4.9,4.10

4.9类型组合

 1 #include<iostream>
 2 struct years
 3 {
 4     int year;
 5 };
 6 
 7 int main()
 8 {
 9     using namespace std;
10 
11     years s1,s2,s3;
12     s1.year=1998;
13 
14     years *pa=&s2;
15     pa->year=1999;
16 
17     years trio[3];//array of 3 structures
18     trio[0].year=2003;
19     cout<<trio->year<<endl;
20 
21     const years *arp[3]={&s1,&s2,&s3};
22     cout<<arp[1]->year<<endl;
23 
24     const years **ppa=arp;
25     auto ppb=arp;//c++11 automatic type deduction
26     cout<<(*ppa)->year<<endl;
27     cout<<(*(ppa+1))->year<<endl;
28 
29     return 0;
30 }

4.10数组的替代品

vector

头文件<vector>，使用using namespace std

1 #include<vector>
2 using namespace std;
3 vector<int> vi;//设置初始长度为0
4 int n;
5 cin>>n;
6 vector<double> vd(n);//n个double的数组

模板：vector<typename>vt<n_elem>

n_elem可以是常量也可以是变量

array（c++11）

头文件<array>,使用using namespace std；

1 #include<array>
2 using namespace std;
3 array<int,5>ai;
4 array<double ,a>ad={1.2,2.1,6.43,6.5};

模板：array<typename,n_elem>arr;

n_elem不能使变量。

例子：

 1 #include<iostream>
 2 #include<vector>
 3 #include<array>
 4 int main()
 5 {
 6     using namespace std;
 7     double a[4]={1.2,2.4,3.6,4.8};
 8 
 9     vector<double>b(4);
10     b[0]=1.0/3.0;
11     b[1]=1.0/5.0;
12     b[2]=1.0/7.0;
13     b[3]=1.0/9.0;
14 
15     array<double,4>c={3.14,2.72,1.62,1.41};
16     array<double,4>d;
17     d=c;
18 
19     cout<<"a[2]:"<<a[2]<<" at "<<&a[2]<<endl;
20     cout<<"b[2]:"<<b[2]<<" at "<<&b[2]<<endl;
21     cout<<"c[2]:"<<c[2]<<" at "<<&c[2]<<endl;
22     cout<<"d[2]:"<<d[2]<<" at "<<&d[2]<<endl;
23 
24     a[-2]=20.2;//危¡ê险?
25     cout<<"a[-2]:"<<a[-2]<<" at "<<&a[-2]<<endl;
26     cout<<"c[2]:"<<c[2]<<" at "<<&c[2]<<endl;
27     cout<<"d[2]:"<<d[2]<<" at "<<&d[2]<<endl;
28 
29     return 0;
30 }

a[-2]=20.2;

意思是a所指的位置向前移2个位，存储20.2，存储在数组外面。c++并不检查这种越界错误。这种行为是很危险的。