# std::basic_spanbuf

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< cpp‎ | io

Defined in header `<spanstream>` |  |
template< class CharT, class Traits = std::char_traits<CharT> > class basic_spanbuf : public std::basic_streambuf<CharT, Traits> |  | (since C++23)

`std::basic_spanbuf` is a std::basic_streambuf whose associated character sequence is a memory-resident sequence of arbitrary characters, which can be initialized from or made available as an instance of std::span<CharT>.

`std::basic_spanbuf` performs I/O on a fixed buffer, and therefore it does not attempt to obtain a new buffer when the underlying buffer is exhausted.

Typical implementations of `std::basic_spanbuf` add two member subobjects: a std::ios_base::openmode recording the open mode, and a std::span<CharT> referencing the underlying buffer.

Several typedefs for common character types are provided:

Defined in header `<spanstream>`
Type | Definition
`std::spanbuf` | std::basic_spanbuf<char>
`std::wspanbuf` | std::basic_spanbuf<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

##### Public member functions
(constructor) (C++23) | constructs a `basic_spanbuf` object (public member function)
(destructor) [virtual] (C++23) | destroys the `basic_spanbuf` object (virtual public member function)
operator= (C++23) | assigns a `basic_spanbuf` object (public member function)
swap (C++23) | swaps two `basic_spanbuf` objects (public member function)
span (C++23) | obtains or initializes an underlying buffer according to mode (public member function)
##### Protected member functions
setbuf [virtual] (C++23) | attempts to replace the controlled character sequence with an array (virtual protected member function)
seekoff [virtual] (C++23) | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function)
seekpos [virtual] (C++23) | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function)

### Non-member functions

std::swap(std::basic_spanbuf) (C++23) | specializes the std::swap algorithm (function template)

## Inherited from std::basic_streambuf

### 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 of `std::basic_streambuf<CharT,Traits>`)
##### Locales
pubimbue | invokes imbue() (public member function of `std::basic_streambuf<CharT,Traits>`)
getloc | obtains a copy of the associated locale (public member function of `std::basic_streambuf<CharT,Traits>`)
##### Positioning
pubsetbuf | invokes setbuf() (public member function of `std::basic_streambuf<CharT,Traits>`)
pubseekoff | invokes seekoff() (public member function of `std::basic_streambuf<CharT,Traits>`)
pubseekpos | invokes seekpos() (public member function of `std::basic_streambuf<CharT,Traits>`)
pubsync | invokes sync() (public member function of `std::basic_streambuf<CharT,Traits>`)
##### Get area
in_avail | obtains the number of characters immediately available in the get area (public member function of `std::basic_streambuf<CharT,Traits>`)
snextc | advances the input sequence, then reads one character without advancing again (public member function of `std::basic_streambuf<CharT,Traits>`)
sbumpc stossc (removed in C++17) | reads one character from the input sequence and advances the sequence (public member function of `std::basic_streambuf<CharT,Traits>`)
sgetc | reads one character from the input sequence without advancing the sequence (public member function of `std::basic_streambuf<CharT,Traits>`)
sgetn | invokes xsgetn() (public member function of `std::basic_streambuf<CharT,Traits>`)
##### Put area
sputc | writes one character to the put area and advances the next pointer (public member function of `std::basic_streambuf<CharT,Traits>`)
sputn | invokes xsputn() (public member function of `std::basic_streambuf<CharT,Traits>`)
##### Putback
sputbackc | puts one character back in the input sequence (public member function of `std::basic_streambuf<CharT,Traits>`)
sungetc | moves the next pointer in the input sequence back by one (public member function of `std::basic_streambuf<CharT,Traits>`)

### 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 of `std::basic_streambuf<CharT,Traits>`)
##### Positioning
setbuf [virtual] | replaces the buffer with user-defined array, if permitted (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
seekoff [virtual] | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
seekpos [virtual] | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
sync [virtual] | synchronizes the buffers with the associated character sequence (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
##### Get area
showmanyc [virtual] | obtains the number of characters available for input in the associated input sequence, if known (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
underflow [virtual] | reads characters from the associated input sequence to the get area (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
uflow [virtual] | reads characters from the associated input sequence to the get area and advances the next pointer (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
xsgetn [virtual] | reads multiple characters from the input sequence (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
eback gptr egptr | 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 of `std::basic_streambuf<CharT,Traits>`)
overflow [virtual] | writes characters to the associated output sequence from the put area (virtual protected member function of `std::basic_streambuf<CharT,Traits>`)
pbase pptr epptr | 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 of `std::basic_streambuf<CharT,Traits>`)

### Notes

`std::basic_spanbuf` does not own the underlying buffer.

It is the responsibility of programmers to ensure the underlying buffer is in its lifetime when used by a `std::basic_spanbuf` object. Additional synchronization may be needed if more than one thread operates the same underlying buffer through different `std::basic_spanbuf` objects.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_spanstream` | 202106L | (C++23) | `std::spanbuf`, std::spanstream

### See also

basic_stringbuf | implements raw string device (class template)
strstreambuf (deprecated in C++98) | implements raw character array device (class)

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