preprocessor

[themadme]

Hi, im creating a program that will use either OpenGL or Direct3D. Is there any predefined preprocessor

so i can say

#if OPENGL
/// code ///
#elif DIRECT3D
// code ///
#endif 

if not, is there any possible way of making your own, with out modifying the complier source

[JosAH]

Sure, you can define that macro on the command line of the compiler:
-DOPENGL or -DDIRECTX3D. Read your compiler user manual.

kind regards,

Jos

[donbock]

Are you asking if you can create your own macro to control conditional compilation? If so, then JosAH (#2) gave you the answer.

Are you asking if there are particular macro names that are commonly used by programmers everywhere to choose between these options?

Are you asking if the OpenGL and DirectX libraries already provide distinctive macros in their header files that you could use to control conditional compilation? (But then how would you choose which header file to include?)

Are you asking if the OpenGL and DirectX libraries require particular distinctive macros to be defined on the command line?

[themadme]

thanks for the replies.

Good point sorry for not being distinctive. Well what i meant was a could your create your own macro to control conditional compilation, to decide what header files to include and what parts of your programme to be used. Was just checking if like for example visual studios in the library somewhere it had any conditional micros.

The wrapper i have made, when initialising, it takes in an argument to see what render api its going to be using.

I dont know how they did it, i guess i could download the sdk source and search through it. but thought it would quicker to ask if someone knew any information to do such a thing or similar.

So anyway how can it be done?

Thanks

[donbock]

The wrapper i have made, when initialising, it takes in an argument to see what render api its going to be using.

That sounds like run-time configuration. A common way to handle run-time configuration is to abstract a set of device-independent functions, create a data structure of function pointers to each of those functions, then write wrapper functions to make your device-dependent functions match the prototypes of the device-independent functions, then at run-time populate the structure with pointers to the appropriate device-dependent wrapper functions.

On the other hand, you could use conditional compilation. In that case, only one API is ever present at a time. You don't need to specify the API in an argument.

[themadme]

That's a very good point, i guess i could do the wrapper but if i was to use conditional compilation, how would i do that. ive been messing around with it and still end up with the incorrect results. Right now its not my top priority at the moment but it would be useful knowing how to do this properly.

Many thanks for the replies

[donbock]

Refer to message #2 where JosAH explains how to set macros from the command line. Let's assume that you will use this technique to defined either OPENGL or DIRECT3D. You must also be careful to insure that the command-line specifies the -I include-file path and -L object-library path corresponding to the OPENGL/DIRET3D macro.

(I hope the following snippets are ok with the posting guidelines.)

First, confirm the command-line macros are set properly.

#if (defined(OPENGL) && defined(DIRECT3D)) || (!defined(OPENGL) && !defined(DIRECT3d))
#error "Command-line macros OPENGL and DIRECT3D defined inconsistently"
#endif

Next, you need to include the proper header files:

#if defined(OPENGL)
<include the OpenGL headers>
#elif defined(DIRECT3D)
<include the Direct3D headers>
#endif

Approach #1: use wrapper functions.
For each wrapper function, code it something like this ...

<declaration of the wrapper function>
{
#if defined(OPENGL)
   <call the corresponding OpenGL function>
#elif defined(DIRECT3D)
   <call the corresponding Direct3D function>
#endif
}

Approach #2: no wrapper functions.
Every place where you call a graphics function, code it something like this ...

#if defined(OPENGL)
   <call the OpenGL function>
#elif defined(DIRECT3D)
   <call the Direct3D function
#endif

[themadme]

thank you very much, you have been very helpful, thanks again