std::basic_streambuf<CharT,Traits>::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.

# Declarations

protected:
virtual int_type underflow();

# Return value

The value of the character pointed to by the get pointer after the call on success, or Traits::eof() otherwise.

# Example

#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);
}

# See also