任何好的理由赋值运算符是不是一个顺序点？ [英] Any good reason why assignment operator isn't a sequence point?

问题描述

有没有什么好的理由运算符= 不是一个顺序点？无论是在C和C ++。

Is there any good reason for operator = not being a sequence point? Both in C and C++.

我有麻烦思考一个反例。

I have trouble thinking about an counter-example.

推荐答案

通过要求：

在一般情况下，事情需要一个理由为是序列点。他们不需要理由的不可以是一个顺序点;这是默认的。

In general, things need a reason to be a sequence point. They don't need a reason not to be a sequence point; that's the default.

例如，＆放大器;＆安培; 必须是因为短路行为序列点：如果左边是假的，右手边< STRONG>不得进行评估。 （这不只是关于优化;右手边可能有副作用，和/或取决于左侧是真实，如 PTR和放大器;＆安培; ptr-＆GT;数据）。因此左手侧必须首先计算，右手边前，为了查看是否右侧应在所有评估

For example, && must be a sequence point because of short-circuiting behaviour: if the left-hand side is false, the right-hand side must not be evaluated. (This is not just about optimization; the right-hand side could have side effects, and/or depend on the left-hand side being true, as in ptr && ptr->data.) Therefore the left-hand side must be evaluated first, before the right-hand side, in order to see if the right-hand side should be evaluated at all.

这个理由不存在了 = 因为，虽然有评价​​为双方做（尽管有什么可以在两侧出现不同程度的限制：在左侧必须是一个左值 - 的 L不能代表左，顺便说一句，它的全称是位置，如在内存中的位置 - 我们不能分配到临时或文字），它无关紧要哪一方先计算 - 只要双方都实际转让前评估

This reason does not exist for = because, although there is "evaluation" to do for both sides (although there are different restrictions on what can appear on both sides: the left-hand side must be an lvalue - the l doesn't stand for "left", btw; it stands for "location", as in location in memory - we can't assign to a temporary or a literal), it doesn't matter which side is evaluated first - as long as both sides are evaluated before the actual assignment.

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