seekg doesn't work when using istream with custom streambuf

Question

I'm trying to use a stream in order to read from a vector of existing data.

// http://stackoverflow.com/questions/8815164/c-wrapping-vectorchar-with-istream
template<typename CharT, typename TraitsT = std::char_traits<CharT> >
class vectorwrapbuf : public std::basic_streambuf<CharT, TraitsT>
{
public:
    vectorwrapbuf(std::vector<CharT> &vec)
    {
        this->setg(&vec[0], &vec[0], &vec[0] + vec.size());
    }
};

int main()
{
    vector<char> data(100, 1);
    vectorwrapbuf<char> databuf(data);
    std::istream file(&databuf);
    file.exceptions(std::istream::failbit | std::istream::badbit);
    file.seekg(10); // throws
}

However when I call seekg it throws an exception (or reports error normally if not using exceptions). Why is this? 10 is within the 100 chars of input data.

Code needs to work with both C++11 and before compilers.

Live example: http://ideone.com/PfpW0o

score 0 · Accepted Answer · answered Sep 16 '15 at 21:21

0

std::basic_streambuf::seekpos by default returns pos_type(off_type(-1)) according to http://en.cppreference.com/w/cpp/io/basic_streambuf/pubseekpos

istream::seekg() in order threats that as error condition, so you need to override that function in your class vectorwrapbuf and return actual position.

answered Sep 16 '15 at 21:21

Slava

43,454
1
47
90

1

Ok thanks. Can you suggest how I do that, or is it not simple? – Neil Kirk Sep 16 '15 at 21:23
@NeilKirk looks like you need to validate input, call `setg()` with proper positions and return first parameter. – Slava Sep 16 '15 at 21:33

seekg doesn't work when using istream with custom streambuf

1 Answers1