std::basic_streambuf

The class basic_streambuf controls input and output to a character sequence. It includes and provides access to

The I/O stream objects std::basic_istream and std::basic_ostream, as well as all objects derived from them (std::ofstream, std::stringstream, etc), are implemented entirely in terms of std::basic_streambuf.

The controlled character sequence is an array of CharT which, at all times, represents a subsequence, or a "window" into the associated character sequence. Its state is described by three pointers:

A basic_streambuf object may support input (in which case the buffer described by the beginning, next, and end pointers is called get area), output (put area), or input and output simultaneously. In latter case, six pointers are tracked, which may all point to elements of the same character array or two individual arrays.

If the next pointer is less than the end pointer in the put area, a write position is available. The next pointer can be dereferenced and assigned to.

If the next pointer is less than the end pointer in the get area, a read position is available. The next pointer can be dereferenced and read from.

If the next pointer is greater than the beginning pointer in a get area, a putback position is available, and the next pointer may be decremented, dereferenced, and assigned to, in order to put a character back into the get area.

The character representation and encoding in the controlled sequence may be different from the character representations in the associated sequence, in which case a std::codecvt locale facet is typically used to perform the conversion. Common examples are UTF-8 (or other multibyte) files accessed through std::wfstream objects: the controlled sequence consists of wchar_t characters, but the associated sequence consists of bytes.

Typical implementation of the std::basic_streambuf base class holds only the six CharT* pointers and a copy of std::locale as data members. In addition, implementations may keep cached copies of locale facets, which are invalidated whenever imbue() is called. The concrete buffers such as std::basic_filebuf or std::basic_stringbuf are derived from std::basic_streambuf.

Two convenience typedefs are provided by the standard library

Member types

Member functions