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 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 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, 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

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)