std::basic_streambuf<CharT,Traits>::sputbackc

 

C++

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Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

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ios_base

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basic_stringstream

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basic_osyncstream (C++20)

Types

streamoff

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iostream_category (C++11)

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std::basic_streambuf

Public member functions

basic_streambuf::~basic_streambuf

Locales

basic_streambuf::pubimbue

basic_streambuf::getloc

Positioning

basic_streambuf::pubsetbuf

basic_streambuf::pubseekoff

basic_streambuf::pubseekpos

basic_streambuf::pubsync

Get area

basic_streambuf::in_avail

basic_streambuf::snextc

basic_streambuf::sbumpc

basic_streambuf::sgetc

basic_streambuf::sgetn

Put area

basic_streambuf::sputc

basic_streambuf::sputn

Putback

basic_streambuf::sputbackc

basic_streambuf::sungetc

Protected member functions

basic_streambuf::basic_streambuf

basic_streambuf::operator= (C++11)

basic_streambuf::swap (C++11)

Locales

basic_streambuf::imbue

Positioning

basic_streambuf::setbuf

basic_streambuf::seekoff

basic_streambuf::seekpos

basic_streambuf::sync

Get area

basic_streambuf::showmanyc

basic_streambuf::underflow

basic_streambuf::uflow

basic_streambuf::xsgetn

basic_streambuf::ebackbasic_streambuf::gptrbasic_streambuf::egptr

basic_streambuf::gbump

basic_streambuf::setg

Put area

basic_streambuf::xsputn

basic_streambuf::overflow

basic_streambuf::pbasebasic_streambuf::pptrbasic_streambuf::epptr

basic_streambuf::pbump

basic_streambuf::setp

Putback

basic_streambuf::pbackfail

 

int_type sputbackc( char_type c );

Puts back a character back to the get area.

If a putback position is available in the get area (gptr() > eback()), and the character c is equal to the character one position to the left of gptr() (as determined by Traits::eq(c, gptr()[-1]), then simply decrements the next pointer (gptr()).

Otherwise, calls pbackfail(Traits::to_int_type(c)) to either back up the get area or to modify both the get area and possibly the associated character sequence.

The I/O stream function basic_istream::putback is implemented in terms of this function.

Parameters

c

-

character to put back

Return value

If putback position was available, returns the character that the next pointer is now pointing at, converted to int_type with Traits::to_int_type(*gptr()). The next single-character input from this streambuf will return this character.

If putback position was not available, returns what pbackfail() returns, which is Traits::eof() on failure.

Example

Run this code

#include <iostream>
#include <sstream>
 
int main()
{
    std::stringstream s("abcdef"); // gptr() points to 'a' in "abcdef"
    std::cout << "Before putback, string holds " << s.str() << '\n';
    char c1 = s.get(); // c1 = 'a', gptr() now points to 'b' in "abcdef"
    char c2 = s.rdbuf()->sputbackc('z'); // same as s.putback('z')
                                         // gptr() now points to 'z' in "zbcdef"
    std::cout << "After putback, string holds " << s.str() << '\n';
    char c3 = s.get(); // c3 = 'z', gptr() now points to 'b' in "zbcdef"
    char c4 = s.get(); // c4 = 'b', gptr() now points to 'c' in "zbcdef"
    std::cout << c1 << c2 << c3 << c4 << '\n';
 
    s.rdbuf()->sputbackc('b');  // gptr() now points to 'b' in "zbcdef"
    s.rdbuf()->sputbackc('z');  // gptr() now points to 'z' in "zbcdef"
    int eof = s.rdbuf()->sputbackc('x');  // nothing to unget: pbackfail() fails
    if (eof == EOF)
        std::cout << "No room to putback after 'z'\n";
}

Output:

Before putback, string holds abcdef
After putback, string holds zbcdef
azzb
No room to putback after 'z'

See also

sungetc	moves the next pointer in the input sequence back by one (public member function)
putback	puts character into input stream (public member function of std::basic_istream<CharT,Traits>)