Problems writing/reading an vector of Objects to a file [duplicate] - c++

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Need help regarding saving variables via fstream, do I need to use vector?
(2 answers)
Closed 8 years ago.
I got this problem, where I want to make a Devicelist, which I do with a vector of Devices. But I can't make it read/write correctly.
This is my functions:
#include "Devicelist.h"
bool Devicelist::AddDevice(const char *deviceName, char *type)
{
Device tempDevice(deviceName, type, ++id_);
Devicelist_.push_back(tempDevice);
return true;
}
bool Devicelist::deleteDevice(const char *deviceName)
{
for (int i = 0; i < Devicelist_.size(); i++)
{
if (strcmp(deviceName, Devicelist_[i].getName()) == 0)
{
Devicelist_.erase(Devicelist_.begin() + i);
return true;
}
else
{
cout << "No Device found with that Devicename." << endl;
return false;
}
}
}
bool Devicelist::SaveToFile()
{
//remove("Devices.dat");
ofstream SaveFile("Devices.dat", ios::out | ios::binary);
if (!SaveFile)
{
cerr << "File could not be opened." << endl;
return false;
}
for (int i = 0; i < Devicelist_.size(); i++)
SaveFile.write((const char *)(&Devicelist_[i]), sizeof(Devicelist_[i]));
SaveFile.close();
return true;
}
bool Devicelist::LoadFromFile()
{
ifstream LoadFile("Devices.dat", ios::in | ios::binary);
if (!LoadFile)
{
cerr << "File could not be opened." << endl;
return false;
}
for (int i = 0; i < Devicelist_.size(); i++)
LoadFile.read((char *)(&Devicelist_[i]), sizeof(Devicelist_[i]));
LoadFile.close();
return true;
}
Device Devicelist::findDevice(const char *deviceName)
{
for (int i = 0; i < Devicelist_.size(); i++)
{
if (strcmp(deviceName, Devicelist_[i].getName()) == 0)
return Devicelist_[i];
else
cout << "Couldn't find device." << endl;
}
}
And this is my main:
#include "Devicelist.h"
void main()
{
Devicelist list;
list.AddDevice("Lampe3", "Lampe");
list.AddDevice("Lampe4", "Lampe");
list.SaveToFile();
Devicelist list2;
list2.LoadFromFile();
Device lampe = list2.findDevice("Lampe");
cout << lampe.getName() << endl;
cout << lampe.getID() << endl;
cout << lampe.getType() << endl;
}
Can anyone see what my problem is?
Thanks in advance!
Edit1:
My .h-file for Devicelist is:
#ifndef DEVICELIST_H
#define DEVICELIST_H
#include "Device.h"
#include <vector>
#include <string>
#include <iostream>
#include <fstream>
class Devicelist
{
public:
Devicelist();
bool SaveToFile();
bool LoadFromFile();
bool AddDevice(const char *deviceName, char *type);
bool deleteDevice(const char *deviceName);
Device findDevice(const char *devicename);
private:
vector<Device> Devicelist_;
int id_;
};
#endif
This is my Device .h-file:
#ifndef DEVICE_H
#define DEVICE_H
#include <iostream>
#include <string>
#define DNAME_SIZE 33
using namespace std;
class Device
{
public:
Device(const char *devicename = "Default", char *type = "type", int id = 0);
const char *getName();
int getID();
int getType();
private:
char deviceName_[DNAME_SIZE];
int id_;
int type_;
};
#endif
And Device .cpp-file:
#include "Device.h"
Device::Device(const char *deviceName, char *type, int id)
{
strncpy_s(deviceName_, deviceName, DNAME_SIZE);
if (type == "Lampe")
type_ = 1;
else if (type == "Roegalarm")
type_ = 2;
else if (type == "Tyverialarm")
type_ = 3;
else
{
type_ = 0;
cout << "Type does not exists." << endl;
}
id_ = id;
}
const char *Device::getName()
{
return deviceName_;
}
int Device::getID()
{
return id_;
}
int Device::getType()
{
return type_;
}
The problem is when I try to read view what's saved in the file, this is the output:
http://imgur.com/P8WEAKq

Answer 1

The problem is here in the LoadFromFile function, I think:
for (int i = 0; i < Devicelist_.size(); i++)
LoadFile.read((char *)(&Devicelist_[i]), sizeof(Devicelist_[i]));
The DeviceList is empty, or has the wrong size.
You could write something similar as in AddDevice:
Device tempDevice();
LoadFile.read((char *)(&tempDevice), sizeof(tempDevice));
Devicelist_.push_back(tempDevice);
but the size (how many devices) of the file needs to be known.
As mentioned in a comment, this works only if Device doesn't have pointers to other objects for example. It should be a POD (Plain Old Data) object.
An elegant solution would be to implement streaming operators on Device

Answer 2

You will need to to have Device::StoreToFile write the number of devices before writing out each device. This will enable your Device::LoadFromFile to know how many device instances to read from the file.
I highly recommend you do not write a class or structure bit for bit to the file. If your class or struct has any advanced data structures, such as std::string or std::vector, you can't write them in binary. They may use pointers to memory, which will not be the same when you load them.
Here is a better solution:
1. Pass a buffer of uint8_t to the load and store methods.
2. The methods load from the buffer or store into the buffer (append).
3. The caller reads or writes the buffer to file.
4. The class also has a "size on file" method that returns the space occupied on a file, which may be different than the structure size.
Also search the web for "boost::serialization".