将可选参数与reference_wrapper结合使用是否有意义?

问题描述

我想到在C ++中可以使用 std :: optional< std :: reference_wrapper< T>> 类型.这种类型的对象实质上是对 T 类型的对象的引用，或者是一个空值，即几乎是一个指针.我的问题:

It occurred to me that in C++ it is possible to use the type std::optional<std::reference_wrapper<T>>. An object of this type is essentially a reference to an object of type T or a null value, i.e., pretty much a pointer. My questions:

1. std :: optional< std :: reference_wrapper< T>> 和 T * 之间有什么概念上的区别吗?

1. Is there any conceptual difference between std::optional<std::reference_wrapper<T>> and T*?

有实际区别吗?在某些情况下，建议在 T * 上选择 std :: optional< std :: reference_wrapper< T>> ?

Is there any practical difference? Are there situations where it might be advisable to choose std::optional<std::reference_wrapper<T>> over T*?

推荐答案

顾名思义，

std :: optional<> 旨在在我们可以有值或根本没有任何值时使用.

std::optional<>, as the name already suggest, is meant to be used when we could have a value or might not have any value at all.

等效于无值的 T * 对象将为其分配 nullptr ，即:指针将指向无处，而不是某处(甚至任何地方，即:未初始化).可以说， std :: optional<> 导出 nullptr 的概念以指向任何任意类型的指针.因此，我想说它们在概念上非常相似，是 std :: option<> 方法的一种概括.

The equivalent of having no value for a T* object would be assigning nullptr to it, i.e.: the pointer will point to nowhere, as opposed to somewhere (or even anywhere, i.e.: uninitialized). It can be said that std::optional<> exports the concept of nullptr for pointers to any arbitrary type. So, I would say they are conceptually very similar, being the std::option<> approach a kind of generalization.

我能想到的大小. std :: optional<> 包含一个内部标志，用于指示值的存在/不存在，而对于 T * ， nullptr 被编码直接作为指针可以存储的值之一.因此， std :: optional< std :: reference_wrapper< T>> 对象将大于 T * .

I can think of the size. std::optional<> contains an internal flag for indicating the presence/absence of a value, whereas for T* the nullptr is encoded directly as one of the values the pointer can store. So a std::optional<std::reference_wrapper<T>> object will be larger than a T*.

在安全性方面，与 T * 不同， std :: optional<> 提供了成员函数 value()如果没有值，则为异常(它提供了与 T * 一样的不安全的 operator *()).

When it comes to safety, unlike T*, std::optional<> provides the member function value() which throws an exception if there is no value (it provides as well as the unsafe operator*() as T* does).

此外，例如，使用 std :: optional< std :: reference_wrapper< T>> 代替 T * ，例如，函数的返回值可能会在一种更明确的方式，可能根本没有任何价值.

Also, using std::optional<std::reference_wrapper<T>> instead of T* , for example, as a function's return value may indicate in a more explicit way that there might be no value at all.

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