C/C++ on MacOS

Compiler choice

Usually in MacOS, the default compiler is Clang. Even the g++ command in the terminal is just a wrapper around Clang.

Although Clang will work just fine for most of the time, sometimes it just misbehaves completely compared to GCC. Since online judges usually use GCC, it is a good idea to install GCC and tell rbx to use it.

To use the GNU compiler, you need to install it separately.

brew install gcc

But, still after installing it, you need to tell rbx to use it. Find at the end of the brew command which version of g++ was installed.

For instance, if you see:

Installing gcc 14.2.0

This means that g++-14 is probably available as a command to run in your terminal.

To tell rbx to use it, you can run rbx config edit and add the following to the file (replace g++-14 with whatever version you have installed):

command_substitutions:
  g++: g++-14
  gcc: gcc-14

This will tell rbx to use gcc-14 and g++-14 as the C and C++ compilers.

A caveat to this is that GNU GCC sanitizers do not work on MacOS. If you need sanitizers in MacOS, you will need to tell rbx to fall back to Clang when sanitizing.

sanitizers:
  command_substitutions:
    g++: clang++
    gcc: clang

Floating-point precision

If you're on a Mac with Apple Sillicon (ARM), be aware you might run into floating-point precision issues. These two different architectures apply math optimizations differently. Whilst rbx tries its best to disable some of them in ARM, it's not always possible to completely match the behaviour of x86.

Another important thing to notice is that long double is a 64-bit float in ARM, as opposed to the standard extended 80-bit float in x86. Thus, for all effects, long double == double in ARM. If you have solutions that are heavily dependent on the precision of long double, you might need to adjust them when running on ARM, or simply accept they might fail.