std::basic_streambuf - cppreference.com

template<classCharT,classTraits=std::char_traits<CharT>>classbasic_streambuf;The class basic_streambuf controls input and output to a character sequence. It includes and provides access to

The controlled character sequence, also called the buffer, which may contain input sequence (also called get area) for buffering the input operations and/or output sequence (also called put area) for buffering the output operations.

The associated character sequence, also called source (for input) or sink (for output). This may be an entity that is accessed through OS API (file, TCP socket, serial port, other character device), or it may be an object (

std::vector

array

string literal

), that can be interpreted as a character source or sink.

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:

The beginning pointer, always points at the lowest element of the buffer.

The next pointer, points at the element that is the next candidate for reading or writing.

The end pointer, points one past the end of the buffer.

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

std::basic_stringbuf

are derived from std::basic_streambuf.

Several typedefs for common character types are provided:

Type Definition std::streambufstd::basic_streambuf<char>std::wstreambufstd::basic_streambuf<wchar_t>Member types

Member type Definition char_typeCharT