性病：基本[医]filebuf：：Sekpos

#include <fstream>
#include <iostream>
 
struct mybuf : std::filebuf
{
    pos_type seekpos(pos_type sp, std::ios_base::openmode which) {
         std::cout << "Before seekpos(" << sp << "), size of the get area is "
                   << egptr()-eback() << " with "
                   << egptr()-gptr() << " read positions available\n";
         pos_type rc = std::filebuf::seekpos(sp, which
         std::cout << "seekpos() returns " << rc << ".\nAfter the call, "
                   << "size of the get area is "
                   << egptr()-eback() << " with "
                   << egptr()-gptr() << " read positions available\n";
// uncomment if get area is emptied by seekpos()
//         std::filebuf::underflow(
//         std::cout << "after forced underflow(), size of the get area is "
//                   << egptr()-eback() << " with "
//                   << egptr()-gptr() << " read positions available\n";
        return rc;
    }
};
 
int main()
{
    mybuf buf;
    buf.open("test.txt", std::ios_base::in
    std::istream stream(&buf
    stream.get( // read one char to force underflow()
    stream.seekg(2
}

Before seekpos(2), size of the get area is 110 with 109 read positions available
seekpos() returns 2.
After the call, size of the get area is 110 with 108 read positions available

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