This chapter is devoted to string handling, parsing, and printing of arbitrary data. For such jobs, STL provides its I/O stream library. The library basically consists of the following classes, which are each depicted in gray boxes:

The arrows show the inheritance scheme of the classes. This might look very overwhelming at first, but we will get to use most of these classes in this chapter and get familiar with them class by class. When looking at those classes in the C++ STL documentation, we will not find them directly with these exact names. That is because the names in the diagram are what we see as application programmers, but they are really mostly just typedefs of classes with a basic_ class name prefix (for example, we will have an easier job searching the STL documentation for basic_istream rather than istream). The basic_* I/O stream classes are templates that can be specialized for different character types. The classes in the diagram are specialized on char values. We will use these specializations throughout the book. If we prefix those class names with the w character, we get wistream, wostream, and so on--these are the specialization typedefs for wchar_t instead of char, for example.

At the top of the diagram, we see std::ios_base. We will basically never use it directly, but it is listed for completeness because all other classes inherit from it. The next specialization is std::ios which embodies the idea of an object which maintains a stream of data, that can be in good state, run empty of data state (EOF), or some kind of fail state.

The first specializations we are going to actually use are std::istream and std::ostream. The "i" and the "o" prefix stand for input and output. We have seen them in our earliest days of C++ programming in the simplest examples in form of the objects std::cout and std::cin (but also std::cerr). These are instances of those classes, which are always globally available. We do data output via ostream and input via istream.

A class which inherits from both istream and ostream is iostream. It combines both input and output capabilities. When we understand how all classes from the trio consisting of istream, ostream and iostream can be used, we basically are ready to immediately put all following ones to use, too:

ifstream, ofstream and fstream inherit from istream, ostream and iostream respectively, but lift their capabilities to redirect the I/O from and to files from the computer's filesystem.

The istringstream, ostringstream and iostringstream work pretty analogously. They help build strings in memory, and/or consuming data from them.