这== NULL //这怎么可能？ [英] this == null // How can it be possible?

问题描述

最近我遇到了我的应用程序的一些奇怪的行为。它已经开发主要在C＃但CLI / C ++也用于实现更好的性能。我是在时间跨度比较得到一个System.NullReferenceException在一个非常简单的方法：

Recently I came across some strange behaviour of my application. It has been developed mainly in C# but CLI/C++ was also used to achieve better performance. I was getting a System.NullReferenceException in a very simple method at the TimeSpan comparison:

TimeSpan _timestamp;
void UpdateFrame(TimeSpan timestamp)
{
    if(TimeSpan::Equals(_timestamp, timestamp) == false) 

很明显，在这个前pression使用的唯一参照的是隐含的这个（this._timestamp）。我添加了一个断言语句，它原来，这实际上是空。经过简短的调查，我设法ppared短节目presenting这种现象$ P $。这是C ++ / CLI。

It was obvious that the only reference used in this expression was implicit this (this._timestamp). I added an assert statement and it turned out that this is actually null. After short investigation I managed to prepared short program presenting this phenomenon. It is C++/CLI.

using namespace System;
using namespace System::Reflection;

public class Unmanaged
{
public:
    int value;
};

public ref class Managed
{
public:
    int value;

    Unmanaged* GetUnmanaged()
    {
        SampleMethod();
        return new Unmanaged();
    }

    void SampleMethod()
    {
        System::Diagnostics::Debug::Assert(this != nullptr);
        this->value = 0;
    }
};

public ref class ManagedAccessor
{
public:
    property Managed^ m;
};

int main(array<System::String ^> ^args)
{
    ManagedAccessor^ ma = gcnew ManagedAccessor();
    // Confirm that ma->m == null
    System::Diagnostics::Debug::Assert(ma->m == nullptr);
    // Invoke method on the null reference
    delete ma->m->GetUnmanaged();
    return 0;
}

有谁知道这怎么可能？它是在编译器中的错误？

Does anybody know how can it be possible? Is it a bug in the compiler?

推荐答案

在C ++（和presumably在C ++ / CLI）没有什么$ P $从试图调用一个空指针的方法pventing你。在大多数实现中，一个虚方法调用会崩溃的调用点，因为运行时将无法读取虚拟方法表。但是，的非虚的方法调用只是一些参数，其中一个是这指针函数调用。如果它是零，那么这就是被传递给函数。

In C++ (and presumably in C++/CLI) there's nothing preventing you from trying to call methods on a NULL pointer. In most implementations, a virtual method call will crash at the point of the call because the runtime won't be able to read the virtual method table. However, a non-virtual method call is just a function call with some parameters, one of which is the this pointer. If it's null, then that is what is passed to the function.

我相信呼吁一个 NULL （或 nullptr ）的指针是正式未定义的任何成员函数的结果行为。

I believe the result of calling any member function on a NULL (or nullptr) pointer is officially "undefined behaviour".

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