Question

Can I use an elsewhere opened

FILE* f = ...

thing to initialize some std::ostream instance? Like this (pseudocode):

FILE *f = ...;
std::ostream os;
os./*bind_to_f*/( f );    // HOW?
os << "Hello world" << std::endl;

Answer 1

With GCC you can just pass the existing file descriptor to the constructor (ref):

FILE *f = ...;
std::ofstream os(fileno(f));

Constructor: ofstream::ofstream (int fd)

Make an ofstream for writing to a file that was already open, using file descriptor fd.