Some of the standard iomanip functions take take a parameter.

I'd like to know how this is accomplished, for instance, can I do something similar with a function? That's really the solution that I needed for this answer, but I couldn't figure out how to do this.

When I looked up the definition for setw function for example in http://en.cppreference.com it lists the return type as "unspecified", and it also only lists one argument, rather than also taking a stream& parameter. How does this work?

Note to the reader:

This question has an excellent answer but to the individual looking up this question; this answer will only be useful in combination with the following functionality provided by ios_base:

xalloc
iword
pword

Here is a simple example of a user-defined manipulator that takes one parameter defined using a class:

#include <iostream>

class putX // injects some `X`s into the stream
{
    std::size_t _n;
public:
    explicit putX(std::size_t n): _n(n) {}
    std::size_t getn() const {return _n;}
    friend std::ostream& operator<<(std::ostream& os, const putX& obj)
    {
        std::size_t n = obj.getn();
        for (std::size_t i = 0; i < n; ++i)
            os << 'X';
        return os;
    }
};

int main()
{
    std::cout << putX(10) << " test " << putX(10);
}

Manipulators that take no parameters can simply be implemented as

std::ostream& custom_manip(std::ostream& os) { // do something with os and return os;}

That's because basic_ostream::operator<< has an overload that takes a pointer-to-function std::ostream& (*fp)(std::ostream&) as its right hand side (e.g., a manipulator)

PS: The C++ Standard Library by N. Josuttis describes how manipulators/custom manipulators work in great detail, see Sec. 15.6.3 User-Defined Manipulators