c++ - 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   237   {     ?   ?   {   ? 025      
0000010   (   ?   ?     4   ?   ?     8   ?   ?     j   e   r   r
0000020   y     ?     ?   ?   |   026         @   ?   ?  
0000030   L   ?   ?     P   ?   ?     j   i   m   m   y      
0000040   ?   6   ?   027         X   ?   ?     d   ?   ?  
0000050   h   ?   ?                                                  
0000054