Minimalist std::streambuf subclass for custom reading

streambuf.cpp

#include <sstream>
#include <fstream>
#include <iostream>
#include <array>

class Buffer : public std::streambuf {
    // we use ifstream here only for the sake of example
    std::ifstream file;
    std::array<char, 1024> data;
    
    bool reload() {
        unsigned count = file.readsome(&data[0], data.size());
        if (not count) {
            file.read(&data[0], data.size());
            count = file.gcount();
        }
        setg(&data[0], &data[0], &data[count]);
        return count;
    }
    
    std::streamsize xsgetn(char* s, std::streamsize n) {
        for (std::streamsize i = 0; i != n; ++i) {
            if (gptr() == egptr() and not reload()) {
                return i;
            }
            *s++ = *_M_in_cur++;
        }
        return n;
    }
    
    int_type underflow() {
        return (gptr() == egptr() and not reload()) ? traits_type::eof() : *_M_in_cur;
    }
    
    int_type uflow() {
        return (gptr() == egptr() and not reload()) ? traits_type::eof() : *_M_in_cur++;
    }
    
public:
    Buffer(const std::string &filename): file(filename) {
    }
};

int main() {
    Buffer buf("streambuf.cpp");
    std::istream stream(&buf);
    std::string tmp;
    while (buf.sgetc() != Buffer::traits_type::eof()) {
        stream >> tmp;
    }
    std::cout << tmp << std::endl;
    return 0;
}