std::strstreambuf::underflow

C++

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Language support library

Concepts library (C++20)

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Technical Specifications

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated)

basic_syncbuf (C++20)

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ios_base

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basic_istringstream

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basic_stringstream

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istrstream (deprecated)

ostrstream (deprecated)

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Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

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Error category interface

iostream_category (C++11)

io_errc (C++11)

std::strstreambuf

Public member functions

strstreambuf::strstreambuf

strstreambuf::~strstreambuf

strstreambuf::freeze

strstreambuf::str

strstreambuf::pcount

Protected member functions

strstreambuf::underflow

strstreambuf::pbackfail

strstreambuf::overflow

strstreambuf::setbuf

strstreambuf::seekoff

strstreambuf::seekpos

protected: virtual int_type underflow();

Reads the next character from the get area of the buffer.

If the input sequence has a read position available (gptr() < egptr(), returns (unsigned char)(*gptr()).

Otherwise, if pptr() is not null and pptr() > egptr() (there is a put area and it is located after the get area), extends the end of the get area to include the characters that were recently written into the put area by incrementing egptr() to some value between gptr() and pptr(), and then returns (unsigned char)(*gptr()).

Otherwise, returns EOF to indicate failure.

Parameters

(none)

Return value

The next character in the get area, (unsigned char)(*gptr()) on success, EOF on failure

Example

Run this code

#include <strstream>
#include <iostream>

struct mybuf : std::strstreambuf
{
    int_type overflow(int_type c) 
    {
        std::cout << "Before overflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        int_type rc = std::strstreambuf::overflow(c);
        std::cout << "After overflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        return rc;
    }

    int_type underflow() 
    {
        std::cout << "Before underflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        int_type ch = std::strstreambuf::underflow();
        std::cout << "After underflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        if (ch == EOF) {
            std::cout << "underflow() returns EOF\n";
        } else {
            std::cout << "underflow() returns '" << char(ch) << "'\n";
        }
        return ch;
    }
};

int main()
{
    mybuf sbuf; // read-write dynamic strstreambuf
    std::iostream stream(&sbuf);

    int n;
    stream >> n;
    stream.clear();
    stream << "123";
    stream >> n;
    std::cout << n << '\n';
}

Possible output:

Before underflow(): size of the get area is 0 size of the put area is 0
After underflow(): size of the get area is 0 size of the put area is 0
underflow() returns EOF
Before overflow(): size of the get area is 0 size of the put area is 0
After overflow(): size of the get area is 0 size of the put area is 32
Before underflow(): size of the get area is 0 size of the put area is 32
After underflow(): size of the get area is 3 size of the put area is 32
underflow() returns '1'
Before underflow(): size of the get area is 3 size of the put area is 32
After underflow(): size of the get area is 3 size of the put area is 32
underflow() returns EOF
123

See also

underflow [virtual]	reads characters from the associated input sequence to the get area (virtual protected member function of std::basic_streambuf)
underflow [virtual]	returns the next character available in the input sequence (virtual protected member function of std::basic_stringbuf)
underflow [virtual]	reads from the associated file (virtual protected member function of std::basic_filebuf)
sgetc	reads one character from the input sequence without advancing the sequence (public member function of std::basic_streambuf)
get	extracts characters (public member function of std::basic_istream)