第一步检查文件是否打开，用到头文件：

#include <fstream>
#include <sstream>

文件打开的函数为

file.isopen() 

信息输入到文件应该为

file << "" << value;

注意是file<<  

如图

定义file

ofstream file(filename);

完整代码演示

void write_data_to_file(const string& filename, char separator, const Household* household, int household_number) {
    string files,x;
    ofstream file(filename);

    if (household == nullptr) {
        file << "A4;" << household_number << ";" << household->city << ";" << household->waterheating << ";" <<
         household->price_perKWh << endl;
    }
    
    
    if (!file.is_open()) {
        cout << "Error: Unable to open file for writing." << endl;
        return;
    }

    file << "A4;" << household_number << ";" << household->city << ";" << household->waterheating << ";"
         << household->persons << ";" << household->squaremetres << endl;

    file << "household;" << household_number << ";" << household->city << ";" << household->waterheating << ";"
         << household->persons << ";" << household->squaremetres << endl;


    cout << "Output file \"" << filename << "\" closed." << endl;
}

 文件参数为：文件名，分隔符，变量，变量的长度，也可以简化使用，在主函数的调用如下

string filename;

cout << "Input file name: ";
cin >> filename;

char separator;
cout << "Input separator character: ";
cin >> separator;

write_data_to_file(filename, separator, households[index], index);