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Showing content from https://en.cppreference.com/w/cpp/language/../io/basic_stringbuf/../basic_streambuf/pbump.html below:

std::basic_streambuf<CharT,Traits>::pbump - cppreference.com

protected:
void pbump( int count );

Repositions the put pointer (pptr()) by count characters, where count may be positive or negative. No checks are done for moving the pointer outside the put area [pbase(), epptr()).

If the pointer is advanced and then overflow() is called to flush the put area to the associated character sequence, the effect is that extra count characters with undefined values are output.

(none)

Because this function takes an int, it cannot manipulate buffers larger than std::numeric_limits<int>::max() characters (LWG issue 255).

#include <fstream>
#include <iostream>
#include <string>
 
struct showput_streambuf : std::filebuf
{
    using std::filebuf::pbump; // expose protected
    std::string showput() const
    {
        return std::string(pbase(), pptr());
    }
};
 
int main()
{
    showput_streambuf mybuf;
    mybuf.open("test.txt", std::ios_base::out);
    std::ostream str(&mybuf);
    str << "This is a test" << std::flush << "1234";
    std::cout << "The put area contains: " << mybuf.showput() << '\n';
    mybuf.pbump(10);
    std::cout << "after pbump(10), it contains " << mybuf.showput() << '\n';
}

Output:

The put area contains: 1234
after pbump(10), it contains 1234 is a test

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