c++ decltype和auto对比学习 

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 int main()
 6 {
 7 
 8     int a=1;
 9     int b=2;
10     int &ra = a;
11     int *ptr = &a;
12     /*
13     *1.decltype对于括号和解引用操作符*，会解析为引用＆
14     *int &ra = a;
15     *decltype(ra) da; //int &
16     *decltype(a) db;  //int
17     *decltype((a)) dc;//int &,必须初始化，否则报错
18     */
19     //  decltype(ra) refa; //int &,error:reference must initialized
20     decltype(a) testb; //int
21     //  decltype((a)) refb; //int &,error:reference must initialized
22     //  decltype(*ptr) refc;//int &,error:reference must initialized
23 
24     /*
25     *２．decltype和auto对于const的处理对比
26     *decltype 保留底层const和顶层const
27     *auto只保留底层const
28     *auto不得不保留底层const是因为从底层const常量转换为没有底层const的变量是错误的
29     *const int *cpa = &i;
30     *auto a = cpa; //a是const int *
31     */
32     //顶层const,decltype保留，auto忽略
33     const int cona = 1;
34     decltype(cona) td = 1; // const int
35     auto hd = cona;   //int
36     hd = 1; //ok
37     //  td = 1; //error:const int read-only
38 
39     //底层const，decltype保留，auto保留
40     const int *conb = &a;
41     decltype(conb) te = &a;//const int *
42     //  *te = 2; //error:const int * read-only locatin
43     auto fe = conb; //const int *
44     //  *fe = 3; //error:assignment of read-only location
45 
46     //底层和顶层const
47     const int * const conc = &a;
48     decltype(conc) tf = &a; //const int * const
49     //tf = &b; //error:assignment of read-only variable
50     //*tf = 3; //error:assignment of read-only location
51     auto ft = conc; //const int *
52     ft = &b; //ok
53     //  *ft = 4;  //error:assignment of read-only location
54     return 0;
55 }

 

posted on 2018-04-19 11:06  CreatorKou  阅读(132)  评论(0)  编辑  收藏  举报

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