basic_streambuf::overflow

virtual int_type overflow(int_type c = T::eof());

If c does not compare equal to T::eof(), the protected virtual member function endeavors to insert the element T::to_char_type(c) into the output stream. It can do so in various ways:

• If a write position is available, it can store the element into the write position and increment the next pointer for the output buffer.
• It can make a write position available by allocating new or additional storage for the output buffer.
• It can make a write position available by writing out, to some external destination, some or all of the elements between the beginning and next pointers for the output buffer.

If the function cannot succeed, it returns T::eof() or throws an exception. Otherwise, it returns T::not_eof(c). The default behavior is to return T::eof().