VS2013默认初始化vs值初始化 [英] VS2013 default initialization vs value initialization

问题描述

请考虑下面的代码

  struct B 
 {
 B（）：member {} { }; 
 int member [10]; 
} 
 
 int main（）
 {
 B b; 
} 
  

VS2013编译器提供以下警告：

警告C4351：新行为：数组'B :: member'的元素将是
默认初始化1> test.vcxproj - >
C： \\Users\asaxena2\documents\visual studio
2013\Projects\test\Debug\test.exe

这是此处的说明

使用C ++ 11，并应用默认初始化的概念，意味着B.member的元素不会被初始化。

但我相信 member {} 应该执行值初始化而不是默认初始化。是VS2013编译器坏了吗？

$ 8.5 / 6

类型 T 的对象意味着：
- 如果 T 是（可能是cv限定的）类类型（条款9），调用 T 的默认构造函数（如果 T 没有;
- 如果 T 是数组类型，每个元素都默认初始化;

-
如果程序调用 const - 限定类型 T ， T 是一个具有用户提供的默认构造函数的类类型。

$ 8.5.1

类型 T 定义如下：\\
- 如果初始化器列表没有元素，并且 T

- 否则，如果 T 是聚合，则执行聚合初始化（8.5.1）

如果列表中的初始化子句数少于聚合中的成员数，那么没有显式初始化的每个成员都应该从一个空的初始化列表.4）。 [示例：

  struct S {int a; const char * b; int c; }; 
 S ss = {1，asdf}; 
  

使用<$ c初始化 ss.a $ c> 1 ， ss.b 与asdf $ c> cs.c 与 int（）形式的表达式的值，即 0 。 -end example ]

解决方案是一个不正确的措辞的警告消息（我惊讶的是在第一个地方打印警告），但行为正确。 B :: member 正在进行值初始化，对于 int 的数组，变为零初始化。这可以通过以下方式证明：

  #include< iostream> 
 
 struct B 
 {
 B（）：member {} {}; 
 int member [10]; 
} 
 
 struct C 
 {
 C（）{}; 
 int member [10]; 
} 
 
 int main（）
 {
 B b; 
 for（auto const& a：bmember）std :: cout<< a<<< ''; 
 std :: cout<< std :: endl; 
 
 C c; 
 for（auto const& a：cmember）std :: cout<< a<< ''; 
 std :: cout<< std :: endl; 
} 
  

如果您在调试模式下编译并运行导致输出：

  0 0 0 0 0 0 0 0 0 0 
 -858993460 -858993460 -858993460  - 858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 
  

第二行中的数字为 0xCCCCCCCC ，调试模式VC ++编译器在调试模式下填充内存。因此， B :: member 正在进行零初始化，而不对 C :: member 执行初始化。 p>

免责声明：我知道从未初始化的变量读取未定义的行为，但这是最好的证明。

Consider the code below

struct B
{
    B() : member{}{};
    int member[10];
};

int main()
{
    B b;
}

VS2013 compiler gives the following warning:

warning C4351: new behavior: elements of array 'B::member' will be default initialized 1> test.vcxproj -> C:\Users\asaxena2\documents\visual studio 2013\Projects\test\Debug\test.exe

This is documented here

With C++11, and applying the concept of 'default initialization', means that elements of B.member will not be initialized.

But I believe that member{} should perform value initialization and not default initialization. Is the VS2013 compiler broken?

$8.5/6

To default-initialize an object of type T means: — if T is a (possibly cv-qualified) 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, no initialization is performed.
If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

$8.5.1

List-initialization of an object or reference of type T is defined as follows:
— If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
— Otherwise, if T is an aggregate, aggregate initialization is performed (8.5.1).

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from an empty initializer list (8.5.4). [ Example:

  struct S { int a; const char* b; int c; };
  S ss = { 1, "asdf" };

initializes ss.a with 1, ss.b with "asdf", and ss.c with the value of an expression of the form int(), that is, 0. —end example ]

解决方案

It seems to be an incorrectly worded warning message (and I'm surprised it is printing a warning in the first place), but the behavior is correct. B::member is being value initialized, which for an array of int turns into zero initialization. This can be demonstrated using the following:

#include <iostream>

struct B
{
    B() : member{}{};
    int member[10];
};

struct C
{
    C() {};
    int member[10];
};

int main()
{
    B b;
    for(auto const& a : b.member) std::cout << a << ' ';
    std::cout << std::endl;

    C c;
    for(auto const& a : c.member) std::cout << a << ' ';
    std::cout << std::endl;
}

If you compile and run in Debug mode this results in the output:

0 0 0 0 0 0 0 0 0 0
-858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460

The numbers in the second line are 0xCCCCCCCC, the debug pattern the VC++ compiler fills memory with in Debug mode. Thus B::member is being zero-initialized, while no initialization is performed for C::member.

Disclaimer: I know that reading from an uninitialized variable is undefined behavior, but this is the best proof I could come up with.

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