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

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

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

#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";
}

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

See also