Use a #define macro as path in #include directive in shader?

Hi guys,

In a Unity shader, it doesn't seem to be possible to do the following:

Code (CSharp):

1. #define MYMACRO "UnityCG.cginc"
2. #include MYMACRO

It fails to compile:

Code (CSharp):

1. Shader error in '***': syntax error : unexpected token 'MYMACRO'

Do you why it's not possible to use a macro in an include preprocessor directive?
Is there a way to do something similar?

Thanks

 

This is because of how #include and c style preprocessor macros work.

My understanding of how it goes is something like this.

Step 1: load .shader text.
Step 2: scan text for the first #include line and find the appropriate file.
Step 3: load the text from the found file. insert that text into the previously loaded code at the position the #include for it existed.
Step 4: Repeat steps 1 through 3 until no #include lines are found.

Note, at no point yet has the text been parsed or compiled in any way beyond just looking for those #inlcude lines. This is just building one big giant file with all of the include files inlined into the shader. Imagine manually going through and replacing all of your #include lines with the code copy/pasted from those files. That’s all it’s doing.

Step 5 then takes that one giant file and probably looks for #pragma multi_compile lines and other ShaderLab specific stuff, then Step 6 and beyond is actually parsing the #if and #define lines to generate code variants before eventually compiling those many shader variants.

TLDR version, #include lines are parsed before #define lines, so it doesn’t yet know what that MYMACRO is supposed to be something else.

 

Thanks for your reply.
The weird thing is that it works in C++.
That's why I was wondering why the behavior is different in shader lab.
Thanks

 

Well, this isn’t c++, or even c, it’s just something Unity implemented themselves in the same written style as c.