C++输入输出流

class Sales_item {
    /* 重载流输入 */
    friend std::istream& operator>>(std::istream&, Sales_item&);
    
    /* 重载流输出 */
    friend std::ostream& operator<<(std::ostream&, const Sales_item&);
}

std::ostream& operator<<(std::ostream& os, const Sales_item& item) {
        os << item.member;
        return os;
}

#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>

using namespace std;

void test_ostringstream() {
        int num = 1234;
        string payload = "神马都是浮云";

        //设置显示宽度和填充字符
        ostringstream os;
        os << std::setw(8) << std::hex << num;
        os << std::setw(32) << std::setfill(' ') << payload.substr(0, 32);

        std::cout << os.str() << std::endl;
}