fifo.tcc compile errors

[mjsmithers]

When attempting to test fifo.hpp and fifo.tcc on macOS and windows 10 in a basic command line app, the compilers on both platforms fail on fifo.tcc line 194
std::vector slots = {};
macOS: error: expected member name or ';' after declaration specifiers
visual studio on Windows 10: error C2059: syntax error: '=', error C2334: unexpected token(s) preceding '{'; skipping apparent function body