什么样是字节对齐为什么样要对齐？为什么样我觉得 Objective-C 的内存管理比 C++ 要复杂得多

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初级c++开发者，不是很理解字节对齐的原理。

Computer Systems: A Programmer\'s Perspective:

③.⑨.③ Data Alignment

Many computer systems place restrictions on the allowable addresses for the primitive data types, requiring that the address for some type of object must be a multiple of some value K (typically ② · ④ · or ⑧). Such alignment restrictions simplify the design of the hardware forming the interface between the processor and the memory system. For example, suppose a processor always fetches ⑧ bytes from memory with an address that must be a multiple of ⑧. If we can guarantee that any double will be aligned to have its address be a multiple of ⑧ · then the value can be read or written with a single memory operation. Otherwise, we may need to perform two memory accesses, since the object might be split across two ⑧-byte memory blocks.

The IA③② hardware will work correctly regardless of the alignment of data. However, Intel recommends that data be aligned to improve memory system performance. Linux follows an alignment policy where ②-byte data types (e.g., short) must have an address that is a multiple of ② · while any larger data types (e.g., int, int *, float, and double) must have an address that is a multiple of ④. Note that this requirement means that the least significant bit of the address of an object of type short must equal zero. Similarly, any object of type int, or any pointer, must be at an address having the low-order ② bits equal to zero.

Aside: A case of mandatory alignment

For most IA③② instructions, keeping data aligned improves efficiency, but it does not affect program behavior. On the other hand, some of the SSE instructions for implementing multimedia operations will not work correctly with unaligned data. These instructions operate on ①⑥-byte blocks of data, and the instructions that transfer data between the SSE unit and memory require the memory addresses to be multiples of ①⑥. Any attempt to access memory with an address that does not satisfy this alignment will lead to an exception, with the default behavior for the program to terminate.

This is the motivation behind the IA③② convention of making sure that every stack frame is a multiple of ①⑥ bytes long (see the aside of page ②②⑥). The compiler can allocate storage within a stack frame in such a way that a block can be stored with a ①⑥-byte alignment.

Aside: Alignment with Microsoft Windows

Microsoft Windows imposes a stronger alignment requirement—any primitive object of K bytes, for K = ② · ④ · or ⑧ · must have an address that is a multiple of K. In particular, it requires that the address of a double or a long long be a multiple of ⑧. This requirement enhances the memory performance at the expense of some wasted space. The Linux convention, where ⑧-byte values are aligned on ④-byte boundaries was probably good for the i③⑧⑥ · back when memory was scarce and memory interfaces were only ④ bytes wide. With modern processors, Microsoft’s alignment is a better design decision. Data type long double, for which gcc generates IA③② code allocating ①② bytes (even though the actual data type requires only ①⓪ bytes) has a ④-byte alignment requirement with both Windows and Linux.

C++ 其实是放弃了内存管理作为语言特性，而把责任推给了库。这么做有几个负面的历史遗产：

让整整①代程序员认为 OOP 这种东西是不需要 GC 的（注意我认为 reference-counting 也是①种 GC）。让 value-copy 成为①种编程风格。让采用不同内存管理模式的人在①个项目里互相制造麻烦。①个 ①⓪ 人的团队，只要①个人某天头脑发热用了①个 raw pointer ，整个团队所有的 shared_ptr 的用法就会整个失败。

具体可看：

其实从原理上来说，RC这种应该是更简单也更明了，完全遵守了谁创建(拥有)谁释放的原则。

反观C++则做不到：经常有对象需要在其作用域后仍旧存在，比如线程池封装的task对象等等。

如果说趋势的话未必，GC才是趋势，RC或者ARC这种半自动的形式只能说是①种过渡。 (前面也有童鞋认为RC或者ARC是GC的①种...如果这么说的话的确是趋势)

基本可以把C++的方式 RC/ARC 和 GC③种类比为汽车的手动档，手自①体和自动档。

不考虑耗油方面的因素的话，自动档必然是趋势。

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