Thread: Streambuf overloading with sputbackc

I want to have a simple way to replace a istream or ostream in order to do on-line filtering of the content (for instance: add 1 to each character when reading, remove 1 when writing). It has to support operations like unget and putback

I think the easiest way might be to overload streambuf

I thought this would be something very easy, but it seems most of the code I'm doing results in error when code tries to do a unget

Could someone show me how to subclass a streambuf to do online filtering (char by char) which I can then put in a istream or ostrea?

Here is some code I'm using

Code:

class FilteringBuf : public std::streambuf
{
    char readBuf_; // current char
    std::streambuf* pExternBuf_; // source buffer


    int_type underflow()
    {
        if (gptr() == &readBuf_)
            return traits_type::to_int_type(readBuf_);


        int_type nextChar = pExternBuf_->sbumpc();
        if (nextChar == traits_type::eof())
            return traits_type::eof();
        readBuf_ = traits_type::to_char_type(nextChar);


        // replace character
        readBuf_ += 1;


        setg(&readBuf_, &readBuf_, &readBuf_ + 1);
        return traits_type::to_int_type(readBuf_);
    }


    int_type overflow(int_type ch) {
        // replace character
        ch = ch - 1;
        return pExternBuf_->sputc(ch);
    }


    int sync() {
        return pExternBuf_->pubsync();
    }


    public:
        FilteringBuf(std::streambuf* pExternBuf)
            : pExternBuf_(pExternBuf)
        {
            reset();
        }


        ~FilteringBuf() {
            sync();
        }


        void reset() {
            setg(0, 0, 0);
            setp(0, 0);
        }
};

I do not know, no, I'm not a code master.
And provide all the code, I'm interested, and I'll compile your entire project, and I'll think and read.