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

 

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std::basic_streambuf

Public member functions basic_streambuf::~basic_streambuf      Locales basic_streambuf::pubimbue basic_streambuf::getloc Positioning basic_streambuf::pubsetbuf basic_streambuf::pubseekoff basic_streambuf::pubseekpos basic_streambuf::pubsync Get area basic_streambuf::in_avail basic_streambuf::snextc basic_streambuf::sbumpc basic_streambuf::sgetc basic_streambuf::sgetn Put area basic_streambuf::sputc basic_streambuf::sputn Putback basic_streambuf::sputbackc basic_streambuf::sungetc

Protected member functions basic_streambuf::basic_streambuf basic_streambuf::operator= (C++11) basic_streambuf::swap (C++11) Locales basic_streambuf::imbue Positioning basic_streambuf::setbuf basic_streambuf::seekoff basic_streambuf::seekpos basic_streambuf::sync Get area basic_streambuf::showmanyc basic_streambuf::underflow basic_streambuf::uflow basic_streambuf::xsgetn basic_streambuf::ebackbasic_streambuf::gptrbasic_streambuf::egptr basic_streambuf::gbump basic_streambuf::setg Put area basic_streambuf::xsputn basic_streambuf::overflow basic_streambuf::pbasebasic_streambuf::pptrbasic_streambuf::epptr basic_streambuf::pbump basic_streambuf::setp Putback basic_streambuf::pbackfail

 

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