4.9,4.10

4.9类型组合

#include<iostream>
struct years
{
    int year;
};

int main()
{
    using namespace std;

    years s1,s2,s3;
    s1.year=1998;

    years *pa=&s2;
    pa->year=1999;

    years trio[3];//array of 3 structures
    trio[0].year=2003;
    cout<<trio->year<<endl;

    const years *arp[3]={&s1,&s2,&s3};
    cout<<arp[1]->year<<endl;

    const years **ppa=arp;
    auto ppb=arp;//c++11 automatic type deduction
    cout<<(*ppa)->year<<endl;
    cout<<(*(ppa+1))->year<<endl;

    return 0;
}

4.10数组的替代品

vector

头文件<vector>，使用using namespace std

#include<vector>
using namespace std;
vector<int> vi;//设置初始长度为0
int n;
cin>>n;
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；

#include<array>
using namespace std;
array<int,5>ai;
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：

#include<iostream>
#include<vector>
#include<array>
int main()
{
    using namespace std;
    double a[4]={1.2,2.4,3.6,4.8};

    vector<double>b(4);
    b[0]=1.0/3.0;
    b[1]=1.0/5.0;
    b[2]=1.0/7.0;
    b[3]=1.0/9.0;

    array<double,4>c={3.14,2.72,1.62,1.41};
    array<double,4>d;
    d=c;

    cout<<"a[2]:"<<a[2]<<" at "<<&a[2]<<endl;
    cout<<"b[2]:"<<b[2]<<" at "<<&b[2]<<endl;
    cout<<"c[2]:"<<c[2]<<" at "<<&c[2]<<endl;
    cout<<"d[2]:"<<d[2]<<" at "<<&d[2]<<endl;

    a[-2]=20.2;//危¡ê险?
    cout<<"a[-2]:"<<a[-2]<<" at "<<&a[-2]<<endl;
    cout<<"c[2]:"<<c[2]<<" at "<<&c[2]<<endl;
    cout<<"d[2]:"<<d[2]<<" at "<<&d[2]<<endl;

    return 0;
}

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。