What exactly are cout/cin?

Question

I know Java and now want to learn C++. I can't understand what are cout (character output stream) and cin (character input). Are these global variables? Then why

"My message">>cout; 

doesn't work? But

cout<<"My message";

works.

Answer 1

cout is an instance of the class std::ostream, and yes, it's a global variable. But operator>>(char *, ostream& os); hasn't been declared by the relevant header, so "My message">>cout; will give an error of something like "can't find an operator >> which takes arguments const char * and std::ostream" (and possibly a lot more errors because sometimes compilers get very confused by these sort of things).

cin is the same thing, except std::istream

If you really want to mess with peoples heads, you could do:

template<typename T>
std::ostream& operator>>(T x, std::ostream& os)
{
   os << x;
   return os;
}

Of course, it won't work for "My Message " >> "Some other string" >> cout;, which is probably one of the reasons it's not done that way.

Note that this is simply slight abuse of the operator overloading, where we have a custom type as the left-hand side, and standard or non-standard type on the right hand side. cout is no different from some other variable of a custom type.