At the same time I investigated why openxcom couldn't open my xcom files, I noticed you use
#define DATA_FOLDER "./DATA/"
instead of something like
const std::string DATA_FOLDER = "./DATA/";
Using const instead of #define has advantages over #define and, apart from the work of changing an existing code base, has no disadvantages at all.
The first advantage is that you gain type checking for your constant variable, something you don't have with a #define.
The second advantage is that when using const, scope is respected and you don't pollute the global namespace.
I know you are trying to just get things done and leaving premature optimizations and such for later - but think of this as an investment for the future in the form of more readable code and maintainable code.