类组件初始化顺序 [英] Class component order of initialisation

问题描述

  D类：A 
 {
 B obj; 
 C obj2; 
} 
  

这里的构造顺序是什么？ / p>

我知道D将在A，B和C之后构造，但我真正想知道的是A是否被保证

我知道你可以有一个明确的初始化列表：

/ p>

  D（）：A（），B（），C（）
 {} 
  

但是该初始化列表是否确定初始化的顺序

？

此外，是否任何组件有或没有默认构造函数？

解决方案

从C ++ 03标准ISO / IEC 14882：2003（E）§12.6.2/ 5 [class.base.init]：

初始化将按照以下顺序进行：

- 首先，只有最大派生类的构造函数如下所述，虚拟基类才会按照它们出现在深度 - 首先从左到右遍历基类的有向无环图，其中从左到右是派生类中的基类类名的出现顺序 base-specifier-list 。

- 然后，直接基类应按照声明顺序初始化，因为它们出现在 base-specifier-list 中（不管 mem- ）。

- 然后，非静态数据成员将按照它们在类定义中声明的顺序进行初始化（同样不考虑 mem-initializers ）。

- 最后，构造函数的主体被执行。

[注意：声明顺序是强制的，以确保base和成员子对象以与初始化的相反顺序被破坏。 ]

因此，在这种情况下，您可以保证初始化的顺序将首先是基类 A ，那么子对象 B （因为它首先出现在类定义中的类成员列表中），那么子对象 C 。初始化器列表的顺序是无关紧要的，无论任何成员是否具有默认构造函数 - 如果成员没有默认构造函数，并且没有在初始化器列表中显式初始化，那么它具有一个未指定的值。

class D: A
{
    B obj;
    C obj2;
}

What order of construction here is guaranteed?

I know that D will be constructed after A, B and C, but what I really want to know is whether A is guaranteed to be constructed before B or C, or even whether B is guaranteed to be constructed before C.

I know you can have an explicit initialiser list:

D(): A(), B(), C()
{}

but does that initialiser list determine the order of initialisation?

Also, does whether or not any of the components do or don't have a default constructor?

解决方案

From the C++03 standard ISO/IEC 14882:2003(E) §12.6.2/5 [class.base.init]:

Initialization shall proceed in the following order:
— First, and only for the constructor of the most derived class as described below, virtual base classes shall be initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where "left-to-right" is the order of appearance of the base class names in the derived class base-specifier-list.
— Then, direct base classes shall be initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
— Then, nonstatic data members shall be initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
— Finally, the body of the constructor is executed.
[Note: the declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. ]

So in this case, you are guaranteed that the order of initialization will be first the base class A, then the subobject B (since it appears first in the list of class members in the class definition), then the subobject C. The order of the initializer list is irrelevant, as is whether or not any of the members do or do not have a default constructor—if a member does not have a default constructor and it is not explicitly initialized in an initializer list, then it has an unspecified value.

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