How to read / write a struct in Binary Files?

Question

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.

Answer 1

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