4 General principles [intro]

4.1 Implementation compliance [intro.compliance]

The set of diagnosable rules consists of all syntactic and semantic rules in this International Standard except for those rules containing an explicit notation that “no diagnostic is required” or which are described as resulting in “undefined behavior”.

Although this International Standard states only requirements on C++ implementations, those requirements are often easier to understand if they are phrased as requirements on programs, parts of programs, or execution of programs. Such requirements have the following meaning:

  • If a program contains no violations of the rules in this International Standard, a conforming implementation shall, within its resource limits, accept and correctly execute2 that program.

  • If a program contains a violation of any diagnosable rule or an occurrence of a construct described in this International Standard as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.

  • If a program contains a violation of a rule for which no diagnostic is required, this International Standard places no requirement on implementations with respect to that program.

[Note: During template argument deduction and substitution, certain constructs that in other contexts require a diagnostic are treated differently; see [temp.deduct]. end note]

For classes and class templates, the library Clauses specify partial definitions. Private members are not specified, but each implementation shall supply them to complete the definitions according to the description in the library Clauses.

For functions, function templates, objects, and values, the library Clauses specify declarations. Implementations shall supply definitions consistent with the descriptions in the library Clauses.

The names defined in the library have namespace scope ([basic.namespace]). A C++ translation unit obtains access to these names by including the appropriate standard library header.

The templates, classes, functions, and objects in the library have external linkage. The implementation provides definitions for standard library entities, as necessary, while combining translation units to form a complete C++ program ([lex.phases]).

Two kinds of implementations are defined: a hosted implementation and a freestanding implementation. For a hosted implementation, this International Standard defines the set of available libraries. A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries ([compliance]).

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that use such extensions that are ill-formed according to this International Standard. Having done so, however, they can compile and execute such programs.

Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support and defines all locale-specific characteristics.3

“Correct execution” can include undefined behavior, depending on the data being processed; see Clause [intro.defs] and [intro.execution].

This documentation also defines implementation-defined behavior; see [intro.execution].

4.2 Structure of this document [intro.structure]

Clauses [lex] through [cpp] describe the C++ programming language. That description includes detailed syntactic specifications in a form described in [syntax]. For convenience, Annex [gram] repeats all such syntactic specifications.

Clauses [language.support] through [thread] and Annex [depr] (the library clauses) describe the C++ standard library. That description includes detailed descriptions of the entities and macros that constitute the library, in a form described in Clause [library].

Annex [implimits] recommends lower bounds on the capacity of conforming implementations.

Annex [diff] summarizes the evolution of C++ since its first published description, and explains in detail the differences between C++ and C. Certain features of C++ exist solely for compatibility purposes; Annex [depr] describes those features.

Throughout this document, each example is introduced by “[Example: ” and terminated by “end example]”. Each note is introduced by “[Note: ” and terminated by “end note]”. Examples and notes may be nested.

4.3 Syntax notation [syntax]

In the syntax notation used in this document, syntactic categories are indicated by italic type, and literal words and characters in constant width type. Alternatives are listed on separate lines except in a few cases where a long set of alternatives is marked by the phrase “one of”. If the text of an alternative is too long to fit on a line, the text is continued on subsequent lines indented from the first one. An optional terminal or non-terminal symbol is indicated by the subscript “opt”, so

{ expressionopt }

indicates an optional expression enclosed in braces.

Names for syntactic categories have generally been chosen according to the following rules:

  • X-name is a use of an identifier in a context that determines its meaning (e.g., class-name, typedef-name).

  • X-id is an identifier with no context-dependent meaning (e.g., qualified-id).

  • X-seq is one or more X's without intervening delimiters (e.g., declaration-seq is a sequence of declarations).

  • X-list is one or more X's separated by intervening commas (e.g., identifier-list is a sequence of identifiers separated by commas).

4.4 The C++ memory model [intro.memory]

The fundamental storage unit in the C++ memory model is the byte. A byte is at least large enough to contain any member of the basic execution character set and the eight-bit code units of the Unicode UTF-8 encoding form and is composed of a contiguous sequence of bits,4 the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit. The memory available to a C++ program consists of one or more sequences of contiguous bytes. Every byte has a unique address.

[Note: The representation of types is described in [basic.types]. end note]

A memory location is either an object of scalar type or a maximal sequence of adjacent bit-fields all having nonzero width. [Note: Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation. end note] Two or more threads of execution can access separate memory locations without interfering with each other.

[Note: Thus a bit-field and an adjacent non-bit-field are in separate memory locations, and therefore can be concurrently updated by two threads of execution without interference. The same applies to two bit-fields, if one is declared inside a nested struct declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field declaration. It is not safe to concurrently update two bit-fields in the same struct if all fields between them are also bit-fields of nonzero width. end note]

[Example: A structure declared as

struct {
  char a;
  int b:5,
  struct {int ee:8;} e;

contains four separate memory locations: The field a and bit-fields d and e.ee are each separate memory locations, and can be modified concurrently without interfering with each other. The bit-fields b and c together constitute the fourth memory location. The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be. end example]

The number of bits in a byte is reported by the macro CHAR_­BIT in the header <climits>.

4.5 The C++ object model [intro.object]

The constructs in a C++ program create, destroy, refer to, access, and manipulate objects. An object is created by a definition, by a new-expression, when implicitly changing the active member of a union, or when a temporary object is created ([conv.rval], [class.temporary]). An object occupies a region of storage in its period of construction ([class.cdtor]), throughout its lifetime, and in its period of destruction ([class.cdtor]). [Note: A function is not an object, regardless of whether or not it occupies storage in the way that objects do. end note] The properties of an object are determined when the object is created. An object can have a name. An object has a storage duration which influences its lifetime. An object has a type. Some objects are polymorphic; the implementation generates information associated with each such object that makes it possible to determine that object's type during program execution. For other objects, the interpretation of the values found therein is determined by the type of the expressions (Clause [expr]) used to access them.

Objects can contain other objects, called subobjects. A subobject can be a member subobject ([class.mem]), a base class subobject (Clause [class.derived]), or an array element. An object that is not a subobject of any other object is called a complete object. If an object is created in storage associated with a member subobject or array element e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:

  • the lifetime of e's containing object has begun and not ended, and

  • the storage for the new object exactly overlays the storage location associated with e, and

  • the new object is of the same type as e (ignoring cv-qualification).

[Note: If the subobject contains a reference member or a const subobject, the name of the original subobject cannot be used to access the new object ([basic.life]). end note] [Example:

struct X { const int n; };
union U { X x; float f; };
void tong() {
  U u = {{ 1 }};
  u.f = 5.f;                          // OK, creates new subobject of u ([class.union])
  X *p = new (&u.x) X {2};            // OK, creates new subobject of u
  assert(p->n == 2);                  // OK
  assert(*std::launder(&u.x.n) == 2); // OK
  assert(u.x.n == 2);                 // undefined behavior, u.x does not name new subobject

end example]

If a complete object is created ([expr.new]) in storage associated with another object e of type “array of N unsigned char” or of type “array of N std​::​byte” ([cstddef.syn]), that array provides storage for the created object if:

  • the lifetime of e has begun and not ended, and

  • the storage for the new object fits entirely within e, and

  • there is no smaller array object that satisfies these constraints.

[Note: If that portion of the array previously provided storage for another object, the lifetime of that object ends because its storage was reused ([basic.life]). end note] [Example:

template<typename ...T>
struct AlignedUnion {
  alignas(T...) unsigned char data[max(sizeof(T)...)];
int f() {
  AlignedUnion<int, char> au;
  int *p = new (au.data) int;     // OK, au.data provides storage
  char *c = new (au.data) char(); // OK, ends lifetime of *p
  char *d = new (au.data + 1) char();
  return *c + *d; // OK

struct A { unsigned char a[32]; };
struct B { unsigned char b[16]; };
A a;
B *b = new (a.a + 8) B;      // a.a provides storage for *b
int *p = new (b->b + 4) int; // b->b provides storage for *p
                             // a.a does not provide storage for *p (directly),
                             // but *p is nested within a (see below)

end example]

An object a is nested within another object b if:

  • a is a subobject of b, or

  • b provides storage for a, or

  • there exists an object c where a is nested within c, and c is nested within b.

For every object x, there is some object called the complete object of x, determined as follows:

  • If x is a complete object, then the complete object of x is itself.

  • Otherwise, the complete object of x is the complete object of the (unique) object that contains x.

If a complete object, a data member, or an array element is of class type, its type is considered the most derived class, to distinguish it from the class type of any base class subobject; an object of a most derived class type or of a non-class type is called a most derived object.

Unless it is a bit-field, a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size. An object of trivially copyable or standard-layout type shall occupy contiguous bytes of storage.

Unless an object is a bit-field or a base class subobject of zero size, the address of that object is the address of the first byte it occupies. Two objects a and b with overlapping lifetimes that are not bit-fields may have the same address if one is nested within the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they have distinct addresses.5


static const char test1 = 'x';
static const char test2 = 'x';
const bool b = &test1 != &test2;      // always true

end example]

[Note: C++ provides a variety of fundamental types and several ways of composing new types from existing types ([basic.types]). end note]

Under the “as-if” rule an implementation is allowed to store two objects at the same machine address or not store an object at all if the program cannot observe the difference ([intro.execution]).

4.6 Program execution [intro.execution]

The semantic descriptions in this International Standard define a parameterized nondeterministic abstract machine. This International Standard places no requirement on the structure of conforming implementations. In particular, they need not copy or emulate the structure of the abstract machine. Rather, conforming implementations are required to emulate (only) the observable behavior of the abstract machine as explained below.6

Certain aspects and operations of the abstract machine are described in this International Standard as implementation-defined (for example, sizeof(int)). These constitute the parameters of the abstract machine. Each implementation shall include documentation describing its characteristics and behavior in these respects.7 Such documentation shall define the instance of the abstract machine that corresponds to that implementation (referred to as the “corresponding instance” below).

Certain other aspects and operations of the abstract machine are described in this International Standard as unspecified (for example, evaluation of expressions in a new-initializer if the allocation function fails to allocate memory ([expr.new])). Where possible, this International Standard defines a set of allowable behaviors. These define the nondeterministic aspects of the abstract machine. An instance of the abstract machine can thus have more than one possible execution for a given program and a given input.

Certain other operations are described in this International Standard as undefined (for example, the effect of attempting to modify a const object). [Note: This International Standard imposes no requirements on the behavior of programs that contain undefined behavior. end note]

A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input. However, if any such execution contains an undefined operation, this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation).

An instance of each object with automatic storage duration is associated with each entry into its block. Such an object exists and retains its last-stored value during the execution of the block and while the block is suspended (by a call of a function or receipt of a signal).

The least requirements on a conforming implementation are:

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

  • At program termination, all data written into files shall be identical to one of the possible results that execution of the program according to the abstract semantics would have produced.

  • The input and output dynamics of interactive devices shall take place in such a fashion that prompting output is actually delivered before a program waits for input. What constitutes an interactive device is implementation-defined.

These collectively are referred to as the observable behavior of the program. [Note: More stringent correspondences between abstract and actual semantics may be defined by each implementation. end note]

[Note: Operators can be regrouped according to the usual mathematical rules only where the operators really are associative or commutative.8 For example, in the following fragment

int a, b;
/* ... */
a = a + 32760 + b + 5;

the expression statement behaves exactly the same as

a = (((a + 32760) + b) + 5);

due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in which overflows produce an exception and in which the range of values representable by an int is [-32768, +32767], the implementation cannot rewrite this expression as

a = ((a + b) + 32765);

since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce an exception while the original expression would not; nor can the expression be rewritten either as

a = ((a + 32765) + b);


a = (a + (b + 32765));

since the values for a and b might have been, respectively, 4 and -8 or -17 and 12. However on a machine in which overflows do not produce an exception and in which the results of overflows are reversible, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur. end note]

A constituent expression is defined as follows:


struct A { int x; };
struct B { int y; struct A a; };
B b = { 5, { 1+1 } };

The constituent expressions of the initializer used for the initialization of b are 5 and 1+1. end example]

The immediate subexpressions of an expression e are

A subexpression of an expression e is an immediate subexpression of e or a subexpression of an immediate subexpression of e. [Note: Expressions appearing in the compound-statement of a lambda-expression are not subexpressions of the lambda-expression. end note]

A full-expression is

If a language construct is defined to produce an implicit call of a function, a use of the language construct is considered to be an expression for the purposes of this definition. Conversions applied to the result of an expression in order to satisfy the requirements of the language construct in which the expression appears are also considered to be part of the full-expression. For an initializer, performing the initialization of the entity (including evaluating default member initializers of an aggregate) is also considered part of the full-expression. [Example:

struct S {
  S(int i): I(i) { }       // full-expression is initialization of I
  int& v() { return I; }
  ~S() noexcept(false) { }
  int I;

S s1(1);                   // full-expression is call of S​::​S(int)
void f() {
  S s2 = 2;                // full-expression is call of S​::​S(int)
  if (S(3).v())            // full-expression includes lvalue-to-rvalue and
                           // int to bool conversions, performed before
                           // temporary is deleted at end of full-expression
  { }
  bool b = noexcept(S());  // exception specification of destructor of S
                           // considered for noexcept
  // full-expression is destruction of s2 at end of block
struct B {
      B(S = S(0));
   B b[2] = { B(), B() };  // full-expression is the entire initialization
                           // including the destruction of temporaries

end example]

[Note: The evaluation of a full-expression can include the evaluation of subexpressions that are not lexically part of the full-expression. For example, subexpressions involved in evaluating default arguments are considered to be created in the expression that calls the function, not the expression that defines the default argument. end note]

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.

Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a single thread, which induces a partial order among those evaluations. Given any two evaluations A and B, if A is sequenced before B (or, equivalently, B is sequenced after A), then the execution of A shall precede the execution of B. If A is not sequenced before B and B is not sequenced before A, then A and B are unsequenced. [Note: The execution of unsequenced evaluations can overlap. end note] Evaluations A and B are indeterminately sequenced when either A is sequenced before B or B is sequenced before A, but it is unspecified which. [Note: Indeterminately sequenced evaluations cannot overlap, but either could be executed first. end note] An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

Every value computation and side effect associated with a full-expression is sequenced before every value computation and side effect associated with the next full-expression to be evaluated.9

Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced. [Note: In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed consistently in different evaluations. end note] The value computations of the operands of an operator are sequenced before the value computation of the result of the operator. If a side effect on a memory location is unsequenced relative to either another side effect on the same memory location or a value computation using the value of any object in the same memory location, and they are not potentially concurrent, the behavior is undefined. [Note: The next section imposes similar, but more complex restrictions on potentially concurrent computations. end note]


void g(int i) {
  i = 7, i++, i++;    // i becomes 9

  i = i++ + 1;        // the value of i is incremented
  i = i++ + i;        // the behavior is undefined
  i = i + 1;          // the value of i is incremented

end example]

When calling a function (whether or not the function is inline), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function. For each function invocation F, for every evaluation A that occurs within F and every evaluation B that does not occur within F but is evaluated on the same thread and as part of the same signal handler (if any), either A is sequenced before B or B is sequenced before A.10 [Note: If A and B would not otherwise be sequenced then they are indeterminately sequenced. end note] Several contexts in C++ cause evaluation of a function call, even though no corresponding function call syntax appears in the translation unit. [Example: Evaluation of a new-expression invokes one or more allocation and constructor functions; see [expr.new]. For another example, invocation of a conversion function can arise in contexts in which no function call syntax appears. end example] The sequencing constraints on the execution of the called function (as described above) are features of the function calls as evaluated, whatever the syntax of the expression that calls the function might be.

If a signal handler is executed as a result of a call to the std​::​raise function, then the execution of the handler is sequenced after the invocation of the std​::​raise function and before its return. [Note: When a signal is received for another reason, the execution of the signal handler is usually unsequenced with respect to the rest of the program. end note]

This provision is sometimes called the “as-if” rule, because an implementation is free to disregard any requirement of this International Standard as long as the result is as if the requirement had been obeyed, as far as can be determined from the observable behavior of the program. For instance, an actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no side effects affecting the observable behavior of the program are produced.

This documentation also includes conditionally-supported constructs and locale-specific behavior. See [intro.compliance].

Overloaded operators are never assumed to be associative or commutative.

As specified in [class.temporary], after a full-expression is evaluated, a sequence of zero or more invocations of destructor functions for temporary objects takes place, usually in reverse order of the construction of each temporary object.

In other words, function executions do not interleave with each other.

4.7 Multi-threaded executions and data races [intro.multithread]

A thread of execution (also known as a thread) is a single flow of control within a program, including the initial invocation of a specific top-level function, and recursively including every function invocation subsequently executed by the thread. [Note: When one thread creates another, the initial call to the top-level function of the new thread is executed by the new thread, not by the creating thread. end note] Every thread in a program can potentially access every object and function in a program.11 Under a hosted implementation, a C++ program can have more than one thread running concurrently. The execution of each thread proceeds as defined by the remainder of this International Standard. The execution of the entire program consists of an execution of all of its threads. [Note: Usually the execution can be viewed as an interleaving of all its threads. However, some kinds of atomic operations, for example, allow executions inconsistent with a simple interleaving, as described below. end note] Under a freestanding implementation, it is implementation-defined whether a program can have more than one thread of execution.

For a signal handler that is not executed as a result of a call to the std​::​raise function, it is unspecified which thread of execution contains the signal handler invocation.

An object with automatic or thread storage duration is associated with one specific thread, and can be accessed by a different thread only indirectly through a pointer or reference.

4.7.1 Data races [intro.races]

The value of an object visible to a thread T at a particular point is the initial value of the object, a value assigned to the object by T, or a value assigned to the object by another thread, according to the rules below. [Note: In some cases, there may instead be undefined behavior. Much of this section is motivated by the desire to support atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for more restricted programs. end note]

Two expression evaluations conflict if one of them modifies a memory location and the other one reads or modifies the same memory location.

The library defines a number of atomic operations and operations on mutexes that are specially identified as synchronization operations. These operations play a special role in making assignments in one thread visible to another. A synchronization operation on one or more memory locations is either a consume operation, an acquire operation, a release operation, or both an acquire and release operation. A synchronization operation without an associated memory location is a fence and can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write operations, which have special characteristics. [Note: For example, a call that acquires a mutex will perform an acquire operation on the locations comprising the mutex. Correspondingly, a call that releases the same mutex will perform a release operation on those same locations. Informally, performing a release operation on A forces prior side effects on other memory locations to become visible to other threads that later perform a consume or an acquire operation on A. “Relaxed” atomic operations are not synchronization operations even though, like synchronization operations, they cannot contribute to data races. end note]

All modifications to a particular atomic object M occur in some particular total order, called the modification order of M. [Note: There is a separate order for each atomic object. There is no requirement that these can be combined into a single total order for all objects. In general this will be impossible since different threads may observe modifications to different objects in inconsistent orders. end note]

A release sequence headed by a release operation A on an atomic object M is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first operation is A, and every subsequent operation

  • is performed by the same thread that performed A, or

  • is an atomic read-modify-write operation.

Certain library calls synchronize with other library calls performed by another thread. For example, an atomic store-release synchronizes with a load-acquire that takes its value from the store ([atomics.order]). [Note: Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a requirement would sometimes interfere with efficient implementation. end note] [Note: The specifications of the synchronization operations define when one reads the value written by another. For atomic objects, the definition is clear. All operations on a given mutex occur in a single total order. Each mutex acquisition “reads the value written” by the last mutex release. end note]

An evaluation A carries a dependency to an evaluation B if

[Note: “Carries a dependency to” is a subset of “is sequenced before”, and is similarly strictly intra-thread. end note]

An evaluation A is dependency-ordered before an evaluation B if

  • A performs a release operation on an atomic object M, and, in another thread, B performs a consume operation on M and reads a value written by any side effect in the release sequence headed by A, or

  • for some evaluation X, A is dependency-ordered before X and X carries a dependency to B.

[Note: The relation “is dependency-ordered before” is analogous to “synchronizes with”, but uses release/consume in place of release/acquire. end note]

An evaluation A inter-thread happens before an evaluation B if

  • A synchronizes with B, or

  • A is dependency-ordered before B, or

  • for some evaluation X

    • A synchronizes with X and X is sequenced before B, or

    • A is sequenced before X and X inter-thread happens before B, or

    • A inter-thread happens before X and X inter-thread happens before B.

[Note: The “inter-thread happens before” relation describes arbitrary concatenations of “sequenced before”, “synchronizes with” and “dependency-ordered before” relationships, with two exceptions. The first exception is that a concatenation is not permitted to end with “dependency-ordered before” followed by “sequenced before”. The reason for this limitation is that a consume operation participating in a “dependency-ordered before” relationship provides ordering only with respect to operations to which this consume operation actually carries a dependency. The reason that this limitation applies only to the end of such a concatenation is that any subsequent release operation will provide the required ordering for a prior consume operation. The second exception is that a concatenation is not permitted to consist entirely of “sequenced before”. The reasons for this limitation are (1) to permit “inter-thread happens before” to be transitively closed and (2) the “happens before” relation, defined below, provides for relationships consisting entirely of “sequenced before”. end note]

An evaluation A happens before an evaluation B (or, equivalently, B happens after A) if:

  • A is sequenced before B, or

  • A inter-thread happens before B.

The implementation shall ensure that no program execution demonstrates a cycle in the “happens before” relation. [Note: This cycle would otherwise be possible only through the use of consume operations. end note]

An evaluation A strongly happens before an evaluation B if either

  • A is sequenced before B, or

  • A synchronizes with B, or

  • A strongly happens before X and X strongly happens before B.

[Note: In the absence of consume operations, the happens before and strongly happens before relations are identical. Strongly happens before essentially excludes consume operations. end note]

A visible side effect A on a scalar object or bit-field M with respect to a value computation B of M satisfies the conditions:

  • A happens before B and

  • there is no other side effect X to M such that A happens before X and X happens before B.

The value of a non-atomic scalar object or bit-field M, as determined by evaluation B, shall be the value stored by the visible side effect A. [Note: If there is ambiguity about which side effect to a non-atomic object or bit-field is visible, then the behavior is either unspecified or undefined. end note] [Note: This states that operations on ordinary objects are not visibly reordered. This is not actually detectable without data races, but it is necessary to ensure that data races, as defined below, and with suitable restrictions on the use of atomics, correspond to data races in a simple interleaved (sequentially consistent) execution. end note]

The value of an atomic object M, as determined by evaluation B, shall be the value stored by some side effect A that modifies M, where B does not happen before A. [Note: The set of such side effects is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. end note]

If an operation A that modifies an atomic object M happens before an operation B that modifies M, then A shall be earlier than B in the modification order of M. [Note: This requirement is known as write-write coherence. end note]

If a value computation A of an atomic object M happens before a value computation B of M, and A takes its value from a side effect X on M, then the value computed by B shall either be the value stored by X or the value stored by a side effect Y on M, where Y follows X in the modification order of M. [Note: This requirement is known as read-read coherence. end note]

If a value computation A of an atomic object M happens before an operation B that modifies M, then A shall take its value from a side effect X on M, where X precedes B in the modification order of M. [Note: This requirement is known as read-write coherence. end note]

If a side effect X on an atomic object M happens before a value computation B of M, then the evaluation B shall take its value from X or from a side effect Y that follows X in the modification order of M. [Note: This requirement is known as write-read coherence. end note]

[Note: The four preceding coherence requirements effectively disallow compiler reordering of atomic operations to a single object, even if both operations are relaxed loads. This effectively makes the cache coherence guarantee provided by most hardware available to C++ atomic operations. end note]

[Note: The value observed by a load of an atomic depends on the “happens before” relation, which depends on the values observed by loads of atomics. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here. end note]

Two actions are potentially concurrent if

  • they are performed by different threads, or

  • they are unsequenced, at least one is performed by a signal handler, and they are not both performed by the same signal handler invocation.

The execution of a program contains a data race if it contains two potentially concurrent conflicting actions, at least one of which is not atomic, and neither happens before the other, except for the special case for signal handlers described below. Any such data race results in undefined behavior. [Note: It can be shown that programs that correctly use mutexes and memory_­order_­seq_­cst operations to prevent all data races and use no other synchronization operations behave as if the operations executed by their constituent threads were simply interleaved, with each value computation of an object being taken from the last side effect on that object in that interleaving. This is normally referred to as “sequential consistency”. However, this applies only to data-race-free programs, and data-race-free programs cannot observe most program transformations that do not change single-threaded program semantics. In fact, most single-threaded program transformations continue to be allowed, since any program that behaves differently as a result must perform an undefined operation. end note]

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.

[Note: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this International Standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the coherence rules. end note]

[Note: Transformations that introduce a speculative read of a potentially shared memory location may not preserve the semantics of the C++ program as defined in this International Standard, since they potentially introduce a data race. However, they are typically valid in the context of an optimizing compiler that targets a specific machine with well-defined semantics for data races. They would be invalid for a hypothetical machine that is not tolerant of races or provides hardware race detection. end note]

4.7.2 Forward progress [intro.progress]

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.

[Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. end note]

Executions of atomic functions that are either defined to be lock-free or indicated as lock-free are lock-free executions.

  • If there is only one thread that is not blocked in a standard library function, a lock-free execution in that thread shall complete. [Note: Concurrently executing threads may prevent progress of a lock-free execution. For example, this situation can occur with load-locked store-conditional implementations. This property is sometimes termed obstruction-free. end note]

  • When one or more lock-free executions run concurrently, at least one should complete. [Note: It is difficult for some implementations to provide absolute guarantees to this effect, since repeated and particularly inopportune interference from other threads may prevent forward progress, e.g., by repeatedly stealing a cache line for unrelated purposes between load-locked and store-conditional instructions. Implementations should ensure that such effects cannot indefinitely delay progress under expected operating conditions, and that such anomalies can therefore safely be ignored by programmers. Outside this document, this property is sometimes termed lock-free. end note]

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.

An invocation of a standard library function that blocks is considered to continuously execute execution steps while waiting for the condition that it blocks on to be satisfied. [Example: A library I/O function that blocks until the I/O operation is complete can be considered to continuously check whether the operation is complete. Each such check might consist of one or more execution steps, for example using observable behavior of the abstract machine. end example]

[Note: Because of this and the preceding requirement regarding what threads of execution have to perform eventually, it follows that no thread of execution can execute forever without an execution step occurring. end note]

A thread of execution makes progress when an execution step occurs or a lock-free execution does not complete because there are other concurrent threads that are not blocked in a standard library function (see above).

For a thread of execution providing concurrent forward progress guarantees, the implementation ensures that the thread will eventually make progress for as long as it has not terminated. [Note: This is required regardless of whether or not other threads of executions (if any) have been or are making progress. To eventually fulfill this requirement means that this will happen in an unspecified but finite amount of time. end note]

It is implementation-defined whether the implementation-created thread of execution that executes main and the threads of execution created by std​::​thread provide concurrent forward progress guarantees. [Note: General-purpose implementations are encouraged to provide these guarantees. end note]

For a thread of execution providing parallel forward progress guarantees, the implementation is not required to ensure that the thread will eventually make progress if it has not yet executed any execution step; once this thread has executed a step, it provides concurrent forward progress guarantees.

[Note: This does not specify a requirement for when to start this thread of execution, which will typically be specified by the entity that creates this thread of execution. For example, a thread of execution that provides concurrent forward progress guarantees and executes tasks from a set of tasks in an arbitrary order, one after the other, satisfies the requirements of parallel forward progress for these tasks. end note]

For a thread of execution providing weakly parallel forward progress guarantees, the implementation does not ensure that the thread will eventually make progress.

[Note: Threads of execution providing weakly parallel forward progress guarantees cannot be expected to make progress regardless of whether other threads make progress or not; however, blocking with forward progress guarantee delegation, as defined below, can be used to ensure that such threads of execution make progress eventually. end note]

Concurrent forward progress guarantees are stronger than parallel forward progress guarantees, which in turn are stronger than weakly parallel forward progress guarantees. [Note: For example, some kinds of synchronization between threads of execution may only make progress if the respective threads of execution provide parallel forward progress guarantees, but will fail to make progress under weakly parallel guarantees. end note]

When a thread of execution P is specified to block with forward progress guarantee delegation on the completion of a set S of threads of execution, then throughout the whole time of P being blocked on S, the implementation shall ensure that the forward progress guarantees provided by at least one thread of execution in S is at least as strong as P's forward progress guarantees. [Note: It is unspecified which thread or threads of execution in S are chosen and for which number of execution steps. The strengthening is not permanent and not necessarily in place for the rest of the lifetime of the affected thread of execution. As long as P is blocked, the implementation has to eventually select and potentially strengthen a thread of execution in S. end note] Once a thread of execution in S terminates, it is removed from S. Once S is empty, P is unblocked.

[Note: A thread of execution B thus can temporarily provide an effectively stronger forward progress guarantee for a certain amount of time, due to a second thread of execution A being blocked on it with forward progress guarantee delegation. In turn, if B then blocks with forward progress guarantee delegation on C, this may also temporarily provide a stronger forward progress guarantee to C. end note]

[Note: If all threads of execution in S finish executing (e.g., they terminate and do not use blocking synchronization incorrectly), then P's execution of the operation that blocks with forward progress guarantee delegation will not result in P's progress guarantee being effectively weakened. end note]

[Note: This does not remove any constraints regarding blocking synchronization for threads of execution providing parallel or weakly parallel forward progress guarantees because the implementation is not required to strengthen a particular thread of execution whose too-weak progress guarantee is preventing overall progress. end note]

An implementation should ensure that the last value (in modification order) assigned by an atomic or synchronization operation will become visible to all other threads in a finite period of time.

4.8 Acknowledgments [intro.ack]

The C++ programming language as described in this document is based on the language as described in Chapter R (Reference Manual) of Stroustrup: The C++ Programming Language (second edition, Addison-Wesley Publishing Company, ISBN 0-201-53992-6, copyright ©1991 AT&T). That, in turn, is based on the C programming language as described in Appendix A of Kernighan and Ritchie: The C Programming Language (Prentice-Hall, 1978, ISBN 0-13-110163-3, copyright ©1978 AT&T).

Portions of the library Clauses of this document are based on work by P.J. Plauger, which was published as The Draft Standard C++ Library (Prentice-Hall, ISBN 0-13-117003-1, copyright ©1995 P.J. Plauger).

POSIX® is a registered trademark of the Institute of Electrical and Electronic Engineers, Inc.

ECMAScript® is a registered trademark of Ecma International.

All rights in these originals are reserved.