Writing Objects into files in binary mode

[Sujan Dhakal]

Writing Object Succeeds with the following code

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

class Data
{
public:
string name;
int id;
public:
Data(){}
Data(string n,int id){name=n;this->id=id;}
};

int main() {

Data objdata("sujan dhakal",1);
fstream file("c:\\\\test.txt",ios::out | ios::binary);
if(file.is\_open())
{
	file.write((char\*)&objdata,sizeof(Data));
	cout<<"Success";
	file.close();
}
else
{
	cout<<"Error";
}

return 0;

}

but why reading not working #include <iostream> #include <fstream> using namespace std; class Data { public: string name; int id; public: Data(){} Data(string n,int id){name=n;this->id=id;} }; int main() { Data objdata; fstream file("c:\\test.txt",ios::in | ios::binary); if(file.is_open()) { file.read((char*)&objdata,sizeof(Data)); cout<<objdata.name<<objdata.id<<endl; cout<<"Success"; file.close(); } else { cout<<"Error"; } return 0; } help me

[Cedric Moonen]

I suggest you read this series of articles[^]. They are about serrialization, this is what you need to do in order to save/load objects.

Cédric Moonen Software developer
Charting control [v3.0] OpenGL game tutorial in C++

[Sujan Dhakal]

i'm writing a simple program using Ansi C++. I'm not using Microsoft C++. that doesnot applies to me. what's your suggestion?

[CPallini]

You cannot do that. You're trying to serialize an object (see "Serialization" topic at Wikipedia [^]) in a wrong way. Since std::string is not a scalar type, the object instance (objdata) holds just a reference to it (and you're saving just such a reference, i.e. storing an address to disk, clearly it makes no sense). So you cannot save the class as a whole using

file.write((char*)&objdata,sizeof(Data));

instead you have to store each of its members in order to restore appropriately them, for instance:

file.write((char*)&(objdata.id),sizeof(objdata.id));
string::size_type len = objdata.name.length();
file.write((char*) &len, sizeof(len)); // this is useful for reading back the string
file.write((char*)(objdata.name.c_str(), len); //note: not storing null terminator

Can you spot the code for reading back the class? :)

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[My articles]

[Sujan Dhakal]

so it was only the issue of std::string class, thanks i succeed with the following code.

#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;

class Data
{
public:
char name[20];
int id;
public:
Data(){}
Data(char *n,int id){strcpy(name,n);this->id=id;}
};

int main() {

Data objdata;
//fstream file("c:\\\\test.txt",ios::out | ios::binary);
fstream file("c:\\\\test.txt",ios::in | ios::binary);
if(file.is\_open())
{
	//file.write((char\*)&objdata,sizeof(Data));

	file.read((char\*)&objdata,sizeof(Data));
	cout<<objdata.name<<objdata.id;
	cout<<"Success";
	file.close();
}
else
{
	cout<<"Error";
}

return 0;

}