C++中的变量存储在哪里? [英] Where are variables in C++ stored?

问题描述

C++ 中的变量存储在哪里?

Where are variables in C++ stored?

在 RAM 或处理器的缓存中?

Inside the RAM or the processor's cache?

推荐答案

存储命名变量:

• 在堆栈上，如果它们是函数局部变量.
  C++ 将此称为自动存储"¹ 并且不要求它实际上是 asm 调用堆栈，并且在一些罕见的实现中它不是.但在主流实现中确实如此.
• 在每个进程的数据区域中，如果它们是全局的或静态.
  C++ 将此称为静态存储类"；它在 asm 中通过在 section .data、.bss、.rodata 或类似内容中放置/保留字节来实现.

• On the stack, if they're function-local variables.
  C++ calls this "automatic storage"¹ and doesn't require it to actually be the asm call stack, and in some rare implementations it isn't. But in mainstream implementations it is.
• In a per-process data area if they are global or static.
  C++ calls this "static storage class"; it's implemented in asm by putting / reserving bytes in section .data, .bss, .rodata, or similar.

如果变量是用int *p = new int[10];或类似方法初始化的指针，指针变量p会进入自动存储或静态存储如上.内存中的指向对象是:

If the variable is a pointer initialized with int *p = new int[10]; or similar, the pointer variable p will go in automatic storage or static storage as above. The pointed-to object in memory is:

• 在堆上(C++ 称之为动态存储)，用new 或malloc 等分配
  在 asm 中，这意味着调用分配器函数，如果它的空闲列表为空，它最终可能会通过某种系统调用从操作系统获取新内存.堆"不是现代操作系统/C++ 实现中的单个连续区域.

C 和 C++ 不进行自动垃圾收集，命名变量本身不能处于动态存储(堆")中.动态存储中的对象是匿名的，除了被其他对象指向之外，其中一些可能是适当的变量.(结构或类类型的对象，相对于像 int 这样的原始类型，可以让你在这个匿名对象中引用命名的类成员.在成员函数中，它们甚至看起来是相同的.)

C and C++ don't do automatic garbage collection, and named variables can't themselves be in dynamic storage ("the heap"). Objects in dynamic storage are anonymous, other than being pointed-to by other objects, some of which may be proper variables. (An object of struct or class type, as opposed to primitive types like int, can let you refer to named class members in this anonymous object. In a member function they even look identical.)

这就是为什么您不能(安全/有用地)返回指向局部变量的指针或引用的原因.

This is why you can't (safely/usefully) return a pointer or reference to a local variable.

当然，这一切都在 RAM 中.缓存对用户空间进程是透明的，但它可能会明显影响性能.

This is all in RAM, of course. Caching is transparent to userspace processes, though it may visibly affect performance.

编译器可能会优化代码以将变量存储在寄存器中.这是高度依赖于编译器和代码的，但好的编译器会积极地这样做.

Compilers may optimize code to store variables in registers. This is highly compiler and code-dependent, but good compilers will do so aggressively.

脚注 1:有趣的事实:auto 在 C++03 及更早版本中，并且仍然在 C 中，意味着自动存储类，但现在(C++11)它推断类型.

Footnote 1: Fun fact: auto in C++03 and earlier, and still in C, meant automatic storage-class, but now (C++11) it infers types.

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