什么是用C这些声明有何区别？ [英] What is the difference between these declarations in C?

问题描述

在C和C ++什么下面的声明呢？

In C and C++ what do the following declarations do?

const int * i;
int * const i;
const volatile int ip;
const int *i;

是否有任何上述声明的错？

Are any of the above declarations wrong?

如果没有什么是它们之间的含义和区别？

If not what is the meaning and differences between them?

什么是上述声明的有效使用（我的意思是在什么情况下，我们必须用C / C ++ /嵌入式C使用它们）？

What are the useful uses of above declarations (I mean in which situation we have to use them in C/C++/embedded C)?

推荐答案

const int的*我;

I 是一个指向常整型。 I 可以更改为指向一个不同的值，但被指向值由 I 不能改变。

i is a pointer to constant integer. i can be changed to point to a different value, but the value being pointed to by i can not be changed.

为int * const的我;

I 是一个常量指针到非恒定的整数。该值指向 I 是可以改变的，但 I 不能更改指向不同的值。

i is a constant pointer to a non-constant integer. The value pointed to by i can be changed, but i cannot be changed to point to a different value.

const的挥发INT IP;

这一个是棘手的一种。事实上， IP 是常量意味着编译器不会让你改变的价值IP 。但是，它仍可以在理论上修饰，例如通过采取它的地址，并使用的const_cast 运营商。这是非常危险的，不是一个好主意，但它是允许的。在挥发性限定词表示任何时间 IP 被访问时，它应始终从内存重新加载，也就是说，它不应该缓存在寄存器中。这prevents从使某些优化编译器。你想，当你有可能被另一个线程修改的变量使用挥发性预选赛上，或者如果你使用内存映射I / O，或其他类似情况这可能导致行为的编译器可能不被期待的。使用常量和挥发性在同一个变量是相当不寻常的（但合法的） - 你通常会看到一个，但不是其他。

This one is kind of tricky. The fact that ip is const means that the compiler will not let you change the value of ip. However, it could still be modified in theory, e.g. by taking its address and using the const_cast operator. This is very dangerous and not a good idea, but it is allowed. The volatile qualifier indicates that any time ip is accessed, it should always be reloaded from memory, i.e. it should NOT be cached in a register. This prevents the compiler from making certain optimizations. You want to use the volatile qualifier when you have a variable which might be modified by another thread, or if you're using memory-mapped I/O, or other similar situations which could cause behavior the compiler might not be expecting. Using const and volatile on the same variable is rather unusual (but legal) -- you'll usually see one but not the other.

const int的*我;

这是一样的第一个声明。

This is the same as the first declaration.

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