#include <array>
#include <cassert>
#include <cstddef>
#include <locale>
#include <ostream>
#include <streambuf>
#include <string>
class FixedBuffer : public std::streambuf {
    std::array<char, 3> data_{};
protected:
    int_type overflow(int_type c = traits_type::eof()) override {
        if (traits_type::eq_int_type(c, traits_type::eof()))
            return traits_type::not_eof(c);
        return traits_type::eof();
    }
public:
    FixedBuffer() { setp(data_.data(), data_.data() + data_.size()); }
    FixedBuffer(const FixedBuffer&) = delete;
    FixedBuffer& operator=(const FixedBuffer&) = delete;
    std::string text() const { return std::string(pbase(), pptr()); }
};
int main() {
    FixedBuffer buffer;
    std::ostream out(&buffer);
    out.imbue(std::locale::classic());
    out << 42 << '!';
    assert(out.good());
    assert(buffer.text() == "42!");
    out.put('?');
    assert(out.bad());
    assert(buffer.text() == "42!");
}