# std::basic_streambuf

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< cpp‎ | io

Defined in header `<streambuf>` |  |
template< class CharT, class Traits = std::char_traits <CharT> > class basic_streambuf; |  |

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

1) 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.
2) 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:

1) The beginning pointer, always points at the lowest element of the buffer
2) The next pointer, points at the element that is the next candidate for reading or writing
3) 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 or std::basic_stringbuf are derived from `std::basic_streambuf`.

Two convenience typedefs are provided by the standard library

Defined in header `<streambuf>`
Type | Definition
`streambuf` | basic_streambuf<char>
`wstreambuf` | basic_streambuf<wchar_t>

### Member types

Member type | Definition
`char_type` | CharT
`traits_type` | Traits; the program is ill-formed if `Traits::char_type` is not `CharT`.
`int_type` | Traits::int_type
`pos_type` | Traits::pos_type
`off_type` | Traits::off_type

### Member functions

(destructor) [virtual] | destructs the basic_streambuf object (virtual public member function)

##### Locales

pubimbue | invokes imbue() (public member function)
getloc | obtains a copy of the associated locale (public member function)

##### Positioning

pubsetbuf | invokes setbuf() (public member function)
pubseekoff | invokes seekoff() (public member function)
pubseekpos | invokes seekpos() (public member function)
pubsync | invokes sync() (public member function)

##### Get area

in_avail | obtains the number of characters immediately available in the get area (public member function)
snextc | advances the input sequence, then reads one character without advancing again (public member function)
sbumpc | reads one character from the input sequence and advances the sequence (public member function)
stossc (deprecated in C++98)(removed in C++17) | advances the input sequence as if by calling sbumpc() and discarding the result (public member function)
sgetc | reads one character from the input sequence without advancing the sequence (public member function)
sgetn | invokes xsgetn() (public member function)

##### Put area

sputc | writes one character to the put area and advances the next pointer (public member function)
sputn | invokes xsputn() (public member function)

##### Putback

sputbackc | puts one character back in the input sequence (public member function)
sungetc | moves the next pointer in the input sequence back by one (public member function)

### Protected member functions

(constructor) | constructs a basic_streambuf object (protected member function)
operator= (C++11) | replaces a basic_streambuf object (protected member function)
swap (C++11) | swaps two basic_streambuf objects (protected member function)

##### Locales

imbue [virtual] | changes the associated locale (virtual protected member function)

##### Positioning

setbuf [virtual] | replaces the buffer with user-defined array, if permitted (virtual protected member function)
seekoff [virtual] | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function)
seekpos [virtual] | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function)
sync [virtual] | synchronizes the buffers with the associated character sequence (virtual protected member function)

##### Get area

showmanyc [virtual] | obtains the number of characters available for input in the associated input sequence, if known (virtual protected member function)
underflow [virtual] | reads characters from the associated input sequence to the get area (virtual protected member function)
uflow [virtual] | reads characters from the associated input sequence to the get area and advances the next pointer (virtual protected member function)
xsgetn [virtual] | reads multiple characters from the input sequence (virtual protected member function)
ebackgptregptr | returns a pointer to the beginning, current character and the end of the get area (protected member function)
gbump | advances the next pointer in the input sequence (protected member function)
setg | repositions the beginning, next, and end pointers of the input sequence (protected member function)

##### Put area

xsputn [virtual] | writes multiple characters to the output sequence (virtual protected member function)
overflow [virtual] | writes characters to the associated output sequence from the put area (virtual protected member function)
pbasepptrepptr | returns a pointer to the beginning, current character and the end of the put area (protected member function)
pbump | advances the next pointer of the output sequence (protected member function)
setp | repositions the beginning, next, and end pointers of the output sequence (protected member function)

##### Putback

pbackfail [virtual] | puts a character back into the input sequence, possibly modifying the input sequence (virtual protected member function)

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