c ++数组零初始化：这是一个bug，还是这是正确的？ [英] c++ array zero-initialization: Is this a bug, or is this correct?

问题描述

注意：我们在这里谈论（推测）C ++ 98兼容的编译器。这不是C ++ 11的问题。

我们在一个编译器中有一个奇怪的行为，我们不知道这是否OK如果这是一个编译器错误：

  //这个结构有一个默认的构造函数
 struct AAA 
 { 
 AAA（）：value（0）{} 
 int value; 
}; 
 
 //这个结构体有一个类型为AAA的数组和一个int数组，两个都包围
 //由ints 
 struct BBB 
 {
 int嘛 ; 
 AAA m_b; 
 int m_c; 
 int m_d [42]; 
}; 
  

当BBB初始化时：

  BBB bbb = {0}; 
  

我们预计BBB的所有POD成员（包括m_d，int数组）

这在Linux / GCC-3.4的Windows / VisualC ++上的AIX本机编译器上工作。 ...但不是在Solaris / SunStudio上，其中只有非数组成员是零初始化的。

我们做了一点研究，在C ++ 98标准（文件草稿），其中我们发现了以下内容：

[12.6.1 - 2]

当聚合（无论是类还是数组）包含类类型的成员，并且由括号括起来的初始化列表（8.5.1）初始化时，每个这样的成员被复制初始化相应的赋值表达式。 如果初始化器列表中的初始化器比聚合器中的初始化器更少，那么每个未明确初始化的成员都将被默认初始化（8.5）。

然后：

对于类型为T的对象，b

对零初始化存储意味着：

- 如果T是标量类型将值0（零）转换为T;

- 如果T是非联合类类型，则每个非静态数据成员和每个基类子对象的存储为零 - 初始化;

- 如果T是联合类型，其第一个数据成员的存储器89）是零初始化;

- 如果T是数组类型，每个元素的存储均为零初始化;

- 如果T是引用类型，则不执行初始化。

然后：

To default-initialize 一个类型为T的对象意味着：

- 如果T是一个非POD类类型（第9节），T的默认构造函数被称为（如果T没有可访问的默认构造函数，则初始化是错误的）;

- 如果T是数组类型，每个元素都是默认初始化的; >
- 否则，对象的存储为零初始化。

我读的方式：SunStudio应该零初始化int数组（BBB :: m_d）

奇怪的事情：如果我们从AAA删除默认构造函数，那么BBB中的所有内容都是零初始化的。

问题：当SunStudio无法初始化一个包含非-荚？或者这是一个编译器错误？

解决方案

这确实是Sun / Solaris的错误。
你写的东西确实是应该发生的，你对你写的一切都是正确的。

Note: We are speaking about (supposedly) C++98 compliant compilers, here. This is not a C++11 question.

We have a strange behavior in one of our compilers and we're not sure if this is Ok or if this is a compiler bug:

// This struct has a default constructor
struct AAA
{
   AAA() : value(0) {}
   int value ;
} ;

// This struct has a member of type AAA and an array of int, both surrounded
// by ints
struct BBB
{
   int m_a ;
   AAA m_b ;
   int m_c ;
   int m_d[42] ;
} ;

When BBB is initialized as such:

BBB bbb = {0} ;

We expected all the POD members of BBB (including m_d, the array of ints) to be zero-initialized, and all the non-POD members of BBB to be constructed.

This worked on the native compiler of AIX, on Linux/GCC-3.4, on Windows/VisualC++... But not on Solaris/SunStudio, where only the non-arrays members are zero-initialized.

We did a little research, in the C++98 standard (a draft document), where we found the following:

[12.6.1 - 2]

When an aggregate (whether class or array) contains members of class type and is initialized by a brace-enclosed initializer-list (8.5.1), each such member is copy-initialized (see 8.5) by the corresponding assignment-expression. If there are fewer initializers in the initializer-list than members of the aggregate, each member not explicitly initialized shall be default-initialized (8.5).

Then:

[8.5 - 5]

To zero-initialize storage for an object of type T means:
— if T is a scalar type (3.9), the storage is set to the value of 0 (zero) converted to T ;
— if T is a non-union class type, the storage for each nonstatic data member and each base-class subobject is zero-initialized;
— if T is a union type, the storage for its first data member 89) is zero-initialized;
— if T is an array type, the storage for each element is zero-initialized;
— if T is a reference type, no initialization is performed.

And then:

To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the storage for the object is zero-initialized.

The way I read it: SunStudio should zero-initialize the array of ints (BBB::m_d)

Strange thing: if we remove the default constructor from AAA, then everything in BBB is zero-initialized.

QUESTION: Is SunStudio behavior standard when it fails to zero-initialize an array of ints of a struct containing a non-POD? Or is this a compiler bug?

解决方案

It is indeed a bug with Sun/Solaris. What you've written is indeed what is supposed to happen and you are correct with everything you write.

这篇关于c ++数组零初始化：这是一个bug，还是这是正确的？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！