c++ - streamsize prec = cout.precision(); 的重点是什么？和以下几行？

Question

#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;
}

在此示例中，“streamsize”的意义何在？为什么要这样设置 cout.precision()？下面几行有“setprecision(3)”，然后在末尾再次设置precision(prec)。这是为什么？

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

Answer 1

代码将 std::cout 的精度重置为原始值，以便后续使用 std::cout 时不使用三位数精度。

我建议使用 scope guard , 因此即使在 setprecision(3) 和 setprecision(prec) 之间抛出异常，精度也会被重置。