C++ In a Nutshell

<iosfwd>

The <iosfwd> header provides forward declarations of the various I/O-related classes and templates. The forward declarations are incomplete type declarations. (Read about complete and incomplete types in Chapter 6.) By using <iosfwd> you can sometimes avoid including the complete definitions of the I/O classes, and thereby save some compilation time.

It is always safe to include <iosfwd> and any of the other I/O headers, even if they both declare the same type.

Because <iosfwd> does not provide any complete type declarations, this section contains only Table 13-18, which lists each type name with a reference to the header (if any) that is required for the complete type.

Table 13-18. Forward type declarations in <iosfwd>

Type declaration	Header
template<typename T> class allocator	<memory>
template<typename charT, typename traits=char_traits<charT> > class basic_filebuf	<fstream>
template<typename charT, typename traits=char_traits<charT> > class basic_fstream	<fstream>
template<typename charT, typename traits=char_traits<charT> > class basic_ifstream	<fstream>
template<typename charT, typename traits=char_traits<charT> > class basic_ios	<ios>
template<typename charT, typename traits=char_traits<charT> > class basic_iostream	<istream>
template<typename charT, typename traits=char_traits<charT> > class basic_istream	<istream>
template<typename charT, typename traits=char_traits<charT>, typename Alloc = allocator<charT> > class basic_istringstream ...