blocks|key|1307679|text|根据标准，除非您在初始化器列表中显式初始化，否则它不会|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307680|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

As per the standard, it doesn't unless you explicitly initialize in initializer list

blocks|key|1092511|text|不是的。默认构造函数分配内存并调用任何父级的无参数构造函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1092512|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

No. The default constructor allocates memory and calls the no-argument constructor of any parents.

blocks|key|1307775|text|从技术上讲，它确实会初始化它们--通过使用它们的默认构造函数，顺便说一下，该构造函数除了为它们分配内存之外什么也不做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307776|如果你想知道的是它们是否被设置为类似于ints的0，那么答案是“否”。|offset|length|style|CODE|1307777|entityMap^0|0|J|3|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Technically it does initialize them -- by using their default constructor, which incidentally does nothing but allocate the memory for them.

If what you wanted to know is whether or not they are set to something sane like 0 for <code>int</code>s, then the answer is "no".

blocks|key|1307861|text|正如前面的发言者所说的那样-不，它们没有初始化。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307862|这实际上是一个非常奇怪的错误的来源，因为现代操作系统倾向于用零填充新分配的内存区域。如果你希望如此，它可能会在第一次工作。然而，随着您的应用程序继续运行，delete-ing和new-ing对象，您迟早会遇到这样一种情况:您期望的是零，但是来自较早的对象的非零的剩余物存在。|offset|length|style|CODE|1307863|那么，为什么不是所有的new-ed数据都是新分配的？是的，但并不总是来自操作系统。操作系统倾向于使用更大的内存块(例如，一次4MB+)，因此所有微小的一个字这里三个字节的分配和释放都是在uyserspace中处理的，因此不会被清零。|1307864|PS。我写的是“倾向于”，也就是说你甚至不能指望第一次成功……|1307865|entityMap^0|0|25|6|2G|3|0|B|3|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@$D|P|E|Q|F|G]|$D|R|E|S|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|T|8|@$D|U|E|V|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|W|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|M|$]]

As previous speakers have stated - no, they are not initialized.

This is actually a source for really strange errors as modern OSs tend to fill newly allocated memory regions with zeroes. If you expect that, it might work the first time. However, as your application keeps running, <code>delete</code>-ing and <code>new</code>-ing objects, you will sooner or later end up in a situation where you expect zeroes but a non-zero leftover from an earlier object sits.

So, why is this then, isn't all <code>new</code>-ed data newly allocated? Yes, but not always from the OS. The OS tends to work with larger chunks of memory (e.g. 4MB at a time) so all the tiny one-word-here-three-bytes-there-allocations and deallocations are handled in uyserspace, and thus not zeroed out.

PS. I wrote "tend to", i.e. you can't even rely on success the first time...

blocks|key|1092792|text|我不太确定你的意思，但是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1092793|struct+A+{+int+x;+};

int+a;+//+a+is+initialized+to+0
A+b;+++//+b.x+is+initialized+to+0

int+main()+{
++++int+c;+++++++++//+c+is+not+initialized
++++int+d+=+int();+//+d+is+initialized+to+0

++++A+e;+++++++++++//+e.x+is+not+initialized
++++A+f+=+A();+++++//+f.x+is+initialized+to+0
}|code-block|syntax|javascript|1092794|在我说“未初始化”的每一种情况下，你可能会发现你的编译器给了它一个一致的值，但标准并不要求它。|1092795|很多人挥舞着手，包括我在内，关于内置类型“实际上”是如何拥有默认构造函数的。实际上，缺省初始化和值初始化是标准中定义的术语，我个人每次都要查阅这些术语。只有在标准中定义的类才具有隐式的默认构造函数。|1092796|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I'm not quite certain what you mean, but:

<pre><code>struct A { int x; };

int a; // a is initialized to 0
A b; // b.x is initialized to 0

int main() {
 int c; // c is not initialized
 int d = int(); // d is initialized to 0

 A e; // e.x is not initialized
 A f = A(); // f.x is initialized to 0
}
</code></pre>

In each case where I say "not initialized" - you might find that your compiler gives it a consistent value, but the standard doesn't require it.

A lot of hand-waving gets thrown around, including by me, about how built-in types "in effect" have a default constructor. Actually default initialization and value initialization are defined terms in the standard, which personally I have to look up every time. Only classes are defined in the standard to have an implicit default constructor.

Does the default constructor (created by the compiler) initialize built-in-types?

Does the default constructor initialize built-in types?

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默认构造函数(由编译器创建)会初始化内置类型吗？

问默认构造函数是否初始化内置类型？
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