std::basic_stringbuf<CharT,Traits,Allocator>::overflow (3) - Linux Manuals

std::basic_stringbuf<CharT,Traits,Allocator>::overflow: std::basic_stringbuf<CharT,Traits,Allocator>::overflow


std::basic_stringbuf<CharT,Traits,Allocator>::overflow - std::basic_stringbuf<CharT,Traits,Allocator>::overflow


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.


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.


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.


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

  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 protected member function of std::basic_streambuf<CharT,Traits>)

underflow returns the next character available in the input sequence
          (virtual protected member function)