-声明时变量的默认值- [英] - Default value of variables at the time of declaration -

问题描述

在初始化变量之前，我想知道变量的默认值是什么...

I was wondering what were the default values of variables before I initialized them...

例如，如果我这样做了：

For example, if I do :

//myClass.h

BOOL myBOOL; // default value ?
NSArray *myArray; // default value ?
NSUInteger myInteger; // default value ?

此处有更多示例：

//myClass.m
// myArray is not initialized, only declared in .h file

if ([myArray count] == 0) { // TRUE or FALSE ?

// do whatever

}

更一般而言，执行此操作时将返回什么：

More generally, what is returned when I do :

[myObjectOnlyDeclaredAndNotInitialized myCustomFunction];

感谢您的回答。

推荐答案

答案是，它取决于定义变量的范围。

The answer is that it depends on the scope in which the variable is defined.

由于分配的内存为零，因此Objective-C对象的实例变量始终初始化为0 / nil / false。

Instance variables of Objective-C objects are always initialised to 0/nil/false because the memory allocated is zeroed.

全局变量为可能初始化为0 / nil / false的原因是，当首次将内存分配给进程时，操作系统也会将其清零。但是，理所当然的是，我从不依赖于此，并且总是自己对它们进行初始化。

Global variables are probably initialised to 0/nil/false to because when memory is first allocated to a process, it is also zeroed by the operating system. However, as a matter of course, I never rely on that and always initialise them myself.

局部变量是未初始化的，并且根据堆栈的增长方式将包含随机数据/ shrunk。

Local variables are uninitialised and will contain random data depending on how the stack has grown/shrunk.

NB用于指向Objective-C对象的指针，您可以安全地将消息发送到nil。因此，例如：

NB for pointers to Objective-C objects, you can safely send messages to nil. So, for instance:

NSArray* foo = nil;
NSLog(@"%@ count = %d", foo, [foo count]);

是完全合法的，并且不会崩溃，输出如下：

is perfectly legal and will run without crashing with output something like:

2010-04-14 11:54:15.226 foo[17980:a0f] (null) count = 0

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