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所有对象占用的内存量都相似吗？ Objective-c

发布于 2024-10-04 21:07:56 字数 114 浏览 8 评论 0原文

如果我使我的对象成为 UIViewController 的子类，它是否比 NSObject 的子类使用更多的内存？一个关于复杂类的子类化与简单类的子类化所使用的开销的大概数字会很棒。编辑：或者我自己找出差异的方法。

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谜兔 2024-10-11 21:07:56

您可以想象，objective-c 对象只是一个如下所示的 C 结构：

typedef struct {
    Class isa;
} NSObject;

该结构的实例在 32 位系统上占用 4 个字节。由于它由单个指针组成 - Class 类似于 id。

NSObject 的子类 MySubclass 具有一个“char”实例变量，看起来像这样：

typedef struct {
    Class isa;
    char singleInstanceVariable.
 } MySubclass;

子类在开头简单地具有其超类的所有实例变量，在末尾加上它自己的实例变量。您可以通过在控制台中键入“p *object”在调试器中看到这一点。

在 32 位系统上，MySubclass 的大小为 5 个字节。一个指针，加一个字符。因此，对象的大小是其所有实例变量的大小之和。要知道的一件重要的事情是对象的大小仅与其实例变量相关。它不受其拥有的方法数量的影响。随着实例化更多实例，这些方法不会消耗任何额外的内存。方法具有固定的初始成本。

另一件需要考虑的事情是，对象通常具有指向其他对象的指针作为实例变量。例如，假设每个 UIView 都有一个 NSMutableArray 来引用其子视图。该数组为空时可能为 12 个字节。因此，空 UIView 将是 UIView 中所有变量的大小，其中包括指向数组的指针的 4 个字节，此外您还可能考虑实际数组实例使用的 12 个附加字节。这只是会计，数组和视图是两个不同的对象，但如果没有数组，视图就无法真正使用。

最后，大多数分配都会向上舍入到某个量子，以便使 malloc 实现更快并满足机器体系结构的某些约束，以便指针正确对齐。此外，对象的实例变量之间可能有空填充，类似于结构填充

You can imagine that an objective-c object is just a C structure that looks like this:

typedef struct {
    Class isa;
} NSObject;

An instance of that structure would take 4 bytes on a 32-bit system. Since it's composed of a single pointer - Class is similar to id.

A subclass of NSObject, MySubclass with one 'char' instance variable would look like this:

typedef struct {
    Class isa;
    char singleInstanceVariable.
 } MySubclass;

A subclass simply has all of the instance variables of its super class at the beginning, plus its own at the end. You can see this in the debugger by typing 'p *object' in the console.

MySubclass's size would be 5 bytes on a 32-bit system. One pointer, plus one char. So, an object's size is the size of all of it's instance variables added together. One important thing to know is that an object's size is only related to its instance variables. It isn't impacted by the number of methods it has. Those methods don't cost any extra memory as more instances are instantiated. Methods have a fixed initial cost.

Another thing to consider is that objects usually have pointers to other objects as instance variables. For example, let's say every UIView has an NSMutableArray to reference its subviews. That array might be 12 bytes when empty. So an empty UIView would be the size of all of the variables in a UIView, which would include 4 bytes for the pointer to an array, plus you might also account for the 12 additional bytes used by the actual array instance. That's all just accounting though, the array and the view are two distinct objects, but the view isn't really usable without the array.

Lastly, most allocations are rounded up to some quantum in order to make the malloc implementation faster and to satisfy some constraints of the architecture of the machine so that pointers are properly aligned. Also an object's instance variables might have empty padding in between them similar to structure padding

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