Why is there assert( sizeof( bool ) == 1 ) in Doom 3 source?

Question

Here's the assert. In what reasonable circumstances can it fail, and why is the game checking it?

Answer 1

Some platforms define bool to be the same size as int. At least older versions of Mac OS X (and likely other RISC BSD ports) were like this. Presumably the code uses bool arrays with an assumption of efficiency. Doom has been ported to a lot of platforms so it's probably very cagey about such things.

It has to be done at runtime because there is no standard macro specifying sizeof(bool), and compile time checks didn't work with non-macro expressions until C++11.

Answer 2

I think I have come across the the answer you were looking for. Doom 3 is cross platform and on x86 platforms bool is defined by gcc with a size of 1. In gcc(compiler used by Apple at the time),on Mac OS X PowerPC on the other hand it defaults to 4. Use the -mone-byte-bool to change it to 1.

From http://linux.die.net/man/1/g++

  -mone-byte-bool
       Override the defaults for "bool" so that "sizeof(bool)==1".  By
       default "sizeof(bool)" is 4 when compiling for Darwin/PowerPC and 1
       when compiling for Darwin/x86, so this option has no effect on x86.

       Warning: The -mone-byte-bool switch causes GCC to generate code
       that is not binary compatible with code generated without that
       switch.  Using this switch may require recompiling all other
       modules in a program, including system libraries.  Use this switch
       to conform to a non-default data model.