Reading/writing binary files?

[vblanco]

Hello, im a c++ programmer, but there is something that i tried to learn and i couldnt find things, how do you guys store binary data?, ogre does it in the mesh format, im talking about writing a 12345 as a single Int and not an array of chars.
Any help will be appreciated.

[bstone]

Why wouldn't you look into the C++ code that Ogre has for reading/writing binary files then?

[vblanco]

I am doing that, but i want some guide or tutorials to learn it the good way instead of reverse engineering. I tried to find some resources about that in the web, but all i get is text based files, and i want full binary. Im learning graphics programming and im making my own engine, so i want to make my own simple format, just to learn. I did a game engine already with OGRE with physics, sound, gui, weapon and vehicle system and more things, but i want to increase my knowledge about 3d graphics, i may help a bit in ogre dev when i learn good 3d rendering

[bstone]

Here's my quick tutorial. Shield your eyes

Code: Select all

#include <fstream>

int main()
{
    std::ofstream os( "myfile.bin", std::ios::out | std::ios::binary );

    const int singleInt = 12345;
    os.write( reinterpret_cast< const char* >( &singleInt ), sizeof( singleInt ) );

    os.close();

    return 0;
}

Edit: I believe you should figure out the reverse process of reading that back.

[vblanco]

I see a const char* there, isnt that a normal string of chars?, i havent tested, but that looks like its making a normal c-string of the number. Also, how to read the values back?

[saejox]

no need to reinvent the wheel

http://www.boost.org/doc/libs/1_52_0/li ... index.html

i have been using it for years. stable, feature rich, flexible and fast.

[bstone]

I see a const char* there, isnt that a normal string of chars?, i havent tested, but that looks like its making a normal c-string of the number. Also, how to read the values back?

That's what you get by using the "learn through tutorials" method. I see that too often lately. My next advice is to get, read, and understand the C++ bible by Bjarne Stroustrup. Without that and some good practice saying "i'm a c++ programmer" doesn't work

[Kojack]

os.write takes the address of a byte in memory (a char is a byte, unless you are on freaky hardware) and a byte count. It will read that many bytes from the address and write them to the file.
This could be a C style string or just a block of bytes, it depends on how you use it, not how it's stored.
In the case above we are getting the address of singleInt, casting that address to pretend it's really pointing to a char instead of an int, then writing out as many bytes as an int has.

Pretty much any simple type can be written to a file like that. Cast it's pointer and use sizeof to get the number of bytes. But be careful of classes or structs that contain pointers or complex objects (so no string, vector, map, list, etc). That will cause problems when loading it back (the pointers will point to the wrong memory). Also never write out a class with virtual methods, the hidden vtable pointer will get written out and loading that back will do very bad things.

[vblanco]

Thank you kojack, thats what i needed to know, i was confused becouse everywhere seems to use that cons char* for normal strings, and i dont use them i allways use std::string, ive never seen using it to another value