删除构造函数的C ++ 14值初始化 [英] C++14 value-initialization with deleted constructor

问题描述

我有一些误解：

让我们将struct A的默认构造函数标记为已删除：

  struct A 
 {
 A（）= delete; 
}; 
  

下一条指令格式正确，效果如何？：

  A a {}; 
  

从 cppreference 值初始化 ：

1）如果T是类类型没有默认构造函数或者用
用户提供的默认构造函数或者删除默认的
构造函数，对象是默认初始化的。

但是默认初始化的效果是：

如果T是类类型，默认构造函数被调用为新对象提供
初始值。

还是聚合初始化？
谢谢！

解决方案

您的 struct A / p>

• 一个类类型包含：
  
  
  
  • 没有用户提供的构造函数 ¹
    
  • 没有私人或受保护的非静态资料成员，
  
  
  • 没有基类，
  
  
  • 没有虚拟成员函数。
  
  

聚合类型，根据第8.5.1 / 1节提供的定义。

然后聚合初始化的值优先于值初始化。该标准说，聚合初始化优先于价值初始化（草案N3936，第8.5.4 / 3，第201页）（强调我）

类型T的对象或引用的列表初始化定义如下：

• 如果T是聚合，


• 否则，如果初始化器列表没有元素，T是具有默认构造函数的类类型，则对象将被初始化。


• [...更多规则...]

^（1） 根据为什么已删除的构造函数不会计为用户定义的注释中的要求， （N3936草案，第8.4.2 / 5页，第198页）：

如果用户声明函数，未在其第一个声明中显式默认或删除。

I have some misunderstanding:

Let's mark default constructor of struct A as deleted:

struct A
{
  A() = delete;
};

The next instruction is well-formed and what's that effect?:

A a{};

From cppreference value initilization:

1) If T is a class type with no default constructor or with a user-provided default constructor or with a deleted default constructor, the object is default-initialized.

but then the effect of default initialization is:

If T is a class type, the default constructor is called to provide the initial value for the new object.

Or it's aggregate initialization? Thanks!

解决方案

Your struct A is :

• a class type that has:
  • no user-provided constructors¹,
  • no private or protected non-static data members,
  • no base classes,
  • no virtual member functions.

It therefore qualifies as an aggregate type, according to the definition provided by § 8.5.1/1.

Then comes the priority of aggregate initialization over value initialization. The standard says that aggregate initialization has precedence over value intialization (draft N3936, § 8.5.4/3, page 201) (emphasis mine)

List-initialization of an object or reference of type T is defined as follows:

• If T is an aggregate, aggregate initialization is performed (8.5.1).
• Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
• [... more rules...]

⁽¹⁾ As requested in the comments on why a deleted constructor does not count as user-defined, here is what the standard says (draft N3936, § 8.4.2/5, page 198):

A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

这篇关于删除构造函数的C ++ 14值初始化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IT屋！