27.streambuf.sgetn.stdcxx-1026.cpp

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#include <cassert>
#include <istream>
#include <streambuf>
 
int main ()
{
    static int x = '0';
 
    struct MyBuf : std::streambuf {
        // unbuffered, always successfully reads one character
        int_type underflow () {
            assert (gptr () == egptr ());
            return x++;
        }
    } sb;
 
    // "endless" stream that never reaches EOF
    std::istream endless (&sb);
 
    char s [4] = "";
 
    endless.read (s, sizeof s);
 
    // expect to find stream in good state (i.e., not EOF) and
    // to have extracted as many characters as requested with
    // just as many calls to underflow()
    assert (endless.good ());
    assert (sizeof s == endless.gcount ());
    assert ('0' == s [0] && '1' == s [1] && '2' == s [2] && '3' == s [3]);
}