Question

我需要你的帮助，因为我似乎无法理解SO上和其他网站上关于插入操作符重载的任何（许多）各种解决方案。

我要创建一个“数据库”，由一个单独的向量组成的员工，除其他外，我必须能够保存到文本文件。

代码必须有一个名为Employees的基类和三个名为Manager，Engineer和Researcher的派生类。

基于：

for (vector<Employees*>::iterator i = midzer.begin(); i != midzer.end(); ++i)
{
    (*i)->printStats(); 
}

/* printStats is a virtual method outputting all data contained in the objects to 
the console window. It's implemented like so for one of the derived classes:*/

void Manager::printStats()
{
    cout << "\nManager." << endl;
    cout << "First name: " << mFirstName << endl; // data member of base class
    cout << "Last name: " << mLastName << endl; // data member of base class
    cout << "Salary: " << mSalary << endl; // data member of base class
    cout << "Meetings per week: " << mNumberOfMeetingsPerWeek << endl; // data member of derived class
    cout << "Vacation Days per year: " << mNumberOfVacationDaysPerYear << endl; //data member of derived class
    cout << endl;
}

我以为我可以像下面那样创建一个ostream对象瞧..

ofstream outfile;
outfile.open("/users/mundt/desktop/EmployeeDatabase.txt");
for (vector<Employees*>::iterator i = midzer.begin(); i != midzer.end(); ++i)
{
    outfile << (*i)->printStats();
}
outfile.close();

正如您（可能）知道这是不可能的，我必须为基类Employees重载插入运算符，并使其多态以某种方式能够访问派生类部分。

我正在寻找的是一种使用printStats（）方法将矢量中所有对象的数据输出到文本文件的方法。

如果有人可以提供一些示例代码来执行此操作，将非常感谢。谢谢。

Answer 1

首先，应告知printStats将数据放在何处，而不是始终将其写入cout。

void Manager::printStats(std::ostream& out) const //function doesn't modify, so const
{
    out << "\nManager." << endl;
    out << "First name: " << mFirstName << endl; // data member of base class
    out << "Last name: " << mLastName << endl; // data member of base class
    out << "Salary: " << mSalary << endl; // data member of base class
    out << "Meetings per week: " << mNumberOfMeetingsPerWeek << endl; // data member of derived class
    out << "Vacation Days per year: " << mNumberOfVacationDaysPerYear << endl; //data member of derived class
    out << endl;
}

然后告诉C ++如何将您的类插入流中：

std::ostream& operator<<(std::ostream& out, const Employees& employee) 
{
   employee.printStats(out);
   return out; //return ostreams by reference when done
}

如果您愿意，可以为矢量添加另一个，这几乎与您的旧循环完全相同

std::ostream& operator<<(std::ostream& out, const vector<Employees*>& midzer) 
{
   for (vector<Employees*>::const_iterator i = midzer.begin(); i != midzer.end(); ++i)
   {
       out << *(*i);
   }
   //or if you have a newer compiler:
   //for (Employees* i : midzer) out << *i;
   return out; //return ostreams by reference when done
}

最后，输出矢量

ofstream outfile;
outfile.open("/users/mundt/desktop/EmployeeDatabase.txt");
outfile << midzer; //well that's easy
outfile.close();

此处的演示演示：http://coliru.stacked-crooked.com/a/03fd3b4e23dc7ee2

Answer 2

这样做有很多选择，我将描述3：

从函数
输出到传递给函数的流
重载operator<<（与上述2之一结合使用）

更多细节：

如果您从printStats()函数返回一个字符串，那么您可以将其写入ofstream：

std::string Manager::printStats()
{
    std::stringstream ss;
    ss << "\nManager." << endl;
    ss << "First name: " << mFirstName << endl; // data member of base class
    ss << "Last name: " << mLastName << endl; // data member of base class
    ss << "Salary: " << mSalary << endl; // data member of base class
    ss << "Meetings per week: " << mNumberOfMeetingsPerWeek << endl; // data member of derived class
    ss << "Vacation Days per year: " << mNumberOfVacationDaysPerYear << endl; //data member of derived class
    ss << endl;

    return ss.str();
}

如果您将ostream传入此功能，则可以直接输出到该功能。（您可能希望将该函数定义为void printStats( std::ostream& os = std::cout );，因此如果您在没有参数的情况下调用它，它将默认为cout：
```
void Manager::printStats( std::ostream& os )
{
    os << "\nManager." << endl;
    //...
}
```
在你的例子中会这样调用：
```
(*i)->printStats( outFile );
```

如果您重载operator<<，则可以直接将其传递给outFile。

std::ostream& operator<<( std::ostream& os, const Employee& e )
{
    os << e.printStats(); 
    // or
    e.printStats( os );

    // for both cases
    return os;
}

然后可以这样称呼：

outFile << *(*i);

Answer 3

cout是函数ostream正在打印的特定PrintStats。您正在寻找的是为您的类型重载operator<<。

    std::ostream & operator<<(std::ostream & s, const Employee& m)
    {
        m.Stream(s);
        return s;
    }

然后这可以用作：

    outfile << *i;

然后在virtual Stream() const中声明一个函数Employee并在Manager

中覆盖它

重载插入运算符（运算符＆lt;＆lt;）

3 个答案: