是否可以保证访问nil指针会引发异常？ [英] There is guarantee that accessing a nil pointer will raise an exception?

问题描述

雷米·勒博（Remy Lebeau）声明无法保证访问nil指针将引发异常。这是未定义的行为。任何事情都可能发生。可能会引发异常，或者您可能只是读取垃圾，或者您可能会浪费内存，或者...。怎么会？
在这种情况下，访问NIL对象的方法不会引发AV并导致内存损坏吗？

Remy Lebeau stated that "There is no guarantee that accessing a nil pointer will raise an exception. It is undefined behavior. Anything could happen. An exception might be raised, or you might just read garbage, or you might trash memory, or ...". How come? In which circumstances, accessing the methods of a NIL object will NOT raise an AV AND lead to memory corruption?

// (Obj is any kind of object. Let's say TStringList) 
Obj = nil;
Obj.LoadFromFile();

推荐答案

没有保证则访问nil指针将引发
异常。

There is no guarantee that accessing nil pointer will raise an exception.

是。

但是，实际行为取决于实际代码。

However, the actual behavior depends on the actual code.

如果您调用动态或虚拟方法，并且在调用时会引发异常尝试直接访问实例字段。

Exception will be raised if you call dynamic or virtual methods and when you try accessing the instance fields directly.

在 nil 引用上调用静态方法并测试<$是非常安全的c $ c> Self 在其中的 nil 。

It is perfectly safe to call static method on nil reference and testing Self for nil inside it.

最好的例子是静态方法是免费。

The best example of such static method is Free.

procedure TObject.Free;
begin
  if Self <> nil then
    Destroy;
end;

很明显，以上行为已得到很好的定义，否则以前编写的每个Delphi程序都会随机崩溃（或时间）。

Obviously, above behavior is well defined, otherwise every Delphi program ever written would crash randomly (or all the time).

通常，访问 nil 引用的字段将导致崩溃。从理论上讲，足够大的类是可能的。

In general, accessing fields of nil reference will result in crash. In theory with large enough class anything is possible.

David Heffernan提供的示例类

Example class provided by David Heffernan

type
   TSillyExample = class
     FStuff: array [0..SomeVeryLargeNumber-1] of Byte;
     FAreYouFeelingLucky: string;
   end;

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