Question

code looks like this:

struct Dog {
  string name;
  unsigned int age;
};

int main()
{
    Dog d = {.age = 3, .name = "Lion"};
    FILE *fp = fopen("dog.txt", "wb");
    fwrite(&d, sizeof(d), 1, fp);   //write d into dog.txt
}

My problem is what’s the point of write a data object or structure into a binary file? I assume it is for making the data generated in a running program persistent, right? If yes, then how can I get the data back? Using fread?

This makes me think of database-like stuff, dose database write data into disk the same way?

Answer 1

You can do it but you will have a lot of issues to care about:

• structure types: all your data needs really be into struct or you can just writing a pointer to some other place.
• structure changes: if you need change your structure you will need write a converter to read old struct and write the new.
• language interoperability: will be hard to access the data using other language

It was a common practice in the early days before relational databases popularization. You can make index files pointing to a record number.

However nowadays I will advice you to make serialization and write strings instead binaries.

NOTE:
if string is something like char[40] your code maybe will survive… but if your question is about C++ and string is a class then kill you child before it grows up! The string object characters are not into your struct but in the heap.