Suppose you write a project which uses some data files, like bitmaps, geometries, tables etc. They are contained in separate files. You may scatter these files over directories (like Qt's Resources).
This makes the program look (actually be) unprofessional. Or, at least, it was considered to be so in the early days of programming. This is why :
- every file can be lost or stolen (joke) during installation or illegal copying what makes the whole project not functional
- every file (e.g. a bitmap) can be easily replaced by another one e.g. a company's logo
- opening multiple files is slow process
That is why it looks reasonable to place groups of files into a single file with programmatic access to every entry in this composite file. I wrote marine simulators before and it was very good to collect data about every ship model (geometry, textures for OpenGL, radar reflection curves, ship dynamics data) into a file.
Of course, you can install and use HDF5. It is fine, but you will have to update it with every new version of it and new version of compiler you use. Usual headache with third-party software.
So, if you wish to have just a simple class to contain multiple files in it, you can use this.
Composite file is a set of files appended to each other and preceded by header according to the following format :
- header specified - header string (without length byte); this header is checked on opening
- total number of files (entries) compiled in the files (2 bytes)
- directory, containing position + length pairs (each 4 + 4 bytes) for every entry
- files one by one.
A composite file can be generated with the CompFiles program. This is one of two VS projects in this solution.
The unusual integer type for total number is used here to keep compatibility with my very old files generated long ago. You can easily change it knowing the file structure.
VS 2019 is the compiler for the whole project.
The base class CompFile does not have anything Windows-specific and can be used in Linux with no or minimum problems.
However, the console project CompFiles compiles in Windows only because after hard 10 years work on C++ standartisation there is still no standard way to list files in a directory (see my efforts in getFileList()).
with a header from all files specified by file mask like this :
>CompFiles RADAR_FILES TestFiles/*.* RADAR.CMP
It takes all files from TestFiles folder, compiles them all into a composite file RADAR.CMP and saves it with header RADAR_FILES.
this second console project shows how to extract entries from a composite file e.g.
> CompFileTest RADAR_FILES RADAR.CMP
where RADAR_FILES is file header; RADAR.CMP is an existing composite file.
This example opens a composite file, extracts all its entries and saves each in a separate file.
The purpose of the example is to show how to read composite file entry data into memory (see the code). It does nothing useful. Maybe only check if all data is stored correctly.