为什么NSArray不为不存在的索引返回NULL？ [英] Why Doesn't NSArray return NULL for nonexistent indexes?

问题描述

我理解Objective C的一个特别的事情是你可以发送消息到NULL，它会忽略它们而不是崩溃。

为什么是NSArray不只是返回一个NULL对象，如果请求的索引超出边界而不是引起NSRangeException？

我期望从Objective C和NSArray如下：

  NSArray * array = [NSArray arrayWithObjects：@Object 1，@Object 2 Object 3，nil]; 
 
 for（int i = 0; i <5; i ++）{
 NSString * string = [array objectAtIndex：i]; 
 
 if（string）{
 NSLog（@Object：％@，string）; 
} 
} 
  

允许我访问数组的索引， t包含对象并简单地返回NULL。然后我可以检查对象是否存在。我可以在其他地方这样做，如检查对象是否已实例化。

  NSArray * array; 
 
 if（！array）{
 array = [NSArray array]; 
} 
  

我发现这是一个基于理论的问题，但我很好奇） / p>

解决方案

到nil的邮件是语言级别的功能，而不是框架API功能。

NSArray （和其他类）抛出一个异常来指示程序员错误是一个有意识的设计决策，当API被设计时（〜1993）。正如Kevin和Richard所指出的， nil eats消息是没有错误的，而超出范围的错误输入。

你在哪里画线？应该 substringWithRange：接受任何旧的输入？ setObject：atIndex：？

通过使NSArray @throw对 indexOfObject：它使API一致; 任何超出范围的索引（或范围）将会 @throw 。

The way I understand it one of the things special about Objective C is that you can send messages to NULL and it will just ignore them instead of crashing.

Why is it that NSArray doesn't just return a NULL object if the index requested is out of bounds instead of causing a NSRangeException?

What I would expect from Objective C and NSArray is the following.

NSArray *array = [NSArray arrayWithObjects:@"Object 1", @"Object 2", @"Object 3", nil];

for (int i = 0; i < 5; i++) {
    NSString *string  = [array objectAtIndex:i];

    if (string) {
        NSLog(@"Object: %@",string);
    }
}

Allowing me to access indexes of the array with don't contain objects and simply returning NULL. Then I can check if the object exists. I can do this in other places such as checking if the object has been instantiated.

NSArray *array;

if (!array) {
    array = [NSArray array];
}

I realize this is a theory based question but I'm curious :)

解决方案

Messages to nil is a language level feature, not a framework API feature.

That NSArray (and other classes) throw an exception to indicate programmer error is a conscious design decision that was made when the API was designed (~1993). As both Kevin and Richard indicate, nil eats message is not erroneous in anyway whereas out of bounds is very much a case of erroneous input.

Where would you draw the line? Should substringWithRange: accept any old input? How about setObject:atIndex:?

By making NSArray @throw on bounds exceptions for indexOfObject: it makes the API consistent; any index (or range) that is out of bound will @throw.

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