).

Returns: desired.

T load(memory_order order = memory_order::seq_cst) const volatile noexcept;
T load(memory_order order = memory_order::seq_cst) const noexcept;

Requires: The order argument shall not be memory_order::release nor memory_order::acq_rel.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by this.

Remarks: The volatile overload shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

operator T() const volatile noexcept;
operator T() const noexcept;

Effects: Equivalent to: return load();

T exchange(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;
T exchange(T desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically replaces the value pointed to by this with desired. Memory is affected according to the value of order. These operations are atomic read-modify-write operations.

Returns: Atomically returns the value pointed to by this immediately before the effects.

Remarks: The volatile overload shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

bool compare_exchange_weak(T& expected, T desired,
                           memory_order success, memory_order failure) volatile noexcept;
bool compare_exchange_weak(T& expected, T desired,
                           memory_order success, memory_order failure) noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order success, memory_order failure) volatile noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order success, memory_order failure) noexcept;
bool compare_exchange_weak(T& expected, T desired,
                           memory_order order = memory_order::seq_cst) volatile noexcept;
bool compare_exchange_weak(T& expected, T desired,
                           memory_order order = memory_order::seq_cst) noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order order = memory_order::seq_cst) volatile noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order order = memory_order::seq_cst) noexcept;

Requires: The failure argument shall not be memory_order::release nor memory_order::acq_rel.

Effects: Retrieves the value in expected. It then atomically compares the value representation of the value pointed to by this for equality with that previously retrieved from expected,eand if true, replaces the value pointed to by this with that in desired. If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure. When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order::acq_rel shall be replaced by the value memory_order::acquire and a value of memory_order::release shall be replaced by the value memory_order::relaxed. If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value pointed to by this during the atomic comparison. If the operation returns true, these operations are atomic read-modify-write operations on the memory pointed to by this. Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

[...]

Specializations for integers [atomics.types.int]

T fetch_key(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;
T fetch_key(T operand, memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations.

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

Remarks: For signed integer types, the result is as if the object value and parameters were converted to their corresponding unsigned types, the computation performed on those types, and the result converted back to the signed type. [ Note: There are no undefined results arising from the computation. —end note ]

T operator op=(T operand) volatile noexcept;
T operator op=(T operand) noexcept;

Effects: Equivalent to: return fetch_key(operand) op operand;

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

Specializations for floating-point types [atomics.types.float]

The following operations perform arithmetic addition and subtraction computations. The key, operator, and computation correspondence are identified in [atomic.arithmetic.computations].

T A::fetch_key(T operand, memory_order order = memory_order_seq_cst) volatile noexcept;
T A::fetch_key(T operand, memory_order order = memory_order_seq_cst) noexcept;

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations.

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

Remarks: If the result is not a representable value for its type the result is unspecified, but the operations otherwise have no undefined behavior. Atomic arithmetic operations on floating-point should conform to the std::numeric_limits<floating-point> traits associated with the floating-point type. The floating-point environment for atomic arithmetic operations on floating-point may be different than the calling thread’s floating-point environment.

T operator op=(T operand) volatile noexcept;
T operator op=(T operand) noexcept;

Effects: Equivalent to: return fetch_key(operand) op operand;

Remarks: If the result is not a representable value for its type the result is unspecified, but the operations otherwise have no undefined behavior. Atomic arithmetic operations on floating-point should conform to the std::numeric_limits<floating-point> traits associated with the floating-point type. The floating-point environment for atomic arithmetic operations on floating-point may be different than the calling thread’s floating-point environment.

Partial specialization for pointers [atomics.types.pointer]

T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;
T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) noexcept;

Requires: T shall be an object type, otherwise the program is ill-formed. [ Note: Pointer arithmetic on \tcode{void} or function pointers is ill-formed. —end note* ]

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations.

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

T* operator op=(ptrdiff_t operand) volatile noexcept;
T* operator op=(ptrdiff_t operand) noexcept;

Effects: Equivalent to: return fetch_key(operand) op operand;

Member operators common to integers and pointers to objects [atomics.types.memop]

T operator++(int) volatile noexcept;
T operator++(int) noexcept;

Effects: Equivalent to: return fetch_add(1);

T operator--(int) volatile noexcept;
T operator--(int) noexcept;

Effects: Equivalent to: return fetch_sub(1);

T operator++() volatile noexcept;
T operator++() noexcept;

Effects: Equivalent to: return fetch_add(1) + 1;

T operator--() volatile noexcept;
T operator--() noexcept;

Effects: Equivalent to: return fetch_sub(1) - 1;

Non-member functions [atomics.nonmembers]

A non-member function template whose name matches the pattern atomic_f or the pattern atomic_f_explicit invokes the member function f, with the value of the first parameter as the object expression and the values of the remaining parameters (if any) as the arguments of the member function call, in order. An argument for a parameter of type atomic<T>::value_type* is dereferenced when passed to the member function call. If no such member function exists, the program is ill-formed.

template<class T>
  void atomic_init(volatile atomic<T>* object, typename atomic<T>::value_type desired) noexcept;
template<class T>
  void atomic_init(atomic<T>* object, typename atomic<T>::value_type desired) noexcept;

Effects: Non-atomically initializes *object with value desired. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: These semantics ensure compatibility with C. —end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race. —end note ]

Remarks: The volatile overloads shall only participate in overload resolution when atomic<T>::is_always:lock_free is true.

[ Note: The non-member functions enable programmers to write code that can be compiled as either C or C++, for example in a shared header file. —end note ]

3.16. Annex D

All deletions above should be added to Annex D, such that volatile is now deprecated in these use cases.