求问大神setprecision操纵符是咋回事啊？为啥结果是这样的?

#include<iostream.h>
#include<iomanip.h>
void main()
{
    double d=123.456789;
    cout <<d<<endl;
    cout <<setprecision(7)<<d<<endl;
    cout <<setprecision(8)<<d<<endl;
    cout <<setprecision(9)<<d<<endl;
}
运行结果：
123.457
123.4568
123.45679
123.456789
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