什么是重载`<<`时`std :: ostream＆（* f）（std :: ostream＆）`，为什么我需要它？

Question

I'm working in a wrapper for boost.log and found this question that seems to be what I want, but instead of std::cout would be something related to the boost stream that I don't really know yet. For that, I stuck wondering why it is needed and what it's actually doing. For example:

MyCout& operator<< (MyCout &s, std::ostream& (*f)(std::ios &)) {
    f(std::cout);
    return s;
}

In this case I understand (or maybe not?) that I'm overloading the operator << for MyCout using the std::ostream& (*f)(std::ios &) , but why is that? What the f(std::cout) actually does and why I would need to overload the operator with this function? It seems that s is not been used at all, just passing through the operator and returning the same as before.

Thanks!

Answer 1

std::ostream& (*f)(std::ios &) is a function pointer named f and it points to function that takes a std::ios & as its only parameter and returns a std::ostream& . This is needed for some of the stream manipulators like std::endl , which is a function, not an object like std::cout is.

With this overload you can stream a function into your stream and have that function do some sort of manipulation of the stream

---

Do not that this function signature isn't what you really want. The input parameter type and return type should be the same. The standard overloads for operator << that take manipulator functions are

basic_ostream& operator<<(
    std::ios_base& (*func)(std::ios_base&) );

basic_ostream& operator<<(
    std::basic_ios<CharT,Traits>& (*func)(std::basic_ios<CharT,Traits>&) );

basic_ostream& operator<<(
    std::basic_ostream<CharT,Traits>& (*func)(std::basic_ostream<CharT,Traits>&) );

basic_istream& operator>>( 
    std::ios_base& (*func)(std::ios_base&) );

basic_istream& operator>>( 
    std::basic_ios<CharT,Traits>& (*func)(std::basic_ios<CharT,Traits>&) );

basic_istream& operator>>( 
    basic_istream& (*func)(basic_istream&) );