Question

I want to use pointer magikry to save a C++ class using the following method that writes byte data into a file:

result Osp::Io::File::Write (const void *buffer, int length);

• Parameters:

  • buffer — A pointer to the user-supplied buffer that contains byte data to be written
  • length — The buffer length in bytes

• Exceptions:

  • E_SUCCESS — The method is successful.
  • E_INVALID_STATE — The file has not been opened as yet.
  • E_ILLEGAL_ACCESS — The file is not opened for write operation, or access is denied due to insufficient permission.
  • E_INVALID_ARG — Either of the following conditions has occurred:
    • The specified buffer contains a null pointer.
    • The specified buffer length is equal or smaller than 0.
    • The file handle is invalid (either the file is closed by another method, or the memory is corrupted).
  • E_STORAGE_FULL — The disk space is full.
  • E_IO — An unexpected device failure has occurred as the media ejected suddenly or file corruption is detected.

I’d rather not assume that there will be any sort of buffering, although I am confident each byte won’t occasion a whole block of flash to be rewritten but I was wondering if there is a niftier way to write all the data fields of a class (and nothing else, eg static fields) by, eg, a pointer to the object (*this)?

Answer 1

In C++ you don’t write “raw” objects into files, but rather serialize them. There’s no magic, you need to write your serialization code yourself (overloading operators << and >>, for convenience).

You can do it the old C-style by just dumping memory, but in addition to the problems this would generally cause with C (alignment, endian issues when transferring data between systems), you also get the problems introduced by C++ (internal class representation, possible “hidden” data members such as a v-table, etc).

If you want to ensure you read and write reliable data that can be transferred between different systems and/or different pieces of software – you better implement the serialization, and don’t look for shortcuts.

You can use libraries like Boost.Serialization for that.