第一眼见到explicit和volatile可能会一愣一愣的觉得可能是c11或者c14新加的标识符。

其实不是这样，volatile和const两个关键字在C语言的第二个版本KR C的时候就被加入了C标准，他们是两个相对的关键字

const 修饰符表示这是一个常量类型，这个变量的值不会被程序改变

volatile 修饰符表示这个变量可能被编译器以外的行为（譬如内联汇编程序）改变。

修饰常量变量只要和类型紧挨着就可以

int const a = 1;
const int a = 1;

修饰指针时以*号为分界符号

#include <iostream>
#include <iostream>

int main() {
    int a = 10;
    int b = 20;

    const int *p1 = &a;
    int const *p2 = &a;
    int *const p3 = &a;
    const int *const p4 = &a;
    int const *const p5 = &a;

    printf("&a = %X\n", &a);
    printf("&b = %X\n", &b);

    printf("p1 = 0x%X , *p1 = %d\n", p1, *p1);
    printf("const int *p1 = &a; \n (*p1) = b;通过p修改a的值\n");
//    (*p1) = b;
    p1 = &b;
    printf("p1 = 0x%X , *p1 = %d\n", p1, *p1);
    printf("p2 = 0x%X , *p2 = %d\n", p2, *p2);
    printf("int const *p2 = &a; \n (*p2) = b;通过p修改a的值\n");
//    (*p2) = b;
    p2 = &b;
    printf("p2 = 0x%X , *p2 = %d\n", p2, *p2);
    printf("p3 = 0x%X , *p3 = %d\n", p3, *p3);
    printf("int *const p3 = &a; \n p3 = &b;修改p的指向\n");
//    p3 = &b;
    (*p3) = b;
    printf("p3 = 0x%X , *p3 = %d\n", p3, *p3);
    printf("p4 = 0x%X , *p4 = %d\n", p4, *p4);
    printf("const int *const p4 = &a; \n 二者都不能修改\n");
//    (*p4) = &b;
//    p4 = &b;
    printf("p4 = 0x%X , *p4 = %d\n", p4, *p4);
    printf("p5 = 0x%X , *p5 = %d\n", p5, *p5);
    printf("int const *const p5 = &a; \n 二者都不能修改\n");
//    (*p5) = &b;
//    p5 = &b;
    printf("p5 = 0x%X , *p5 = %d\n", p5, *p5);
    return 0;
}

gcc报错结果

J:\SITP\alg\main.cpp: In function 'int main()':
J:\SITP\alg\main.cpp:14:11: error: assignment of read-only location '* p1'
     (*p1) = b;
           ^
J:\SITP\alg\main.cpp:17:11: error: assignment of read-only location '* p2'
     (*p2) = b;
           ^
J:\SITP\alg\main.cpp:20:8: error: assignment of read-only variable 'p3'
     p3 = &b;
        ^
J:\SITP\alg\main.cpp:23:11: error: assignment of read-only location '*(const int*)p4'
     (*p4) = &b;
           ^
J:\SITP\alg\main.cpp:23:11: error: invalid conversion from 'int*' to 'int' [-fpermissive]
J:\SITP\alg\main.cpp:24:8: error: assignment of read-only variable 'p4'
     p4 = &b;
        ^
J:\SITP\alg\main.cpp:26:11: error: assignment of read-only location '*(const int*)p5'
     (*p5) = &b;
           ^
J:\SITP\alg\main.cpp:26:11: error: invalid conversion from 'int*' to 'int' [-fpermissive]
J:\SITP\alg\main.cpp:27:8: error: assignment of read-only variable 'p5'
     p5 = &b;
        ^
mingw32-make.exe[3]: *** [CMakeFiles/alg.dir/main.cpp.obj] Error 1
mingw32-make.exe[2]: *** [CMakeFiles/alg.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles/alg.dir/rule] Error 2
CMakeFiles\alg.dir\build.make:61: recipe for target 'CMakeFiles/alg.dir/main.cpp.obj' failed
CMakeFiles\Makefile2:66: recipe for target 'CMakeFiles/alg.dir/all' failed
CMakeFiles\Makefile2:78: recipe for target 'CMakeFiles/alg.dir/rule' failed
mingw32-make.exe: *** [alg] Error 2
Makefile:117: recipe for target 'alg' failed

将非法的行注释掉之后可以看到允许修改的内容

J:\SITP\alg\cmake-build-debug\alg.exe
&a = 72FE24
&b = 72FE20
p1 = 0x72FE24 , *p1 = 10
const int *p1 = &a;
 (*p1) = b;通过p修改a的值
p1 = 0x72FE20 , *p1 = 20
p2 = 0x72FE24 , *p2 = 10
int const *p2 = &a;
 (*p2) = b;通过p修改a的值
p2 = 0x72FE20 , *p2 = 20
p3 = 0x72FE24 , *p3 = 10
int *const p3 = &a;
 p3 = &b;修改p的指向
p3 = 0x72FE24 , *p3 = 20
p4 = 0x72FE24 , *p4 = 20
const int *const p4 = &a;
 二者都不能修改
p4 = 0x72FE24 , *p4 = 20
p5 = 0x72FE24 , *p5 = 20
int const *const p5 = &a;
 二者都不能修改
p5 = 0x72FE24 , *p5 = 20

Process finished with exit code 0