How to write pointers of an object into a file?

Question

I have a situation where I need to store an object's pointer into an file and read it again in the same process. How should I do it?

Right now I write/read like this:

    Myclass* class  = <valid pointer to Myclass>
    FILE* output_file = fopen(filename, "w");
    fwrite(class, sizeof(class), 1, output_file)

// and read it

    FILE* in_file = fopen(filename, "r");
    Myclass* class_read
    fread(class_read, sizeof(class_read), 1, in_file)

I don't see correct values when reading back. I will read and write these files in the same address space.

Answer 1

To read and write the pointer itself, you'll need to pass its address, not the address it points to:

fwrite(&class, sizeof(class), 1, output_file);
fread (&class_read, sizeof(class_read), 1, in_file);
       ^

You're instead writing the first few bytes of whatever class points to, then trying to read them back into whatever class_read points to (failing if it's not yet a valid pointer).