blocks|key|1106534|text|如果您有：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1106535|MyObject+*o+=+NULL;

o->func();|code-block|syntax|javascript|1106536|接下来会发生什么取决于func是否是虚拟的。如果是，那么它将崩溃，因为它需要一个对象来获取vtable。但是如果它不是虚拟的，那么调用就会将this指针设置为NULL。|offset|length|style|CODE|1106537|我相信标准说这是“未定义的行为”，所以任何事情都可能发生，但典型的编译器只是生成代码而不检查指针是否为空。一些众所周知的库依赖于我所描述的行为:+MFC有一个类似SafeGetHandle的函数，可以在空指针上调用，在这种情况下返回null。|1106538|您可能想要编写一个可重用的助手函数：|1106539|void+CheckNotNull(void+*p)
{
++++if+(p+==+NULL)
++++++++throw+NullPointerException();
}|1106540|然后，可以在函数的开头使用它来检查它的所有参数，包括this|1106541|CheckNotNull(this);|1106542|entityMap^0|0|0|B|4|0|29|D|0|0|0|Q|4|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@$I|14|J|15|K|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|18|8|@$I|19|J|1A|K|L]]|9|@]|A|$]]|$1|U|3|V|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|W|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|X|$]]

If you have:

<pre><code>MyObject *o = NULL;

o-&gt;func();
</code></pre>

What happens next depends on whether <code>func</code> is virtual. If it is, then it will crash, because it needs an object to get the vtable from. But if it's not virtual the call proceeds with the this pointer set to NULL.

I believe the standard says this is "undefined behaviour", so anything could happen, but typical compilers just generate the code to not check whether the pointer is NULL. Some well known libraries rely on the behaviour I described: MFC has a function called something like <code>SafeGetHandle</code> that can be called on a null pointer, and returns NULL in that case.

You might want to write a reusable helper function:

<pre><code>void CheckNotNull(void *p)
{
 if (p == NULL)
 throw NullPointerException();
}
</code></pre>

You can then use that at the start of a function to check all its arguments, including <code>this</code>:

<pre><code>CheckNotNull(this);
</code></pre>

blocks|key|491823|text|捕获此类错误的一种方法(通过设计)是使用指针包装器类(类似于shared_ptr)，该类在创建时使用空指针参数抛出。它可能也会在解除引用时抛出，但有点晚了--我想，总比什么都没有好。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|491824|entityMap^0|U|A|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

A way to trap these kind of errors (by design) is using a pointer-wrapper class (resembling a <code>shared_ptr</code>) that throws upon creation with a null pointer argument. It may throw also when dereferenced, but that's a little late - better than nothing, I guess.

blocks|key|3710253|text|if(this+==+null)
+++throw+new+NullPointerException;
if(!this)
+++return+false;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3710254|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>if(this == null)
 throw new NullPointerException;
if(!this)
 return false;
</code></pre>

blocks|key|1106594|text|this有可能为空。我怀疑这段代码试图(严重地)检测到对象尚未完成初始化或已被删除的竞争条件。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1106595|entityMap^0|0|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

It's possible for <code>this</code> to be null. I suspect that this code is trying to (badly) detect a race condition, where the object is not yet finished being initialized, or has been deleted.

blocks|key|4556712|text|(此==+NULL)是根据标准的未定义行为。我认为您应该删除此检查:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4556713|假设我们进行以下调用：|4556714|((CSomeClass*)+0)->method();|code-block|syntax|javascript|4556715|行为已经是未定义的，那么为什么还要费心在CSomeClass::method中检查这个==+NULL呢？|4556716|如果你不使用成员变量，我假设你的编译器会处理(0+==+)，但是它在哪里能找到虚拟表指针呢？在这种情况下，您的类不能使用多义性。|offset|length|style|BOLD|4556717|entityMap^0|0|0|0|0|S|10|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@$M|X|N|Y|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

(this == NULL) is undefined behaviour according to the standard. I think you should remove this check :)

Assume we make the following call:

<pre><code>((CSomeClass*) 0)-&gt;method();
</code></pre>

The behaviour is already undefined, so why bother doing the check for this == NULL in CSomeClass::method ?

EDITED:
I Assume that your compiler will handle (0 == this) if you don't use member variables, but where would it find the virtual table pointer? In this case, your class can not use polymoprhism.

blocks|key|3710341|text|在类似以下情况下，此指针可能变为null：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3710342|class+Test
{

public:
++++bool+f();

private:
++++int+m_i;
};


bool+Test::f()
{
++++if(!this)
++++{
++++++++return+false;
++++}

++++m_i+=+0;
++++return+true;

}


int+main(int+argc,+char+**argv)
{
++++Test*+p+=+new+Test;
++++delete+p;
++++p+=+NULL;

++++p->f();
}|code-block|syntax|javascript|3710343|我猜有人做了一个快速的黑客攻击，以避免访问冲突异常。|3710344|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

this pointer can become null in cases like these:

<pre><code>class Test
{

public:
 bool f();

private:
 int m_i;
};


bool Test::f()
{
 if(!this)
 {
 return false;
 }

 m_i = 0;
 return true;

}


int main(int argc, char **argv)
{
 Test* p = new Test;
 delete p;
 p = NULL;

 p-&gt;f();
}
</code></pre>

I guess somebody did a quick hack to avoid to the access violation exception.

blocks|key|1112021|text|只有当你在一个已删除的对象上调用一个方法，或者有一些东西不应该写到内存中(特别是覆盖对象的==指针)时，才会出现这个this+null。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1112022|你应该调查你的代码到底出了什么问题，而不是试图像这样解决它。|1112023|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

this == null should only occur if you are calling a method on a deleted object, or if something is writing to memory that it should not (and overwriting the object's this pointer in particular).

You should investigate what is really wrong with your code rather than trying to work around it like this.

I stumbled upon this piece of code which seems totaly broken to me, but it does happen that <code>this</code> is <code>null</code>. I just don't get how this can be <code>null</code>

it is inside a normal method call such as

<pre><code>myObject-&gt;func();
</code></pre>

inside <code>MyObject::func()</code> we have

<pre><code>if (!this) { return false; }
</code></pre>

is there any way I can have the first line to throw a <code>NullPointerException</code> instead of going inside the <code>null</code>(?) method?

if (!this) { return false; }

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我偶然发现了这段代码，这段代码对我来说似乎完全崩溃了，但它确实是this的null。我就是不明白这怎么会是null它在一个普通的方法调用中，比如myObject->func();在MyObject::func()内部，我们有if (!this) { return false; }有没有办法让我的第一行抛出NullPointerException，而不是进入null(?)方法？

问如果(!this) { return false；}
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