#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
    double a=654.3210456789;
    cout<<a<<endl;
    cout<<setprecision(9)<<a<<endl;
    cout<<setiosflags(ios::fixed)<<setprecision(8)<<a<<endl;
    cout<<setiosflags(ios::scientific)<<a<<endl;
    cout<<setiosflags(ios::fixed)<<setprecision(3)<<a<<endl;
    system("pause");
    return 0;
}

结果是

654.321

654.321046

654.32104568

8.00532e-307

8.00532e-307

为什么后面两个会出现这样的结果啊