What is the header?

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It’s so you can declare, in your own headers, methods that rely on the declarations of iostream types without having to #include the iostream headers themselves, which are large, complex and slow to compile against.

Here’s a simple example:

// foo.h
#include <iosfwd>

void sucker(std::iostream& is);

 

// foo.cc
#include <iostream>

void sucker(std::iostream& is) {
    is >> somevar;
}

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