使用结构读取和写入二进制文件

Question

I am attempting read from a binary file and dump the information into a structure. Before I read from it I write into the file from a vector of structures. Unfortunately I am not able to get the new structure to receive the information from the file.

I have tried switching between vectors and individual structures. Also tried messing with the file pointer, moving it back and forth and also leaving it as is to see if that has been the problem. Using vectors because it is supposed to take unlimited values. Also allows me to test what the output should look like when I look up a specific structure in the file.

struct Department{
    string departmentName;
    string departmentHead;
    int departmentID;
    double departmentSalary;
};

int main()
{
    //...
   vector<Employee> emp;
   vector<Department> dept;
   vector<int> empID;
   vector<int> deptID;
        if(response==1){
            addDepartment(dept, deptID);
            fstream output_file("departments.dat", ios::in|ios::out|ios::binary);
            output_file.write(reinterpret_cast<char *>(&dept[counter-1]), sizeof(dept[counter-1]));
            output_file.close();
        }
        else if(response==2){
            addEmployee(emp, dept, empID);
        }
        else if(response==3){
            Department master;
            int size=dept.size();
            int index;
            cout << "Which record to EDIT:\n";
            cout << "Please choose one of the following... 1"<< " to " << size << " : ";
            cin >> index;
            fstream input_file("departments.dat", ios::in|ios::out|ios::binary);
            input_file.seekg((index-1) * sizeof(master), ios::beg);
            input_file.read(reinterpret_cast<char *>(&master), sizeof(master));
            input_file.close();
            cout<< "\n" << master.departmentName;
        }
        else if(response==4){

        }


//...

Answer 1

Files are streams of bytes. If you want to write something to a file and read it back reliably, you need to define the contents of the file at the byte level. Have a look at the specifications for some binary file formats (such a GIF) to see what such a specification looks like. Then write code to convert to and from your class instance and a chunk of bytes.

Otherwise, it will be hit or miss and, way too often, miss. Punch "serialization C++" into your favorite search engine for lots of ideas on how to do this.

Your code can't possibly work for an obvious reason. A string can contain a million bytes of data. But you're only writing sizeof(string) bytes to your file. So you're not writing anything that a reader can make sense out of.

Say sizeof(string) is 32 on your platform but the departmentHead is more than 32 bytes. How could the file's contents possibly be right? This code makes no attempt to serialize the data into a stream of bytes suitable for writing to a file which is ... a stream of bytes.