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
