流大小 prec = cout.precision(） 的意义何在;和以下行

#include <algorithm>
#include <iomanip>
#include <ios>
#include <iostream>
#include <string>
#include <vector>
using std::cin; using std::sort;
using std::cout; using std::streamsize;
using std::endl; using std::string;
using std::setprecision; using std::vector;
int main()
{
    cout << "Please enter your midterm and final exam grades: ";
    double midterm, final;
    cin >> midterm >> final;
    cout << "Enter all your homework grades, "
        "followed by end-of-file: ";
    vector<double> homework;
    double x;
    while (cin >> x)
        homework.push_back(x);
        int size = homework.size();
        if (size == 0) {
                    cout << endl << "You must enter your grades. "
                    "Please try again." << endl;
                    return 1;
                        }
    sort(homework.begin(), homework.end());
    int mid = size/2;
    double median;
    median = size % 2 == 0 ? (homework[mid] + homework[mid-1]) / 2
            : homework[mid];
    streamsize prec = cout.precision();
    cout << "Your final grade is " << setprecision(3)
        << 0.2 * midterm + 0.4 * final + 0.4 * median
        << setprecision(prec) << endl;
return 0;
}

在这个例子中，"streamsize"的意义是什么，为什么cout.precision（）是这样设置的？以下行有"setprecision（3）"，然后在末尾再次设置precision（prec）。为什么？

streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
    << 0.2 * midterm + 0.4 * final + 0.4 * median
    << setprecision(prec) << endl;