std::basic_streambuf::underflow(3)

NAME
std::basic_streambuf::underflow - std::basic_streambuf::underflow

Synopsis
   protected:
   virtual int_type underflow();
   Ensures that at least one character is available in the input area by updating the
   pointers to the input area (if needed) and reading more data in from the input
   sequence (if applicable). Returns the value of that character (converted to int_type
   with Traits::to_int_type(c)) on success or Traits::eof() on failure.
   The function may update gptr, egptr and eback pointers to define the location of
   newly loaded data (if any). On failure, the function ensures that either gptr() ==
   nullptr or gptr() == egptr.
   The base class version of the function does nothing. The derived classes may
   override this function to allow updates to the get area in the case of exhaustion.

Parameters
   (none)

Return value
   The value of the character pointed to by the get pointer after the call on success,
   or Traits::eof() otherwise.
   The base class version of the function returns traits::eof().

Note
   The public functions of std::streambuf call this function only if gptr() == nullptr
   or gptr() >= egptr().

Example
// Run this code
 #include <iostream>
 #include <sstream>
 class null_filter_buf : public std::streambuf {
     std::streambuf* src;
     char ch; // single-byte buffer
 protected:
     int underflow() {
         traits_type::int_type i;
         while ((i = src->sbumpc()) == '\0') ; // skip zeroes
         if (!traits_type::eq_int_type(i, traits_type::eof())) {
             ch = traits_type::to_char_type(i);
             setg(&ch, &ch, &ch+1); // make one read position available
         }
         return i;
     }
 public:
     null_filter_buf(std::streambuf* buf) : src(buf) {
         setg(&ch, &ch+1, &ch+1); // buffer is initially full
     }
 };
 void filtered_read(std::istream& in)
 {
     std::streambuf* orig = in.rdbuf();
     null_filter_buf buf(orig);
     in.rdbuf(&buf);
     for(char c; in.get(c); )
             std::cout << c;
     in.rdbuf(orig);
 }
 int main()
 {
     char a[] = "This i\0s \0an e\0\0\0xample";
     std::istringstream in(std::string(std::begin(a), std::end(a)));
     filtered_read(in);
 }

Output:
 This is an example

See also
   uflow reads characters from the associated input sequence to the get area and
   [virtual] advances the next pointer
             (virtual protected member function)
   overflow writes characters to the associated output sequence from the put area
   [virtual] (virtual protected member function)
   underflow reads from the associated file
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
   underflow returns the next character available in the input sequence
   [virtual] (virtual protected member function of
             std::basic_stringbuf<CharT,Traits,Allocator>)
   underflow reads a character from the input sequence without advancing the next
   [virtual] pointer
             (virtual protected member function of std::strstreambuf)