C++ 嵌套条件运算符的求值顺序

Question

对于像这样的表达式

x = a ? b : c ? d : e;

我理解因为 ?: 运算符具有正确的结合性，所以表达式被分组为

x = a ? b : (c ? d : e);

但是，评估的顺序呢？结合性是否意味着 (c ? d : e) 分支首先求值，然后它的答案作为参数传递给左边的 ?: 运算符？还是先评估 a，然后根据返回 b 或评估 (c ? d : e) 分支？还是未定义？

Answer 1

n3376 5.16/1

Conditional expressions group right-to-left. The first expression is contextually converted to bool (Clause 4). It is evaluated and if it is true, the result of the conditional expression is the value of the second expression, otherwise that of the third expression. Only one of the second and third expressions is evaluated. Every value computation and side effect associated with the first expression is sequenced before every value computation and side effect associated with the second or third expression.