Order of evaluation
From cppreference.com
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There is no concept of left-to-right or right-to-left evaluation in C, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression f1() + f2() + f3() is parsed as (f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to f3 may be evaluated first, last, or between f1() or f2() at run time.
Definitions
Evaluations
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- value computation: calculation of the value that is returned by the expression. This may involve determination of the identity of the object (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 27 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) or reading the value previously assigned to an object (rvalue evaluation)
- side effect: access (read or write) to an object designated by a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 26 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE lvalue, modification (writing) to an object, atomic synchronization(since 哋它亢11), modifying a file, modifying the floating-point environment (if supported), or calling a function that does any of those operations.
If no side effects are produced by an expression and the compiler can determine that the value is not used, the expression MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 24 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
Ordering
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- If a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 22 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is present between the subexpressions E1 and E2, then both value computation and side effects of E1 are sequenced-before every value computation and side effect of E2
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(since 哋它亢11) |
Rules
1) There is a sequence point after the evaluation of all function arguments and of the function designator, and before the actual function call.
2) There is a sequence point after evaluation of the first (left) operand and before evaluation of the second (right) operand of the following binary operators:
&& (logical AND), || (logical OR), and , (comma).3) There is a sequence point after evaluation of the first (left) operand and before evaluation of the second or third operand (whichever is evaluated) of the conditional operator
?:4) There is a sequence point after the evaluation of a full expression (an expression that is not a subexpression: typically something that ends with a semicolon or a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 21 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of
if/switch/while/do) and before the next full expression.|
5) There is a sequence point at the end of a full declarator.
6) There is a sequence point immediately before the return of a library function.
7) There is a sequence point after the action associated with each conversion specifier in formatted I/O (in particular, it is well-formed for MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 20 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE to write different fields into the same variable and for MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 18 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE to read and modify or modify the same variable more than once using %n)
8) There are sequence points before and immediately after each call to a comparison function made by the library functions MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 17 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 16 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, as well as between any call to the comparison function and the movement of the associated objects made by MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
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(since 哋它亢99) |
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9) The value computations (but not the side-effects) of the operands to any operator are sequenced before the value computation of the result of the operator (but not its side-effects).
10) The side effect (modification of the left argument) of the direct assignment operator and of all compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments.
11) The value computation of the postincrement and postdecrement operators is sequenced before its side-effect.
12) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (CPU instructions that constitute different function calls cannot be interleaved, even if the functions are inlined)
13) In MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE list expressions, all evaluations are indeterminately sequenced
14) With respect to an indeterminately-sequenced function call, the operation of compound assignment operators, and both prefix and postfix forms of increment and decrement operators are single evaluations.
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(since 哋它亢11) |
Undefined behavior
1) If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 03 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
i = ++i + i++; // undefined behavior i = i++ + 1; // undefined behavior f(++i, ++i); // undefined behavior f(i = -1, i = -1); // undefined behavior
2) If a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object, the behavior is undefined.
f(i, i++); // undefined behavior a[i] = i++; // undefined bevahior
3) The above rules apply as long as at least one allowable ordering of subexpressions permits such an unsequenced side-effect.
See also
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