I don't know for sure, but C++ is always a good guess with that kind of software.

I downloaded IrfanView and ran "strings" on the exe file, and one of the strings in there is "Microsoft Visual C++ Runtime Library" - so that would point towards C++ (although it's not certain from that alone).

Yeah, Irfanview dates to a time when MS’s C compiler was horribly out of date (still stuck in C89 compatibility), and the only way to get remotely modern language features was to compile it as C++, even if you never used any C++ features. Everyone was programming in “C++” on windows back then even if they were basically writing C.

So yeah, it’s C++ but that doesn’t necessarily tell you much, it could still very well be basically C.