问题描述
介绍了可选对象的code> nullopt_t 和 nullopt
/ p>
Here is described the nullopt_t
and nullopt
for the optional
object proposed for c++:
struct nullopt_t{see below};
constexpr nullopt_t nullopt(unspecified);
[...]
类型nullopt_t不应该有默认构造函数。它应该是
字面类型。常量nullopt将用一个参数
初始化为字面类型。
[...] Type nullopt_t shall not have a default constructor. It shall be a literal type. Constant nullopt shall be initialized with an argument of literal type.
这个原因在章节:对于 op = {}
是明确的,必须采用一些技巧,其中之一是 nullopt_t
是默认构造的。
The reason for this is explained in the The op = {} syntax chapter of the document: for the op = {}
to be unambiguous some tricks have to be adopted, one of which is that nullopt_t
must not be default constructible.
我的问题是文字类型是什么意思?我发现此。所以它看起来我只是另一个空类会做。它也可以是一个构造函数取 int
?
My question is about what does the literal type means here? I found this SO post. So it looks to me that just another empty class would do. Could it also be a constructor taking a int
?
什么是最小符合 nullopt_t
类看起来像?
What would be a minimal conforming nullopt_t
class look like?
类似这样:
struct nullopt_t_construct_tag_t{};
struct nullopt_t {
nullopt_t() = delete; // I know declaring it as deleted is redundant
constexpr nullopt_t(nullopt_t_construct_tag_t) {};
};
constexpr nullopt_t nullopt(nullopt_t_construct_tag_t{});
或:
struct nullopt_t {
nullopt_t() = delete;
constexpr nullopt_t(int) {};
};
constexpr nullopt_t nullopt(0);
推荐答案
一个最小的实现是
struct nullopt_t {
constexpr nullopt_t(int) {}
};
没有默认构造函数将被隐式声明,[class.ctor] / 4:
No default constructor will be implicitly declared, [class.ctor]/4:
...和 nullopt_t
可以从 int
,一个文字类型构造。
请注意,在你的代码中,默认构造函数存在,虽然被定义为已删除。
... and nullopt_t
can be constructed from int
, a literal type.
Note that in your code a default constructor exists, though being defined as deleted.
上述定义符合文字类型的要求:
The above definition does meet the requirements for a literal type:
- 它有一个简单的析构函数,
- 它是一个聚合类型(8.5.1)或至少有一个
constexpr
constructor [..]不是复制或移动构造函数,并且 - 其所有非静态数据成员和基类都是非 - volatile字面类型。
- it has a trivial destructor,
- it is an aggregate type (8.5.1) or has at least one
constexpr
constructor [..] that is not a copy or move constructor, and - all of its non-static data members and base classes are of non-volatile literal types.
这篇关于experimental ::可选的nullopt_t构造函数的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!