具有类成员默认值的C ++ 11默认构造函数行为

问题描述

这个问题是针对C ++ 11的。

This question is for C++11.

在以下结构 A 中，将始终 x

In the following struct A, will x always be 42, when the default constructor is used?

struct A{
  A() = default;
private:
  int x = 42;
}

简而言之，我想知道默认构造函数是否保证默认成员值会被设置。

In short, I'm wondering if the default constructor guarantees that default member values will be set.

推荐答案

是。

不幸的是下面的措词来自今天的标准草案，但是原理在C ++ 11中是相同的。

Unfortunately the wording below is from the standard draft as of today, but the principle is the same in C++11.

在非委托构造函数中，如果给定的潜在构造子对象未由 mem-initializer-id 指定（包括没有 mem-initializer的情况-list ，因为构造函数没有 ctor-initializer ），则：

[class.base.init]/9 In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then:

1. 如果实体是具有默认成员初始值设定项（ [class.mem] ）和
   

1. 的非静态数据成员构造函数的类是联合（ [class.union] ），并且该联合的其他任何变体成员都没有由 mem-initia指定lizer-id 或


2. 构造函数的类不是联合体，并且，如果实体是匿名联合体的成员，则该联合体的其他成员都不会由 mem-initializer-id ，实体是根据 [dcl.init] 中指定的默认成员初始化程序进行初始化的;

1. the constructor's class is a union ([class.union]), and no other variant member of that union is designated by a mem-initializer-id or
2. the constructor's class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id, the entity is initialized from its default member initializer as specified in [dcl.init];

[..]

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紧随上面引用的段落。

An example of exactly this follows the passage latterly quoted above.

但是，如果要定义 A :: A（）并为 x ，。

However, if you were to define A::A() and provide an initialiser for x, it would take precedence over the inline initialiser.

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