写入和读取.dat文件C ++的记录

Question

I am quite new to C++ and am trying to work out how to write a record in the format of this structure below to a text file:

struct user {
    int id;
    char username [20];
    char password [20];
    char name [20];
    char email [30];
    int telephone;
    char address [70];
    int level;
}; 

So far, I'm able to write to it fine but without an incremented id number as I don't know how to work out the number of records so the file looks something like this after I've written the data to the file.

1 Nick pass Nick email tele address 1
1 user pass name email tele address 1
1 test test test test test test 1
1 user pass Nick email tele addy 1
1 nbao pass Nick email tele 207 1

Using the following code:

ofstream outFile;

outFile.open("users.dat", ios::app);

// User input of data here

outFile << "\n" << 1 << " " << username << " " << password << " " << name << " "
        << email << " " << telephone << " " << address  << " " << 1;
cout << "\nUser added successfully\n\n";

outFile.close();

So, how can I increment the value for each record on insertion and how then target a specific record in the file?

EDIT: I've got as far as being able to display each line:

  if (inFile.is_open())
    {
    while(!inFile.eof())
    {

    cout<<endl;
    getline(inFile,line);
    cout<<line<<endl;

    }
    inFile.close();
    }

Answer 1

Using typical methods at least you will need to use fix size records if you want to have random access when reading the file so say you have 5 characters for name it will be stored as

bob\0\0

or whatever else you use to pad, this way you can index with record number * record size.

To increment the index you in the way you are doing you will need to the read the file to find the high existing index and increment it. Or you can load the file into memory and append the new record and write the file back

std::vector<user> users=read_dat("file.dat");
user user_=get_from_input();
users.push_back(user_);

then write the file back
std::ofstream file("file.dat");
for(size_t i=0; i!=users.size(); ++i) {
    file << users.at(i); 
   //you will need to implement the stream extractor to do this easily
}

Answer 2

What you have so far is not bad, except that it cannot handle cases where there is space in your strings (for example in address!)

What you are trying to do is write a very basic data base. You require three operations that need to be implemented separately (although intertwining them may give you better performance in certain cases, but I'm sure that's not your concern here).

• Insert: You already have this implemented. Only thing you might want to change is the " " to "\\n" . This way, every field of the struct is in a new line and your problem with spaces are resolved. Later when reading, you need to read line by line
• Search: To search, you need to open the file, read struct by struct (which itself consists of reading many lines corresponding to your struct fields) and identifying the entities of your interest. What to do with them is another issue, but simplest case would be to return the list of matching entities in an array (or vector).
• Delete: This is similar to search, except you have to rewrite the file. What you do is basically, again read struct by struct, see which ones match your criteria of deletion. You ignore those that match, and write (like the insert part) the rest to another file. Afterwards, you can replace the original file with the new file.

Here is a pseudo-code:

Write-entity(user &u, ofstream &fout)
    fout << u.id << endl
         << u.username << endl
         << u.password << endl
         << ...

Read-entity(user &u, ifstream &fin)
     char ignore_new_line
     fin >> u.id >> ignore_new_line
     fin.getline(u.username, 20);
     fin.getline(u.password, 20);
     ...
     if end of file
         return fail

Insert(user &u)
     ofstream fout("db.dat");
     Write-entity(u, fout);
     fout.close();

Search(char *username) /* for example */
     ifstream fin("db.dat");
     user u;
     vector<user> results;
     while (Read-entity(u))
         if (strcmp(username, u.username) == 0)
             results.push_back(u);
     fin.close();
     return results;

Delete(int level) /* for example */
     ifstream fin("db.dat");
     ofstream fout("db_temp.dat");
     user u;
     while (Read-entity(u))
         if (level != u.level)
             Write-entity(u, fout);
     fin.close();
     fout.close();
     copy "db_temp.dat" to "db.dat"

Side note: It's a good idea to place the \\n after data has been written (so that your text file would end in a new line )

Answer 3

I suggest to wrap the file handler into a Class, and then overload the operator >> and << for your struct, with this was you will control the in and out. For instance

struct User{
...
};

typedef std::vector<User> UserConT;

struct MyDataFile
{
  ofstream outFile;
  UserConT User_container;

  MyDataFile(std::string const&); //
  MyDataFile& operator<< (User const& user); // Implement and/or process the record before to write
  MyDataFile& operator>> (UserConT & user); // Implement the extraction/parse and insert into container
  MyDataFile& operator<< (UserConT const & user); //Implement extraction/parse and insert into ofstream 
};

MyDataFile& MyDataFile::operator<< (User const& user)
{
  static unsigned myIdRecord=User_container.size();
  myIdRecord++;
  outFile << user.id+myIdRecord << ....;
  return *this;
}



int main()
{
   MydataFile file("data.dat");

   UserConT myUser;
   User a;
   //... you could manage a single record  
   a.name="pepe"; 
   ...
   file<<a;
   ..//

}

Answer 4

A .Dat file is normally a simple text file itself that can be opened with notepad . So , you can simply read the Last Line of the file , read it , extract the first character , convert it into integer . THen increment the value and be done . Some sample code here :

  #include <iostream.h>
  #include <fstream.h>
  using namespace std;

  int main(int argc, char *argv[])
  {
    ifstream in("test.txt");

  if(!in) {
       cout << "Cannot open input file.\n";
       return 1;
    }

     char str[255];

     while(in) {
       in.getline(str, 255);  // delim defaults to '\n'
        //if(in) cout << str << endl;
     }
    // Now str contains the last line , 
    if  ((str[0] >=48) || ( str[0] <=57))
    {
      int i = atoi(str[0]);
      i++;
    } 
    //i contains the latest value , do your operation now
    in.close();

     return 0;
     }

Answer 5

Assuming your file format doesn't not need to be human readable.

You can write the struct out to file such as.

outFile.open("users.dat", ios::app | ios::binary);
user someValue = {};
outFile.write( (char*)&someValue, sizeof(user) );

int nIndex = 0;
user fetchValue = {};
ifstream inputFile.open("user.data", ios::binary);

inputFile.seekg (0, ios::end);

int itemCount = inputFile.tellg() / sizeof(user);

inputFile.seekg (0, ios::beg);

if( nIndex > -1 && nIndex < itemCount){
    inputFile.seekg ( sizeof(user) * nIndex , ios::beg);
    inputFile.read( (char*)&fetchValue, sizeof(user) );
}

Answer 6

The code that writes to the file is a member function of the user struct? Otherwise I see no connection with between the output and the struct.

Possible things to do:

• write the id member instead of 1
• use a counter for id and increment it at each write
• don't write the id and when reading use the line number as id