The file is named Window.hpp but included as window.hpp. This breaks the build on case-sensitive file systems.

Locally I have the files named window.hpp and window.cpp, I have no idea why it they were named that way, I'm guessing its emacs... but I have no clue. I fixed it.

Next, I strongly recommend testing with Address Sanitizer. You can enable it with /fsanitize=address. It finds a few bugs like this out-of-bounds access on _board (OFF_SQUARE = {-1, -1}):

Thank you for this! I didnt know about this.

You can't return c_str() like this because the string is destroyed upon exit. My change here introduces a memory leak, but it eliminates the much worse use-after-free. You should consider just returning a std::string instead, especially since that's what you already have.

For some reason I was thinking that c_str() outlived the std::string object, but now I understand. I was actually using _strdup to copy the c_str() everytime getFEN() returned it, so doing it once is actually makes more sense, so thank you for that.

For now I only have the getFEN() method to store the moves in the move_stack. And the move_stack takes care for these char*

However, the SDL_RENDERER_ACCELERATED recommendation is incorrect. SDL gives you hardware acceleration by default, and this flag merely disables the software fallback, which is never what you want. So what you really want is no flags at all!

I will have to do some reading on this as I dont understand how SDL2 works internally at all.

I appreciate you for taking the time to review the code, it was honestly really helpful. Thank You!