I am facing a small problem. I have a struct, which has a vector. Note that the vector is dynamic per every iteration. Now, in a particular iteration, how do I store the struct which contains a vector of size n to a binary file?
Also, when retrieving, assume that I know how the size of the vector, how to I retrieve from the binary file, the struct variable containing the vector of all the stored elements?
I am able to store something to the binary file (as I can see the size increasing when writing), but when I am trying to retrieve back the elements, I am getting size of vector to be zero.
Unfortunately, I have to achieve this using the standard STL and not use any third-party libraries.
You should have a look at Boost Serialization.
If you can't use 3rd party libraries, you must know that C++ doesn't support serialization directly. This means you'll have to do it yourself.
This article shows a nice way of serializing a custom object to the disk and retrieving it back. And this tutorial shows you how to get started right now with fstream.
This is my attempt:
EDIT: since the OP asked how to store/retrieve more than record I decided to updated the original code.
So, what changed? Now there's an array student_t apprentice[3];
to store information of 3 students. The entire array is serialized to the disk and then it's all loaded back to the RAM where reading/searching for specific records is possible. Note that this is a very very small file (84 bytes). I do not suggest this approach when searching records on huge files.
#include <fstream>
#include <iostream>
#include <vector>
#include <string.h>
using namespace std;
typedef struct student
{
char name[10];
int age;
vector<int> grades;
}student_t;
int main()
{
student_t apprentice[3];
strcpy(apprentice[0].name, "john");
apprentice[0].age = 21;
apprentice[0].grades.push_back(1);
apprentice[0].grades.push_back(3);
apprentice[0].grades.push_back(5);
strcpy(apprentice[1].name, "jerry");
apprentice[1].age = 22;
apprentice[1].grades.push_back(2);
apprentice[1].grades.push_back(4);
apprentice[1].grades.push_back(6);
strcpy(apprentice[2].name, "jimmy");
apprentice[2].age = 23;
apprentice[2].grades.push_back(8);
apprentice[2].grades.push_back(9);
apprentice[2].grades.push_back(10);
// Serializing struct to student.data
ofstream output_file("students.data", ios::binary);
output_file.write((char*)&apprentice, sizeof(apprentice));
output_file.close();
// Reading from it
ifstream input_file("students.data", ios::binary);
student_t master[3];
input_file.read((char*)&master, sizeof(master));
for (size_t idx = 0; idx < 3; idx++)
{
// If you wanted to search for specific records,
// you should do it here! if (idx == 2) ...
cout << "Record #" << idx << endl;
cout << "Name: " << master[idx].name << endl;
cout << "Age: " << master[idx].age << endl;
cout << "Grades: " << endl;
for (size_t i = 0; i < master[idx].grades.size(); i++)
cout << master[idx].grades[i] << " ";
cout << endl << endl;
}
return 0;
}
Outputs:
Record #0
Name: john
Age: 21
Grades:
1 3 5
Record #1
Name: jerry
Age: 22
Grades:
2 4 6
Record #2
Name: jimmy
Age: 23
Grades:
8 9 10
Dump of the binary file:
$ hexdump -c students.data
0000000 j o h n \0 237 { \0 � � { � 025 \0 \0 \0
0000010 ( � � \b 4 � � \b 8 � � \b j e r r
0000020 y \0 � \0 � � | \0 026 \0 \0 \0 @ � � \b
0000030 L � � \b P � � \b j i m m y \0 \0 \0
0000040 � 6 � \0 027 \0 \0 \0 X � � \b d � � \b
0000050 h � � \b
0000054