std::basic_syncbuf<CharT,Traits,Allocator>::sync (3) - Linux Manuals
int sync() override;
First, records that a flush is pending, by updating the appropriate private flag.
Then, if the current emit-on-sync policy is true, makes a call to emit().
Otherwise (if the emit-on-sync policy is false, which is the default), the flush is suspended until emit() is called, such as through std::basic_osyncstream::emit() or std::basic_osyncstream::~basic_osyncstream
sync() or its equivalent is implicitly called by close(), seekoff(), and seekpos() and explicitly called by std::basic_streambuf::pubsync()
This section is incomplete
Reason: no example
sync synchronizes the buffers with the associated character sequence
emit (public member function of std::basic_osyncstream<CharT,Traits,Allocator>)