Order of evaluation of the operands of any C operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.
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
f2() at run time.
There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):
If no side effects are produced by an expression and the compiler can determine that the value is not used, the expression may not be evaluated.
"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types and memory barriers are involved).
| ||(since C11)|
||(logical OR), and
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 ||(since C99)|
|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 initialization 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.||(since C11)|
i = ++i + i++; // undefined behavior i = i++ + 1; // undefined behavior f(++i, ++i); // undefined behavior f(i = -1, i = -1); // undefined behavior
f(i, i++); // undefined behavior a[i] = i++; // undefined bevahior
Operator precedence which defines how expressions are built from their source code representation.
|C++ documentation for Order of evaluation|
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