辨析 const指针 和 指向常量的指针

　　辨析以下几种指针p的定义。

int tmp = 5;

int *p = &tmp;
const int *p = &tmp;
int const* p = &tmp;
int * const p = &tmp;
const int * const p = &tmp;
int const * const p = &tmp;

　　根据文献一，可以采用从右往左读的方式区分。

第一个为普通指针，指向普通int变量；

第二个和第三个相同，都是普通指针，指向const int型变量；

第四个是const指针，指向普通int变量；

第五个和第六个相同，都是const指针，指向const int型变量。

　　实验代码如下：

#include <iostream>


void test1() {
    int tmp = 5;
    int *p = &tmp;
    std::cout << "test1:: p value: " << *p << std::endl;
    // *p and p are common variables.
    *p = 10;  // ok
    int tmp1 = 9;
    p = &tmp1;  // ok
}

void test2() {
    int tmp = 5;
    const int *p = &tmp;
    std::cout << "test2:: p value: " << *p << std::endl;
    // *p is read-only, p is common variable.
//    *p = 10;  // error
    int tmp1 = 9;
    p = &tmp1;  // ok
}

// same with test2
void test3() {
    int tmp = 5;
    int const* p = &tmp;
    std::cout << "test3:: p value: " << *p << std::endl;
    // *p is read-only, p is common variable.
//    *p = 10;  // error
    int tmp1 = 9;
    p = &tmp1;  // ok
}

void test4() {
    int tmp = 5;
    int * const p = &tmp;
    std::cout << "test4:: p value: " << *p << std::endl;
    // p is read-only, *p is common variable.
    *p = 10;  // ok
//    int tmp1 = 9;
//    p = &tmp1;  // error
}

void test5() {
    const int tmp = 5;
    const int * const p = &tmp;
    std::cout << "test5:: p value: " << *p << std::endl;
    // p is read-only, *p is also read-only.
//    *p = 10;  // error
//    int tmp1 = 9;
//    p = &tmp1;  // error
}


// same with test5
void test6() {
    const int tmp = 5;
    int const * const p = &tmp;
    std::cout << "test6:: p value: " << *p << std::endl;
    // p is read-only, *p is also read-only.
//    *p = 10;  // error
//    int tmp1 = 9;
//    p = &tmp1;  // error
}

int main() {
    std::cout << "Hello, World!" << std::endl;
    test1();
    test2();
    test3();
    test4();
    test5();
    test6();
    return 0;
}

References:

(1) https://www.cnblogs.com/bencai/p/8888760.html