在C++中从文件读取类对象/向文件写入类对象

给定文件“ Input.txt”，其中每一行的值均与类的实例变量相同。
将值读入类的对象并进行必要的操作。
理论 ：

The data transfer is usually done using '>>'
and <<' operators. But if you have
a class with 4 data members and want 
to write all 4 data members from its
object directly to a file or vice-versa, 
we can do that using following syntax :

To write object's data members in a file :
// Here file_obj is an object of ofstream
file_obj.write((char *) & class_obj, sizeof(class_obj));

To read file's data members into an object :
// Here file_obj is an object of ifstream
file_obj.read((char *) & class_obj, sizeof(class_obj));

例子：

Input : 
Input.txt :
Michael 19 1806
Kemp 24 2114
Terry 21 2400
Operation : Print the name of the highest 
            rated programmer.

Output : 
Terry

C++

// C++ program to demonstrate read/write of class
// objects in C++.
#include 
#include 
using namespace std;
 
// Class to define the properties
class Contestant {
public:
    // Instance variables
    string Name;
    int Age, Ratings;
 
    // Function declaration of input() to input info
    int input();
 
    // Function declaration of output_highest_rated() to
    // extract info from file Data Base
    int output_highest_rated();
};
 
// Function definition of input() to input info
int Contestant::input()
{
    // Object to write in file
    ofstream file_obj;
 
    // Opening file in append mode
    file_obj.open("Input.txt", ios::app);
 
    // Object of class contestant to input data in file
    Contestant obj;
 
    // Feeding appropriate data in variables
    string str = "Michael";
    int age = 18, ratings = 2500;
 
    // Assigning data into object
    obj.Name = str;
    obj.Age = age;
    obj.Ratings = ratings;
 
    // Writing the object's data in file
    file_obj.write((char*)&obj, sizeof(obj));
 
    // Feeding appropriate data in variables
    str = "Terry";
    age = 21;
    ratings = 3200;
 
    // Assigning data into object
    obj.Name = str;
    obj.Age = age;
    obj.Ratings = ratings;
 
    // Writing the object's data in file
    file_obj.write((char*)&obj, sizeof(obj));
 
    return 0;
}
 
// Function definition of output_highest_rated() to
// extract info from file Data Base
int Contestant::output_highest_rated()
{
    // Object to read from file
    ifstream file_obj;
 
    // Opening file in input mode
    file_obj.open("Input.txt", ios::in);
 
    // Object of class contestant to input data in file
    Contestant obj;
 
    // Reading from file into object "obj"
    file_obj.read((char*)&obj, sizeof(obj));
 
    // max to store maximum ratings
    int max = 0;
 
    // Highest_rated stores the name of highest rated contestant
    string Highest_rated;
 
    // Checking till we have the feed
    while (!file_obj.eof()) {
        // Assigning max ratings
        if (obj.Ratings > max) {
            max = obj.Ratings;
            Highest_rated = obj.Name;
        }
 
        // Checking further
        file_obj.read((char*)&obj, sizeof(obj));
    }
 
    // Output is the highest rated contestant
    cout << Highest_rated;
    return 0;
}
 
// Driver code
int main()
{
    // Creating object of the class
    Contestant object;
 
    // Inputting the data
    object.input();
 
    // Extracting the max rated contestant
    object.output_highest_rated();
 
    return 0;
}

输出：

Terry

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