成员结构可以从构造函数初始化器列表中的零初始化而不调用memset？ [英] Can a member struct be zero-init from the constructor initializer list without calling memset?

问题描述

假设我有以下结构声明（没有构造函数的简单结构体）。

  struct Foo 
 { 
 int x; 
 int y; 
 int z; 
 char szData [DATA_SIZE]; 
}; 
 

现在我们假设这个结构是C ++类的成员，如下：

  class CFoobar 
 {
 Foo _foo; 
 public：
 CFoobar（）; 
}; 
 

如果我声明CFoobar的构造函数如下：

  CFoobar :: CFoobar（）
 {
 printf（_ foo = {％d，％d，％d} \\\
，_foo.x， _foo.y，_foo.z）; 
 for（int x = 0; x< 100; x ++）
 printf（％d\\\
，_foo.szData [x]）; 
} 
  

正如你所期望的，当CFoobar的构造函数运行时，垃圾数据被打印出来显然，容易修复的是memset或ZeroMemory& _foo。这是我一直做的...

然而，我没有注意到如果添加_foo到构造函数的初始化列表，没有参数如下：

  CFoobar :: CFoobar（）
：_foo（）
 {
  / pre> 
 
 
这似乎是清除_foo的成员变量。至少这是g ++在linux上的情况。
 
 
 
 现在这里是我的问题：这是标准C ++，还是这个编译器特定的行为？ 
 
 
 
如果是标准行为，有人可以从官方来源引用我的参考资料吗？任何有关使用更复杂的结构和类的隐式零初始化行为的陷阱？
解决方案
根据标准。 12.6.2 [class.base.init] / 3：如果省略了em-initializer 的 expression-list ，则基类或成员子对象是 value-initialized 。
 
 
 
但是，如果 Foo 不是POD类型，但仍然没有用户声明的构造函数（例如它有一个 std :: string 类型），那么一些非常流行的编译器将无法正确地初始化< 
当您使用时，我知道的所有编译器都能正确执行
 POD成员的 （）作为构造函数initializer-list中的初始化器。
 
Let's say I have the following structure declaration (simple struct with no constructor).
struct Foo
{
    int x;
    int y;
    int z;
    char szData[DATA_SIZE];
};
Now let's say this struct is a member of a C++ class as follows:
class CFoobar
{
     Foo _foo;
public:
     CFoobar();
};
If I declare CFoobar's constructor as follows:
CFoobar::CFoobar()
{
    printf("_foo = {%d, %d, %d}\n", _foo.x, _foo.y,_foo.z);
    for (int x = 0; x < 100; x++)
       printf("%d\n", _foo.szData[x]);
}
As you would expect, when CFoobar's constructor runs, garbage data gets printed out  Obviously, the easy fix is to memset or ZeroMemory &_foo.  It's what I've always done...

However, I did notice that if add _foo to the constructor's initialization list with no parameters as follows:
CFoobar::CFoobar()
: _foo()
{
That this appears to zero-out the member variables of _foo. At least that was the case with g++ on linux.

Now here's my question: Is this standard C++, or is this compiler specific behavior?

If it's standard behavior, can someone quote me a reference from an official source? Any "gotchas" in regards to implicit zero-init behavior with more complicated structs and classes?
解决方案
Yes, this is defined behaviour according to the standard. 12.6.2 [class.base.init] / 3 : "if the expression-list of the mem-initializer is omitted, the base class or member subobject is value-initialized."

Be warned, though, if Foo wasn't a POD-type but still had no user-declared constructor (e.g. it had a std::string type) then some very popular compilers would not correctly value-initialize it.

All compilers that I know of do correctly perform value-initialization of POD members when you use () as the initializer in a constructor initializer-list.

                        
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