std::basic_streambuf::underflow

 

C++

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Regular expressions library (C++11)

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Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated)

Streams

Abstractions

ios_base

basic_ios

basic_istream

basic_ostream

basic_iostream

File I/O

basic_ifstream

basic_ofstream

basic_fstream

String I/O

basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

istrstream (deprecated)

ostrstream (deprecated)

strstream (deprecated)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

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

 

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() {
        while( (ch= src->sbumpc()) == '\0') ; // skip zeroes
        setg(&ch, &ch, &ch+1); // make one read position available
        return ch; // may return EOF
    }
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)
underflow [virtual]	returns the next character available in the input sequence (virtual protected member function of std::basic_stringbuf)
underflow [virtual]	reads a character from the input sequence without advancing the next pointer (virtual protected member function of std::strstreambuf)