如何分配“无"?到std :: optional< T&gt ;? [英] How to assign "nothing" to std::optional<T>?

问题描述

在阅读 std::optional 时，我很困惑将 something 和 nothing 的值分配给optional的标准方法.

Reading about std::optional, I'm confused about what is the standard way to assign a value of something and nothing to an optional.

我想operator=将是为可选值分配值的标准机制.似乎可以工作.这是否意味着基础对象的副本?

I guess operator= would be the standard mechanism to assign a value to an optional. Seems to work. Does this imply a copy of the underlying object?

什么都不分配的标准方法是什么?我看过x = {}(对我来说这没有意义，因为{}是一个空块)和x = std::nullopt.

What is the standard method to assign nothing? I've seen x = {} (which makes no sense to me as {} is an empty block) and x = std::nullopt.

推荐答案

我不会说有一种标准"方式，不向std :: optional分配任何内容.

I wouldn't say there is one "standard" way to assign nothing to std::optional.

如果您阅读了std::optional("A建议添加一个实用程序类来表示可选对象")，作者以这种方式提出了两种方式:

If you read the proposal for std::optional ("A proposal to add a utility class to represent optional objects") the authors present two ways, in this order:

我们在标准语中添加了额外的要求，以确保以下语法可用于重置可选内容:

We put the extra requirements in the standardese to make sure that the following syntax works for resetting the optional:

op = {};

我们认为这将成为在C ++中重置(进入默认构造状态)值的常见习惯用法

We consider that this will become a common idiom for resetting (putting into default-constructed state) values in C++

请注意，这不是脱离可选对象的唯一方法.您还可以使用:

Note that it is not the only way to disengage an optional object. You can also use:

op = std::nullopt;

std::optional::reset在编写该提案时不存在，因此没有提及，但这也是一种有效的实现方式.

std::optional::reset did not exist at the time of writing that proposal, so it was not mentioned, however it is also a valid way to do it.

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