值初始化和非 POD 类型

问题描述

一个小时前我发布了一个答案 这里 根据我的说法是正确的.但是我的回答被 Martin B 否决了.他说

你很幸运并且得到了零，因为我放入的内存恰好是零初始化的.标准不保证这一点.

但是在阅读 Michael Burr 的回答这里并尝试以下示例代码后>

1)

#include 结构 B { ~B();国际米;};int main(){B * b = 新 B();断言(b-> m == 0)；}

我在 MSVC++ 2010 上遇到了调试错误.

当我尝试以下代码时遇到了类似的错误 [我的回答 此处] 在 MSVC++2010 上

2)

#include 结构体{std::string 字符串；int 整数；布尔k;//添加添加添加};结构指令:结构{结构():结构(){}};int main(){指令我;断言(i.k == 0)；}

(1) 和 (2) 都没有在 gcc/Clang 上给出任何这样的错误，这让我觉得 MSVC++2010 是否不支持 C++03.我不确定.

根据 Michael Burr 的帖子 [在 C++03 中]

new B() - 值初始化 B，它对所有字段进行零初始化，因为它的默认构造函数是编译器生成的，而不是用户定义的.

标准说

要对 Tmeans 类型的对象进行值初始化:

——如果 T 是一个类类型(第 9 条)，带有用户声明的构造函数(12.1)，然后调用 T 的默认构造函数(如果没有可访问的默认构造函数);

.....

否则，对象被零初始化

从第一点开始，如果没有用户声明的默认构造函数，编译器合成的默认构造函数将被调用，它将零初始化所有字段(根据最后一点).

那么我错在哪里?我对值初始化的解释是否正确?

Visual Studio 在所有当前版本(2005、2008、2010)中都存在已知错误，即它没有正确实现非 POD 类型的值初始化'没有用户声明的构造函数.

根据语言规则，你们断言都不应该触发，但确实会出现编译器问题.这些是一些错误报告，请注意它们都已关闭或已解决为无法修复".

http://connect.microsoft.com/VisualStudio/反馈/细节/564268/c-value-initialization

http://connect.microsoft.com/VisualStudio/feedback/details/484295/vc-does-not-value-initialize-members-of-derived-classes-without-user-declared-构造函数

http://connect.microsoft.com/VisualStudio/feedback/details/100744/value-initialization-in-new-expression

An hour ago I posted an answer here which according to me was correct. However my answer was downvoted by Martin B. He said

You're just lucky and are getting zeros because the memory that i was placed in happened to be zero-initialized. This is not guaranteed by the standard.

However after reading Michael Burr's answer here and trying the following sample code

1)

#include <cassert>

struct B { ~B(); int m; };

int main()
{
   B * b = new B();
   assert(b->m == 0);
}

I got a debug error on MSVC++ 2010.

I got a similar error when I tried the following code [My answer here] on MSVC++2010

2)

#include <cassert>
struct Struct {
    std::string String;
    int Int;
    bool k;
    // add add add
};

struct InStruct : Struct
{
   InStruct() : Struct() {}
};

int main()
{
   InStruct i;
   assert(i.k == 0);
}

Neither (1) nor (2) gave any such error on gcc/Clang which made me think if MSVC++2010 does not support C++03. I am not sure.

According to Michael Burr's post [in C++03]

new B() - value-initializes B which zero-initializes all fields since its default ctor is compiler generated as opposed to user-defined.

The Standard says

To value-initialize an object of type Tmeans:

— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if Thas no accessible default constructor);

.....

otherwise, the object is zero-initialized

From the first point if there is no user declared default constructor the compiler synthesized default constructor will be called which will zero initialize all the fields (according to last point).

So where am I wrong? Is my interpretation of value initialization correct?

Visual Studio has known bugs in all current versions (2005, 2008, 2010) where it doesn't correctly implement value-initialization for non-POD types that don't have a user declared constructor.

By the language rules none of you asserts should fire but do exhibit the compiler issues. These are some of the bug reports, note that they are all closed or resolved as "Won't Fix".

http://connect.microsoft.com/VisualStudio/feedback/details/564268/c-value-initialization

http://connect.microsoft.com/VisualStudio/feedback/details/484295/vc-does-not-value-initialize-members-of-derived-classes-without-user-declared-constructor

http://connect.microsoft.com/VisualStudio/feedback/details/100744/value-initialization-in-new-expression

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