C++ - explicit和volatile/const的内容

时间：2022-09-03 16:51:39

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 = ;

const int a = ;

修饰指针时以*号为分界符号

#include <iostream>

#include <iostream>

int main() {

    int a = ;

    int b = ;

    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 ;

}

gcc报错结果

J:\SITP\alg\main.cpp: In function 'int main()':

J:\SITP\alg\main.cpp::: error: assignment of read-only location '* p1'

     (*p1) = b;

           ^

J:\SITP\alg\main.cpp::: error: assignment of read-only location '* p2'

     (*p2) = b;

           ^

J:\SITP\alg\main.cpp::: error: assignment of read-only variable 'p3'

     p3 = &b;

        ^

J:\SITP\alg\main.cpp::: error: assignment of read-only location '*(const int*)p4'

     (*p4) = &b;

           ^

J:\SITP\alg\main.cpp::: error: invalid conversion from 'int*' to 'int' [-fpermissive]

J:\SITP\alg\main.cpp::: error: assignment of read-only variable 'p4'

     p4 = &b;

        ^

J:\SITP\alg\main.cpp::: error: assignment of read-only location '*(const int*)p5'

     (*p5) = &b;

           ^

J:\SITP\alg\main.cpp::: error: invalid conversion from 'int*' to 'int' [-fpermissive]

J:\SITP\alg\main.cpp::: error: assignment of read-only variable 'p5'

     p5 = &b;

        ^

mingw32-make.exe[]: *** [CMakeFiles/alg.dir/main.cpp.obj] Error

mingw32-make.exe[]: *** [CMakeFiles/alg.dir/all] Error

mingw32-make.exe[]: *** [CMakeFiles/alg.dir/rule] Error

CMakeFiles\alg.dir\build.make:: recipe for target 'CMakeFiles/alg.dir/main.cpp.obj' failed

CMakeFiles\Makefile2:: recipe for target 'CMakeFiles/alg.dir/all' failed

CMakeFiles\Makefile2:: recipe for target 'CMakeFiles/alg.dir/rule' failed

mingw32-make.exe: *** [alg] Error

Makefile:: 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 =

const int *p1 = &a;

 (*p1) = b;通过p修改a的值

p1 = 0x72FE20 , *p1 =

p2 = 0x72FE24 , *p2 =

int const *p2 = &a;

 (*p2) = b;通过p修改a的值

p2 = 0x72FE20 , *p2 =

p3 = 0x72FE24 , *p3 =

int *const p3 = &a;

 p3 = &b;修改p的指向

p3 = 0x72FE24 , *p3 =

p4 = 0x72FE24 , *p4 =

const int *const p4 = &a;

 二者都不能修改

p4 = 0x72FE24 , *p4 =

p5 = 0x72FE24 , *p5 =

int const *const p5 = &a;

 二者都不能修改

p5 = 0x72FE24 , *p5 = 

Process finished with exit code

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