std::basic_streambuf::underflow — cppreference.com

Original:

Ensures that at least one character is available in the input area by updating the pointers to the input area (if needed). Returns the value of that character on success or traits::eof() on failure.

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Original:

The function may update gptr, egptr and eback pointers to define the location of newly loaded data (if any). On failure, the function ensures that either gptr() == nullptr or gptr() == egptr.

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Original:

The base class version of the function does nothing. The derived classes may override this function to allow updates to the get area in the case of exhaustion.

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Original:

(none)

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Original:

The base class version of the function calls traits::eof().

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Original:

The public functions of std::streambuf call this function only if gptr() == nullptr or gptr() >= egptr().

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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);
}