I have this function:
std::string Room::getUsersAsString(std::vector usersList, User * excludeUser)
{
std::string usersNames = " ";
If you use
if (usersList[i]->getUsername() != excludeUser->getUsername)
instead of
if (usersList[i]->getUsername() != excludeUser->getUsername())
your compiler will think you want to use a function pointer instead of the method itself, and if you would have wanted to use a function pointer, you would still have to get the address of it (using &).
So make sure you don't forget your () after a function call!