C++ Storing objects in a file

Question

I have a list of objects that I would like to store in a file as small as possible for later retrieval. I have been carefully reading this tutorial, and am beginning (I think) to understand, but have several questions. Here is the snippet I am working with:

    static bool writeHistory(string fileName)
{
    fstream historyFile;
    historyFile.open(fileName.c_str(), ios::binary);
    if (historyFile.good())
    {
        list<Referral>::iterator i;
        for(i = AllReferrals.begin(); 
                i != AllReferrals.end();
                i++)
        {
            historyFile.write((char*)&(*i),sizeof(Referral));
        }
        return true;
    } else return false;
}

Now, this is adapted from the snippet

file.write((char*)&object,sizeof(className));

taken from the tutorial. Now what I believe it is doing is converting the object to a pointer, taking the value and size and writing that to the file. But if it is doing this, why bother doing the conversions at all? Why not take the value from the beginning? And why does it need the size? Furthermore, from my understanding then, why does

historyFile.write((char*)i,sizeof(Referral));

not compile? i is an iterator (and isn't an iterator a pointer?). or simply

historyFile.write(i,sizeof(Referral));

Why do i need to be messing around with addresses anyway? Aren't I storing the data in the file? If the addresses/values are persisting on their own, why can't i just store the addresses deliminated in plain text and than take their values later?

And should I still be using the .txt extension? < edit> what should I use instead then? I tried .dtb and was not able to create the file. < /edit> I actually can't even seem to get file to open without errors with the ios::binary flag. I'm also having trouble passing the filename (as a string class string, converted back by c_str(), it compiles but gives an error).

Sorry for so many little questions, but it all basically sums up to how to efficiently store objects in a file?

Answer 1

What you are trying to do is called serialization. Boost has a very good library for doing this.

What you are trying to do can work, in some cases, with some very important conditions. It will only work for POD types. It is only guaranteed to work for code compiled with the same version of the compiler, and with the same arguments.

(char*)&(*i)

says to take the iterator i, dereference it to get your object, take the address of it and treat it as an array of characters. This is the start of what is being written to the file. sizeof(Referral) is the number of bytes that will be written out.

An no, an iterator is not necessarily a pointer, although pointers meet all the requirements for an iterator.

Answer 2

Question #1 why does ... not compile? Answer: Because i is not a Referral* -- it's a list::iterator ;; an iterator is an abstraction over a pointer, but it's not a pointer.

Question #2 should I still be using the .txt extension? Answer: probably not. .txt is associated by many systems to the MIME type text/plain.

Unasked Question: does this work? Answer: if a Referral has any pointers on it, NO. When you try to read the Referrals from the file, the pointers will be pointing to the location on memory where something used to live, but there is no guarantee that there is anything valid there anymore, least of all the thing that the pointers were pointing to originally. Be careful.