C ++空 - 整数成员初始化 - 将内存清零？ [英] C++ empty-paren member initialization - zeroes out memory?

问题描述

我最初编写了这样的代码：

I originally wrote some code like this:

class Foo
{
public:
  Foo() : m_buffer()
    {}

private:
  char   m_buffer[1024];
};

比我更聪明的人说，使用m_buffer（）初始化器会清空内存。我的意图是留下内存未初始化。我没有时间讨论它，但它刺激了我的好奇心。

Someone who is smarter than me said that having the m_buffer() initializer would zero out the memory. My intention was to leave the memory uninitialized. I didn't have time to discuss it further, but it piqued my curiosity.

以前，我一直认为总是列出每个成员在初始化列表是明智的。

Previously, I had thought it was wise to always list each member in the initializer list.

有人可以进一步描述这种行为吗？

Could someone please describe this behavior further?

1）为什么空 - ？

1) Why does the empty-paren initializer fill in memory?

2）它只适用于POD数据类型吗？

2) Does it only hold for POD datatypes? I heard that it was so, but don't have the standard handy.

感谢

推荐答案

如果你有一个初始化的成员，它将值初始化。对于POD也是如此。对于一个结构，每个成员都是以这种方式初始化的，对于一个数组，它的每个元素都是值初始化的。

If you have a member initialized like that, it will be value-initialized. That is also true for PODs. For a struct, every member is value-initialized that way, and for an array, every element of it is value-initialized.

价值初始化为一个标量类型，如指针或整数，你将它初始化为 0 类型。所以你会得到空指针或假或任何具体的类型。

Value-initialization for a scalar type like pointer or integer you will have it inialized to 0 converted to the right type. So you will get null pointers or false or whatever type you have concretely.

请注意，规则从C ++ 98改变为C ++ 03（我们现在的），这可能有惊人的效果。 C ++ 98没有值初始化。它说，默认初始化发生，对于非POD类型总是意味着它的默认构造函数调用。但是如果没有用户声明的构造函数，C ++ 03中的 value-initialization 有特殊的意义：每个元素都是值初始化的。

Note that the rule changed subtly from C++98 to C++03 (what we have right now), which can have surprising effects. C++98 didn't have that value-initialization. It said default initialization happens, which for a non-POD type always meant it's default constructor invokes. But value-initialization in C++03 has special meaning if there is no user-declared constructor: Every element is value-initialized then.

这里是区别：

struct A { int c; ~A() { } }; // non-POD, but no user declared ctor
struct B { A a; B():a(){ } } b;

现在，在C ++ 03中，将保证 / code>为零。而在C ++ 98中， b.a.c 将有一些不确定的值。

Now, in C++03, you will be guaranteed that b.a.c is zero. While in C++98, b.a.c will have some indeterminated value.

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