std::basic_stringbuf<CharT,Traits,Allocator>::basic_stringbuf

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< cpp‎ | io‎ | basic stringbuf
 
C++
 
Input/output library
 
std::basic_stringbuf
 
(1)
explicit basic_stringbuf( std::ios_base::openmode which =
                              std::ios_base::in | std::ios_base::out );
 (until C++11)
explicit basic_stringbuf( std::ios_base::openmode which );
 (since C++11)
basic_stringbuf()
    : basic_stringbuf( std::ios_base::in | std::ios_base::out ) {}
 (2) (since C++11)
explicit

    basic_stringbuf( const std::basic_string<CharT, Traits, Allocator>& s,
                     std::ios_base::openmode which =

                         std::ios_base::in | std::ios_base::out );
 (3)
explicit basic_stringbuf( std::basic_string<CharT, Traits, Allocator>&& s,

                          std::ios_base::openmode which =

                              std::ios_base::in | std::ios_base::out );
 (4) (since C++20)
basic_stringbuf( std::ios_base::openmode which, const Allocator& a );
 (5) (since C++20)
explicit basic_stringbuf( const Allocator& a )
    : basic_stringbuf( std::ios_base::in | std::ios_base::out, a ) {}
 (6) (since C++20)
template< class SAlloc >

explicit basic_stringbuf( const std::basic_string<CharT, Traits, SAlloc>& s,
                          std::ios_base::openmode which =

                              std::ios_base::in | std::ios_base::out );
 (7) (since C++20)
template< class SAlloc >

basic_stringbuf( const std::basic_string<CharT, Traits, SAlloc>& s,

                 std::ios_base::openmode which, const Allocator& a );
 (8) (since C++20)
template< class SAlloc >

basic_stringbuf( const std::basic_string<CharT, Traits, SAlloc>& s,
                 const Allocator& a )

    : basic_stringbuf( s, std::ios_base::in | std::ios_base::out, a ) {}
 (9) (since C++20)
template< class StringViewLike >

explicit basic_stringbuf( const StringViewLike& t,
                          std::ios_base::openmode which =

                              std::ios_base::in | std::ios_base::out );
 (10) (since C++26)
template< class StringViewLike >

basic_stringbuf( const StringViewLike& t,

                 std::ios_base::openmode which, const Allocator& a );
 (11) (since C++26)
template< class StringViewLike >
basic_stringbuf( const StringViewLike& t, const Allocator& a );
 (12) (since C++26)
basic_stringbuf( basic_stringbuf&& rhs );
 (13) (since C++11)
basic_stringbuf( basic_stringbuf&& rhs, const Allocator& a );
 (14) (since C++20)
basic_stringbuf( const basic_stringbuf& rhs ) = delete;
 (15) (since C++11)

The std::basic_streambuf base and the exposition-only data members buf and mode are initialized as follows.

After initializing these subobjects, overloads (3-12) initialize the input and output sequences as if by calling init_buf_ptrs().

 Overload   std::basic_streambuf base  buf mode
(1) default-initialized implementation-defined
(see below) 		which
(2)  std::ios_base::in |
    std::ios_base::out 
(3) s which
(4) std::move(s)
(5) a
(6)  std::ios_base::in |
    std::ios_base::out
(7) s which
(8) {s, a}
(9)  std::ios_base::in |
    std::ios_base::out
(10) {sv, Allocator()} which
(11) {sv, a}
(12)  std::ios_base::in |
    std::ios_base::out
(13) rhs
(copy constructed) 		std::move(rhs).str() rhs.mode
(14)  {std::move(rhs).str(), a} 
1,2) Overload (1)(until C++11)(2)(since C++11) is the default constructor. It is implementation-defined whether the sequence pointers (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) are initialized to null pointers.
5,6) When the construction is complete, str.empty() is true.
7) This overload participates in overload resolution only if std::is_same_v<SAlloc, Allocator> is false.
10-12) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then it is used as above in the table.
These overloads participate in overload resolution only if std::is_convertible_v<const StringViewLike&,
                      std::basic_string_view<CharT, Traits>> is true.
13,14) Overload (13) is the move constructor. It is implementation-defined whether the six sequence pointers in *this obtain the values which rhs had.
When the construction is complete, rhs is empty but usable, and
  • Let rhs_p refer to the state of rhs just prior to this construction, the following expressions will evaluate to true:
  • str() == rhs_p.str()
  • getloc() == rhs_p.getloc()
  • gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
  • egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
  • pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
  • epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
  • Let rhs_a refer to the state of rhs just after this construction, the following expressions will evaluate to true:
  • !eback() || eback() != rhs_a.eback()
  • !gptr() || gptr() != rhs_a.gptr()
  • !egptr() || egptr() != rhs_a.egptr()
  • !pbase() || pbase() != rhs_a.pbase()
  • !pptr() || pptr() != rhs_a.pptr()
  • !epptr() || epptr() != rhs_a.epptr()
15) The copy constructor is deleted; std::basic_stringbuf is not CopyConstructible.

Parameters

s - a std::basic_string used to initialize the buffer
t - an object (convertible to std::basic_string_view) used to initialize the buffer
a - another allocator used to construct the internal std::basic_string
rhs - another basic_stringbuf
which - specifies stream open mode. It is bitmask type, the following constants are defined:
Constant Explanation
app seek to the end of stream before each write
binary open in binary mode
in open for reading
out open for writing
trunc discard the contents of the stream when opening
ate seek to the end of stream immediately after open
noreplace (C++23) open in exclusive mode

Notes

Typically called by the constructor of std::basic_stringstream.

The level of support for the open modes other than std::ios_base::in and std::ios_base::out varies among implementations. C++11 explicitly specifies the support for std::ios_base::ate in str() and in this constructor, but std::ios_base::app, std::ios_base::trunc, and std::ios_base::binary have different effects on different implementations.

Feature-test macro Value Std Feature
__cpp_lib_sstream_from_string_view 202306L (C++26) Interfacing string streams with std::string_view

Example

Demonstrates calling the constructor of std::basic_stringbuf directly:

Run this code
#include <iostream>
#include <sstream>
 
int main()
{
    // default constructor (mode = in | out)
    std::stringbuf buf1;
    buf1.sputc('1');
    std::cout << &buf1 << '\n';
 
    // string constructor in at-end mode (C++11)
    std::stringbuf buf2("test", std::ios_base::in
                              | std::ios_base::out
                              | std::ios_base::ate);
    buf2.sputc('1');
    std::cout << &buf2 << '\n';
 
    // append mode test (results differ among compilers)
    std::stringbuf buf3("test", std::ios_base::in
                              | std::ios_base::out
                              | std::ios_base::app);
    buf3.sputc('1');
    buf3.pubseekpos(1);
    buf3.sputc('2');
    std::cout << &buf3 << '\n';
}

Output: 

1
test1
est12 (Sun Studio) 2st1 (GCC)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 432 C++98 1. overload (1) allocated no array object
2. overload (3) did not specify how the input
    and output sequences are initialized 		1. removed the limitation
2. specified
LWG 562 C++98 overload (3) set epptr() to point one past the last underlying
character if bool(which & std::ios_base::out) == true 		epptr() can be set
beyond that position
P0935R0 C++11 the default constructor was explicit made implicit

See also

 constructs the string stream
(public member function of std::basic_stringstream<CharT,Traits,Allocator>)
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