C++ 中的易失性与可变性

它们绝对不是一回事。可变与 const 交互。如果您有 const 指针，则通常无法更改成员。可变提供了该规则的例外。

另一方面，Volatile 与程序所做的更改完全无关。这意味着内存可能会由于编译器无法控制的原因而更改，因此编译器每次都必须读取或写入内存地址，并且无法将内容缓存在寄存器中。

mutable：可变关键字覆盖任何封闭的 const 语句。可以修改 const 对象的可变成员。

volatile：volatile 关键字是与实现相关的修饰符，在声明变量时使用，可防止编译器优化这些变量。Volatile 应该与其值可能以意外方式（即通过中断）更改的变量一起使用，这可能与编译器可能执行的优化冲突。

源

即使在通过指针或引用访问的对象中，也可以在对象中更改字段，因此编译器知道不要将其存储在 R/O 内存中。位置是可以通过编译器不知道的代码（例如某些内核级驱动程序）更改的位置，因此编译器知道不要优化该值的寄存器分配，因为该值“不可能更改”，因为它上次加载到该寄存器中。向编译器提供非常不同类型的信息，以阻止非常不同类型的无效优化。mutableconstconstvolatile

A variable marked allows for it to be modified in a method declared . mutableconst

A variable marked tells the compiler that it must read/write the variable every time your code tells it too (i.e. it cant optimize away accesses to the variable).volatile

A crude but effective way of thinking of the difference is:

• The compiler knows when a mutable object changes.
• The compiler cannot know when a volatile object changes.

I would like to add that volatile is also very useful when dealing with multithreading applications, i.e, you have your main thread (where main() lives) and you spawn a worker thread that will keep spinning while a variable "app_running" is true. main() controls whether "app_running" is true or false, so if you do not add the volatile attribute to the declaration of "app_running", if the compiler optimizes access to "app_running" in the code ran by the secondary thread, main() might change "app_running" to false but the secondary thread will keep running because the value has been cached. I have seen the same behavior using gcc on Linux and VisualC++. A "volatile" attribute put in "app_running" declaration solved the problem. So, this is scenario where no hardware interrupts or kernel is invoved in changing the value of such variables.