c - 以下表达式的行为是否明确定义？

Question

考虑以下表达式中的序列点

i = (++i,i++,i);

如果我是正确的，执行步骤将如下:

1)

++i, i++

2)

step1,i

3)

i = step2

对于步骤 1 中的求值，i 的值只需修改一次即可将构造定义为已定义(因为在求值逗号运算符之后存在一个序列点)。但我认为这里的情况并非如此。因此它应该是未定义的。 请看这个answer 。这里上述表达式被称为定义。我错过了什么吗？

Answer 1

以下指令的行为已明确定义。

i = (++i, i++, i);

逗号运算符 (,) 的第一个和第二个操作数的计算之间确实存在一个序列点。标准的附录 C 虽然内容丰富，但提供了序列点的描述。

C11, Annex C Sequence points

The following are the sequence points described in 5.1.2.3:

— Between the evaluations of the function designator and actual arguments in a function call and the actual call. (6.5.2.2).

— Between the evaluations of the first and second operands of the following operators: logical AND && (6.5.13); logical OR || (6.5.14); comma , (6.5.17).

— Between the evaluations of the first operand of the conditional ? : operator and whichever of the second and third operands is evaluated (6.5.15). — The end of a full declarator: declarators (6.7.6);

— Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions: an initializer that is not part of a compound literal (6.7.9); the expression in an expression statement (6.8.3); the controlling expression of a selection statement (if or switch) (6.8.4); the controlling expression of a while or do statement (6.8.5); each of the (optional) expressions of a for statement (6.8.5.3); the (optional) expression in a return statement (6.8.6.4).

— Immediately before a library function returns (7.1.4). — After the actions associated with each formatted input/output function conversion specifier (7.21.6, 7.29.2).

— Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call (7.22.5).