c++ - const int *，const int * const和int const *有什么区别？(What is the difference between const int*, const int * const, and int const *?)

Question

I always mess up how to use const int* , const int * const , and int const * correctly.

(我总是搞砸了如何正确使用const int* ， const int * const和int const * 。)

Is there a set of rules defining what you can and cannot do?

(是否有一组规则定义您可以做什么和不能做什么？)

I want to know all the do's and all don'ts in terms of assignments, passing to the functions, etc.

(我想知道在分配，传递给函数等方面所有需要做的事情。)

  ask by translate from so

Answer 1

Read it backwards (as driven by Clockwise/Spiral Rule ):

(向后读取（受“ 顺时针/螺旋规则”驱动）：)

• int* - pointer to int

  (int* -指向int的指针)

• int const * - pointer to const int

  (int const * -指向const int的指针)

• int * const - const pointer to int

  (int * const指向int的const指针)

• int const * const - const pointer to const int

  (int const * const指向const int的const指针)

Now the first const can be on either side of the type so:

(现在，第一个const可以位于类型的任一侧，因此：)

• const int * == int const *

  (const int * == int const *)

• const int * const == int const * const

  (const int * const == int const * const)

If you want to go really crazy you can do things like this:

(如果您想发疯，可以执行以下操作：)

• int ** - pointer to pointer to int

  (int ** -指向int的指针)

• int ** const - a const pointer to a pointer to an int

  (int ** const指向int的指针的const指针)

• int * const * - a pointer to a const pointer to an int

  (int * const * -指向int的const指针)

• int const ** - a pointer to a pointer to a const int

  (int const ** -指向const int的指针)

• int * const * const - a const pointer to a const pointer to an int

  (int * const * const指向int * const * const的const指针的const指针)

• ...

  (...)

And to make sure we are clear on the meaning of const

(并确保我们清楚const的含义)

const int* foo;
int *const bar; //note, you actually need to set the pointer 
                //here because you can't change it later ;)

foo is a variable pointer to a constant integer.

(foo是指向常量整数的变量指针。)

This lets you change what you point to but not the value that you point to.

(这使您可以更改指向的内容，但不能更改指向的值。)

Most often this is seen with C-style strings where you have a pointer to a const char .

(最常见的情况是使用C样式的字符串，其中有指向const char的指针。)

You may change which string you point to but you can't change the content of these strings.

(您可以更改指向的字符串，但是不能更改这些字符串的内容。)

This is important when the string itself is in the data segment of a program and shouldn't be changed.

(当字符串本身位于程序的数据段中并且不应更改时，这一点很重要。)

bar is a constant or fixed pointer to a value that can be changed.

(bar是指向可以更改的值的常量或固定指针。)

This is like a reference without the extra syntactic sugar.

(这就像没有多余语法糖的参考。)

Because of this fact, usually you would use a reference where you would use a T* const pointer unless you need to allow NULL pointers.

(由于这个事实，除非需要允许使用NULL指针，否则通常会在使用T* const指针的地方使用引用。)