4 General principles [intro]

4.6 Program execution [intro.execution]

The semantic descriptions in this document define a parameterized nondeterministic abstract machine.
This document 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 document 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 document as unspecified (for example, evaluation of expressions in a new-initializer if the allocation function fails to allocate memory ([expr.new])).
Where possible, this document 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 document as undefined (for example, the effect of attempting to modify a const object).
[Note
:
This document imposes no requirements on the behavior of programs that contain undefined behavior.
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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 document 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.
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[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);
or
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.
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A constituent expression is defined as follows:
[Example
:
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.
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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) { }
private:
  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
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]
[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.
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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.
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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.
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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.
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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.
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[Example
:
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.
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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.
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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.
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This provision is sometimes called the “as-if” rule, because an implementation is free to disregard any requirement of this document 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.
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.