I inherited a supposedly finalised "core" handling data, but only booleans were dealt with and I had to code the other ones. It was coded in template metaprogramming (MSVC & gcc 4.8 😨), just thinking about it gives me chills down the spine!

It's a bit fascinating (and your normal template problems if you have any will vanish and your loved one will return) but it's, IMO, not for large projects 😬. Impossible to debug (not every error is caught compile time after all).

printf was coded like that (with the variadic template "..."), I don't remember if it was libraries for the old compilers or the new ones... But that's a fun way of checking it out IMO!