成员初始值设定项的顺序

Question

下面的代码给出了正确的输出，如果我声明变量 i 和 j，比如 int i, j;

class A 
{
  int i, j;

public:
    A(int val) : i(val), j(i + 1)
    {
        cout<<i<<endl<<j<<endl;
    }
};

但是如果我声明变量 i 和 j，比如 int j, i;。然后 j 打印 垃圾值 .

class A 
{
  int j, i;

public:
    A(int val) : i(val), j(i + 1)
    {
        cout<<i<<endl<<j<<endl;
    }
};

所以，是否取决于变量声明的顺序？

Answer 1

Is it depend on order of declaration of variables?

是的，数据成员的顺序（即：class A中的i和j） 已初始化，对应于它们被声明的顺序，而不是它们出现在构造函数的成员初始化列表中的顺序.

您的构造函数的成员初始化列表在 class A

A(int val) : i(val), j(i + 1)

没有说明这些数据成员的初始化顺序。

如果 j 在 i 之前声明（即：int j, i），数据成员 j 仍将在 i 之前初始化。在那种情况下 j 正在被初始化为 i + 1，但是此时 i 未初始化，这可能导致 j 包含垃圾。

在 GCC 中，您可以通过提供 -Wreorder 选项在这些情况下显示警告，该选项已通过传递 -Wall 选项启用。

Answer 2

Is it depend on order of declaration of variables?

是的，数据成员总是按照声明的顺序初始化，这与成员初始化列表的顺序无关。

这意味着对于您的第二个代码片段，j 总是在 i 之前初始化；但是当它被成员初始化器初始化时 i 仍然没有初始化。

对象的完整 initialization order 是：

（强调我的）

The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:

1) If the constructor is for the most-derived class, virtual base classes are initialized in the order in which they appear in depth-first left-to-right traversal of the base class declarations (left-to-right refers to the appearance in base-specifier lists)

2) Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list

3) Then, non-static data members are initialized in order of declaration in the class definition.

4) Finally, the body of the constructor is executed

Answer 3

Is it depend on order of declaration of variables?

当然！标准忽略了初始化器在初始化列表中出现的顺序；只有声明的顺序很重要。这样做是为了 "reverse order of initialization" 在析构函数中有意义，即使可能有多个构造函数的初始化列表以不同的顺序排列。

这是 C++ 标准 (12.6.2.10) 的相关部分：

In a non-delegating constructor, initialization proceeds in the following order:

First, and only for the constructor of the most derived class (1.8), virtual base classes are 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 classes in the derived class base-specifier-list.

Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

Finally, the compound-statement of the constructor body is executed.

[ Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. — end note ]

成员初始值设定项的顺序

Order of member initializers

c++

class

member-initialization