std::basic_stringbuf::overflow(3)

NAME
std::basic_stringbuf::overflow - std::basic_stringbuf::overflow

Synopsis
   protected:
   virtual int_type overflow( int_type c = Traits::eof() );
   Appends the character c to the output character sequence.
   If c is the end-of-file indicator (traits::eq_int_type(c, traits::eof()) == true),
   then there is no character to append. The function does nothing and returns an
   unspecified value other than traits::eof().
   Otherwise, if the output sequence has a write position available or this function
   can successfully make a write position available, then calls sputc(c) and returns c.
   This function can make a write position available if the std::stringbuf is open for
   output ((mode & ios_base::out) != 0): in this case, it reallocates (or initially
   allocates) the buffer big enough to hold the entire current buffer plus at least one
   more character. If the std::stringbuf is also open for input ((mode & ios_base::in)
   != 0), then overflow also increases the size of the get area by moving egptr() to
   point just past the new write position.

Parameters
   c - the character to store in the put area

Return value
   Traits::eof() to indicate failure, c if the character c was successfully appended,
   or some value other than Traits::eof() if called with Traits::eof() as the argument.

Notes
   This function is different from a typical overflow() which moves the contents of the
   buffer to the associated character sequence because for a std::basic_stringbuf, the
   buffer and the associated sequence are one and the same.

Example
   In the implementation used to execute this example (e.g. GCC-4.9), overflow()
   over-allocates the put area to 512 bytes: a call to str() would only return the four
   initialized bytes, but the next 508 calls to sputc() would not require new calls to
   overflow(). // Run this code
 #include <sstream>
 #include <iostream>
 struct mybuf : std::stringbuf
 {
     mybuf(const std::string& new_str,
           std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
         : std::stringbuf(new_str, which) {}
     int_type overflow(int_type c = EOF) override
     {
         std::cout << "stringbuf::overflow('" << char(c) << "') called\n"
                   << "Before: size of get area: " << egptr() - eback() << '\n'
                   << " size of put area: " << epptr() - pbase() << '\n';
         int_type ret = std::stringbuf::overflow(c);
         std::cout << "After : size of get area: " << egptr() - eback() << '\n'
                   << " size of put area: " << epptr() - pbase() << '\n';
         return ret;
     }
 };
 int main()
 {
     std::cout << "read-write stream:\n";
     mybuf sbuf(" "); // read-write stream
     std::iostream stream(&sbuf);
     stream << 1234;
     std::cout << sbuf.str() << '\n';
     std::cout << "\nread-only stream:\n";
     mybuf ro_buf(" ", std::ios_base::in); // read-only stream
     std::iostream ro_stream(&ro_buf);
     ro_stream << 1234;
     std::cout << "\nwrite-only stream:\n";
     mybuf wr_buf(" ", std::ios_base::out); // write-only stream
     std::iostream wr_stream(&wr_buf);
     wr_stream << 1234;
 }

Possible output:
 read-write stream:
 stringbuf::overflow('4') called
 Before: size of get area: 3
         size of put area: 3
 After : size of get area: 4
         size of put area: 512
 1234
 read-only stream:
 stringbuf::overflow('1') called
 Before: size of get area: 3
         size of put area: 0
 After : size of get area: 3
         size of put area: 0
 write-only stream:
 stringbuf::overflow('4') called
 Before: size of get area: 0
         size of put area: 3
 After : size of get area: 0
         size of put area: 512
   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 169 C++98 the buffer (re)allocated could only hold one allows more extra
                      extra character characters
                      overflow moved epptr() to point just past the
   LWG 432 C++98 new it is not moved
                      write position if the std::stringbuf is open
                      for input

See also
   overflow writes characters to the associated output sequence from the put area
   [virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)
   underflow returns the next character available in the input sequence
   [virtual] (virtual protected member function)