我什么时候会在 C++ 中使用 const volatile、register volatile、static volatile？
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When would I use const volatile, register volatile, static volatile in C++?
提问by Avraam Mavridis
I am wondering about the different uses of the volatilekeyword in combination with register, constand statickeywords. I am not sure what are the effects, so I think:
我想知道volatile关键字与register、const和static关键字的不同用途。我不确定有什么影响，所以我认为：
register volatile int T=10;
Suggest the compiler to store T in a register and the value of T can be modified from somewhere outside (OS, hardware, another thread)
建议编译器将 T 存储在寄存器中，并且可以从外部（操作系统、硬件、另一个线程）修改 T 的值
const volatile int T=10;
The program itself can not modify T, but T can be modified frow somewhere outside the code.
程序本身不能修改T，但可以在代码之外的某处修改T。
static volatile int T=10;
If T is a data member of a class it means that all the objects of the class have the same value for T and T can be modified from somewhere outside. If T is a global variable in a file, the source code in other files (that are part of the project) cannot access T, but T can be accessed from somewhere outside. If T is a local variable in a function,once it has been initialized remains in the memory until the end of the program and can be modified from somewhere outside.
如果 T 是类的数据成员，则意味着该类的所有对象都具有相同的 T 值，并且可以从外部修改 T。如果 T 是文件中的全局变量，则其他文件（属于项目的一部分）中的源代码无法访问 T，但可以从外部某处访问 T。如果 T 是一个函数中的局部变量，一旦它被初始化，它就会保留在内存中直到程序结束，并且可以从外部的某个地方进行修改。
Are my thoughts correct and can any experienced C++ developer give an example where the above maybe used in real-world applications or it is very rare?
我的想法是否正确，任何有经验的 C++ 开发人员都可以举一个例子，其中上述内容可能在现实世界的应用程序中使用还是非常罕见？
回答by Alok Save
register volatile int T=10;
volatilequalifier means that the compiler cannot apply optimizations or reorder access to T, While registeris a hint to the compiler that Twill be heavily used. If address of Tis taken, the hint is simply ignored by the compiler. Note that registeris deprecated but still used.
volatile限定符意味着编译器不能对 应用优化或重新排序访问T，Whileregister是对T将被大量使用的编译器的提示。如果T取地址，编译器会简单地忽略该提示。请注意，register已弃用但仍在使用。
Practical Usage:
实际用法：
I have never used it never felt the need for it and can't really think of any right now.
我从来没有用过它，从来没有觉得需要它，现在也想不出任何东西。
const volatile int T=10;
constqualifier means that the Tcannot be modified through code. If you attempt to do so the compiler will provide a diagnostic. volatilestill means the same as in case 1. The compiler cannot optimize or reorder access to T.
const限定符意味着T不能通过代码修改。如果您尝试这样做，编译器将提供诊断信息。volatile仍然与情况 1 中的含义相同。编译器无法优化或重新排序对 的访问T。
Practical Usage:
实际用法：
- Accessing shared memory in read-only mode.
- Accessing hardware registers in read-only mode.
- 以只读模式访问共享内存。
- 以只读模式访问硬件寄存器。
static volatile int T=10;
staticstorage qualifier gives Tstatic storage duration (C++11 §3.7) and internal linkage, while volatilestill governs the optimization and reordering.
staticstorage 限定符给出T静态存储持续时间（C++11 §3.7）和内部链接，同时volatile仍然控制优化和重新排序。
Practical Usage:
实际用法：
- Same as
volatileexcept that you need the object to have static storage duration and to be inaccessible from other translation units.
- 一样
volatile，除了你所需要的对象具有静态存储时间和从其他翻译单元无法访问。
