How do Manipulators work?

[teguh123]

http://www.cplusplus.com/reference/iostream/manipulators/oct/

If we do

ostringstream os;

lalu os << oct;

what is oct?

What is the type of oct?

Is << operator defined for os and oct?

Is oct a special character?

Why we don't call oct ()? why oct only?

[hamsterman]

http://www.cplusplus.com/reference/iostream/ostream/operator%3C%3C/
notice the overloads

ostream& operator<< (ostream& ( *pf )(ostream&));
ostream& operator<< (ios& ( *pf )(ios&));
ostream& operator<< (ios_base& ( *pf )(ios_base&));

these take a pointer to a function.
oct itself is declared as inline ios_base& oct(ios_base& _Iosbase){ /*...*/ }
google function pointers if you want to know more.

[teguh123]

Ah I see

So oct is a pointer to a function. That's why we call that oct and not oct();

Hmm.... strange. Why do they do it that way?

I mean why not declare a class called manipulator and define

ostream& operator << manipulator;?

Ah let me guess, the changing of ostream is done within the function it self

Hmm....

[hamsterman]

that's how function pointers work in c++ (c)
func() is a function and func is a pointer to it.

well, then you'd have to have a bunch of global objects or construct them every time you use them. but there's nothing wrong with the idea.

and yes, changes are made in that function.

[jsmith]

It would mean that every time a new manipulator was created, class ostream& would have to be augmented
with another overload for the new manipulator, which would effectively prevent anyone other than the
writers of the C++ I/O streams library from writing a custom manipulator.

[hamsterman]

huh? no.struct manipulator{ virtual ostream& manipulate(ostream&); }; then of course << would have to take manipulator*, but I assume teguh123 just made a mistake about that.
And even if you didn't use polymorphism, you could still overload the << operator.