C++ boost forward declaration question

Answer 1

Forward declarations are needed to reduce compile-time dependencies. For instance, when implementing Pimpl idiom.

One more case is that, for instance, boost::pool^* depends on windows.h on Windows platform. When creating my interface I don't want to force the users of my class to include the system headers by using my interface.

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^*Ok, that's a bad example, because boost/poolfwd.hpp still includes windows.h, but I hope they'll fix this issue. And I hope you get the idea.

Answer 2

I don't know about boost, but these forward declarations also exist in the standard library. For example, <iosfwd> contains forward declarations for streams (which are templates, normally hidden behind typedefs).

You'd benefit from this header, when declaring an overloaded operator<<.

In your header:

#include <iosfwd>

class X { ... };

std::ostream& operator<< (std::ostream& os, const X& x);

Note that the header doesn't require the full definition of ostream (= basic_ostream<char, char_traits<char> >).

The rationale for the header is that those templates are cumbersome to forward declare yourself. For the above example, it would look something like:

namespace std {
    template <class CharT>
    class char_traits;

    template <class CharT, class CharTraits>
    class basic_ostream;

    typedef basic_ostream<char, char_traits<char> > ostream;
}