string_view_with_null_test.cpp

#include <string>
#include <unistd.h>
#include <sys/fcntl.h>
#include <cassert>

template <typename CharT>
class basic_string_view_with_null
{
    static constexpr bool holds_null_termination_info = sizeof(size_t) >= 4;

public:
    basic_string_view_with_null(const std::basic_string<CharT>& string)
            : basic_string_view_with_null(string.data(), string.length(), true)
    {}

    basic_string_view_with_null(const basic_string_view_with_null&) = default;

    basic_string_view_with_null(const CharT* ptr, size_t len, bool is_null_terminated = false)
            : m_ptr(ptr)
            , m_len(len)
    {
        if constexpr (holds_null_termination_info)
        {
            size_t mask = size_t(is_null_terminated) << (sizeof(size_t) * 8 - 1);
            m_len |= mask;
        }
    }

    bool null_terminated()
    {
        if constexpr (holds_null_termination_info)
            return (m_len & (size_t(1) << (sizeof(size_t) * 8 - 1))) != 0;

        return false;
    }

    size_t size() const
    {
        size_t size = m_len;
        if constexpr (holds_null_termination_info)
        {
            size_t mask = ~(size_t(1) << (sizeof(size_t) * 8 - 1));
            size &= mask;
        }
        return size;
    }

    const char* data() const
    {
        return m_ptr;
    }

    explicit operator std::basic_string<CharT>()
    {
        return std::basic_string<CharT>(m_ptr, size());
    }

private:
    const char* m_ptr = nullptr;
    size_t m_len = 0;
};

using string_view_with_null = basic_string_view_with_null<char>;

int foo(string_view_with_null path)
{
    if (path.null_terminated())
        return open(path.data(), O_RDONLY);
    else
        return open(std::string(path).c_str(), O_RDONLY);
}


int main(int argc, char** argv)
{
    std::string path("/etc/passwd");
    string_view_with_null path_view(path);
    assert(path.size() == path_view.size());

    int a = foo(path_view);
    assert(a > 0);
    std::string copied(path_view);
    assert(copied == path);

    return 0;
}