Pointers vs. const (C++11)

1.cosnt pointer

    int39;    constint * pt = &age;

You cannot use PT to change the value of age, that is, *PT is const and cannot be modified.

    1;      // 非法操作    cin >> *pt;    // 非法操作

However, the PT declaration does not imply that the variable it points to (age) is const, and simply that it is not possible to use PT to modify the variable it points to (age).

    20;      // 非法操作    20;      // ok

2. Pointer to const object 2.1 A const pointer to a const object

    constfloat9.80;    constfloat * pe = &g_earth;   // ok

Both G_earth and PE are const and cannot be used to modify their corresponding values (9.80).

2.2 Non-const pointer to const object

C + + does not allow a const address to be assigned to a non-const pointer.

    constfloat1.63;    float * pm = &g_moon;   // 非法操作

Of course, you can also use type conversions to break this limit.

3. Pointers pointing to pointers

    constint **pp2;    int *p1;    constint13;    pp2 = &p1;      // 非法操作。先假设合法。    *pp2 = &n;      // ok，两者都是const，但 p1 = &n 非法。    10;       // 此行的语法是合法的，但不能成功修改n。

Therefore, the operation of assigning a non-const address or pointer to a const pointer applies only in the case of an indirect address operation. In other words, const or non-const data can be assigned to a const pointer, as long as the data itself is not a pointer type.

4.const parameters

    const int months[12] = {312831303130313130313031};    ...    int sum(int arr[], int n);  // 应该是 const int arr[]    12);    // 非法

The above function call attempts to assign a const pointer (months) to a non-const pointer (arr), which results in a compilation error.

Small advice:
As long as you can use const, use it. There are two reasons why it is recommended to add a const to a pointer declaration:
1. Can prevent unintentional modification of data;
2. Use the const allow function to handle const and non-const arguments. If you omit const in a function prototype, the function can only accept data that is not const.
As long as you can, it is recommended to declare the formal parameter as a const pointer.

Pointer to const (C++11)