关于操作数求值顺序的矛盾

Question

当我从 deitel c 学习 C 中的递归函数时，我读到了这句话:

Standard C does not specify the order in which the operands of most operators (including +) are to be evaluated.

但书中也说:

associativity of '+' from left to right

操作数的计算顺序:



谁能解释一下这是为什么？

Answer 1

求值顺序和关联性是两个不同的东西，举个例子:

int x = func1() - func2() - func3(); //having int return types

在这个表达式中你不知道是否 func1()将首先或最后被评估，这意味着，您不知道将调用哪个函数并首先返回其值，但是您知道关联性，如 + , 将从左到右，第一个 func1() - func2()那么减法的结果 - func3() .

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.

https://en.cppreference.com/w/c/language/eval_order