std::basic_streambuf::setg — cppreference.com

Original:

Sets the values of the pointers defining the get area. Specifically, after the call eback() == gbeg, gptr() == gcurr, egptr() == gend

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Original:

(none)

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#include <iostream>
#include <sstream>

class null_filter_buf : public std::streambuf {
    std::streambuf* src;
    char ch; // single-byte buffer
protected:
    int underflow() {
        while( (ch= src->sbumpc()) == '\0') ; // skip zeroes
        setg(&ch, &ch, &ch+1); // make one read position available
        return ch; // may return EOF
    }
public:
    null_filter_buf(std::streambuf* buf) : src(buf) {
        setg(&ch, &ch+1, &ch+1); // buffer is initially full
    }
};

void filtered_read(std::istream& in)
{
    std::streambuf* orig = in.rdbuf();
    null_filter_buf buf(orig);
    in.rdbuf(&buf);
    for(char c; in.get(c); )
            std::cout << c;
    in.rdbuf(orig);
}

int main()
{
    char a[] = "This i\0s \0an e\0\0\0xample";
    std::istringstream in(std::string(std::begin(a), std::end(a)));
    filtered_read(in);
}