Writing manipulators that don’t use arguments requires neither class derivation nor use of complex macros. Suppose your printer requires the pair <ESC>[
to enter bold mode. You can insert this pair directly into the stream:
cout << "regular " << '\033' << '[' << "boldface" << endl;
Or you can define the bold
manipulator, which inserts the characters:
ostream& bold( ostream& os ) {
return os << '\033' << '[';
}
cout << "regular " << bold << "boldface" << endl;
The globally defined bold
function takes an ostream reference argument and returns the ostream reference. It is not a member function or a friend because it doesn’t need access to any private class elements. The bold
function connects to the stream because the stream’s << operator is overloaded to accept that type of function, using a declaration that looks something like this:
ostream& ostream::operator<< ( ostream& (*_f)( ostream& ) ); {
(*_f)( *this );
return *this;
}
You can use this feature to extend other overloaded operators. In this case, it is incidental that bold
inserts characters into the stream. The function is called when it is inserted into the stream, not necessarily when the adjacent characters are printed. Thus, printing could be delayed because of the stream’s buffering.