usingとoperatorについて

下記構文に、

using pointer = typename Traits::pointer;

pointer m_value;

というのがありますが、これはどう違うのですか？
また、
Traits::invalid()

というのが何か所かに出てきますが、
これは何をしていますか？

#ifdef _WIN32
#include <windows.h>
#else
typedef void* HANDLE;

#define DWORD     unsigned long
#ifdef _LP64
#define LONG_PTR  long long
#define ULONG_PTR unsigned long long
#else
#define LONG_PTR  long
#define ULONG_PTR unsigned long
#endif
#define INVALID_HANDLE_VALUE ((HANDLE)(LONG_PTR)-1)

struct SECURITY_ATTRIBUTES
{
    DWORD nLength;
    void* lpSecurityDescriptor;
    int bInheritHandle;
};

struct OVERLAPPED
{
    ULONG_PTR Internal;
    ULONG_PTR InternalHigh;
    union {
        struct {
            DWORD Offset;
            DWORD OffsetHigh;
        } DUMMYSTRUCTNAME;

        void* Pointer;
    } DUMMYUNIONNAME;

    HANDLE hEvent;
};

int CloseHandle(HANDLE hObject) { return 0; }
HANDLE CreateFile(char const*,
    DWORD,
    DWORD,
    SECURITY_ATTRIBUTES*,
    DWORD,
    DWORD,
    HANDLE)
{
    return INVALID_HANDLE_VALUE;
}

int ReadFile(HANDLE,
    void*,
    DWORD,
    DWORD*,
    OVERLAPPED*)
{
    return 0;
}

#define GENERIC_READ           0x80000000L
#define GENERIC_WRITE          0x40000000L
#define CREATE_NEW             1
#define CREATE_ALWAYS          2
#define OPEN_EXISTING          3
#define OPEN_ALWAYS            4
#define TRUNCATE_EXISTING      5
#define FILE_SHARE_READ        1
#define FILE_ATTRIBUTE_NORMAL  0x00000080
#endif

#ifdef _UNICODE
#define _T(x)   L ## x
#else
#define _T(x)
#endif

#include <stdio.h>
#include <algorithm>
#include <vector>
#include <stdexcept>


template <typename Traits>
class unique_handle
{
    using pointer = typename Traits::pointer;

    pointer m_value;

public:
    unique_handle(unique_handle const&) = delete;
    unique_handle& operator=(unique_handle const&) = delete;

    explicit unique_handle(pointer value = Traits::invalid()) noexcept
        :m_value{ value }
    {}

    unique_handle(unique_handle&& other) noexcept
        : m_value{ other.release() }
    {}

    unique_handle& operator=(unique_handle&& other) noexcept
    {
        if (this != &other)
        {
            reset(other.release());
        }

        return *this;
    }

    ~unique_handle() noexcept
    {
        Traits::close(m_value);
    }

    explicit operator bool() const noexcept
    {
        return m_value != Traits::invalid();
    }

    pointer get() const noexcept
    {
        return m_value;
    }

    pointer release() noexcept
    {
        auto value = m_value;
        m_value = Traits::invalid();
        return value;
    }

    bool reset(pointer value = Traits::invalid()) noexcept
    {
        if (m_value != value)
        {
            Traits::close(m_value);
            m_value = value;
        }

        return static_cast<bool>(*this);
    }

    void swap(unique_handle<Traits>& other) noexcept
    {
        std::swap(m_value, other.m_value);
    }
};

template <typename Traits>
void swap(unique_handle<Traits>& left,
    unique_handle<Traits>& right) noexcept
{
    left.swap(right);
}

template <typename Traits>
bool operator==(
    unique_handle<Traits> const& left,
    unique_handle<Traits> const& right) noexcept
{
    return left.get() == right.get();
}

template <typename Traits>
bool operator!=(
    unique_handle<Traits> const& left,
    unique_handle<Traits> const& right) noexcept
{
    return left.get() != right.get();
}

struct null_handle_traits
{
    using pointer = HANDLE;

    static pointer invalid() noexcept
    {
        return nullptr;
    }

    static void close(pointer value) noexcept
    {
        CloseHandle(value);
    }
};

struct invalid_handle_traits
{
    using pointer = HANDLE;

    static pointer invalid() noexcept
    {
        return INVALID_HANDLE_VALUE;
    }

    static void close(pointer value) noexcept
    {
        CloseHandle(value);
    }
};

using null_handle = unique_handle<null_handle_traits>;
using invalid_handle = unique_handle<invalid_handle_traits>;

void function_that_throws()
{
    throw std::runtime_error("an error has occurred");
}

void bad_handle_example()
{
    bool condition1 = false;
    bool condition2 = true;
    HANDLE handle = CreateFile(_T("sample.txt"),
        GENERIC_READ,
        FILE_SHARE_READ,
        nullptr,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL,
        nullptr);

    if (handle == INVALID_HANDLE_VALUE)
        return;

    if (condition1)
    {
        CloseHandle(handle);
        return;
    }

    std::vector<char> buffer(1024);
    unsigned long bytesRead = 0;
    ReadFile(handle, buffer.data(), buffer.size(), &bytesRead, nullptr);

    if (condition2)
    {
        // oops, forgot to close handle
        return;
    }

    // throws exception; the next line will not execute
    function_that_throws();

    CloseHandle(handle);
}

void good_handle_example()
{
    bool condition1 = false;
    bool condition2 = true;

    invalid_handle handle{
        CreateFile(_T("sample.txt"),
                   GENERIC_READ,
                   FILE_SHARE_READ,
                   nullptr,
                   OPEN_EXISTING,
                   FILE_ATTRIBUTE_NORMAL,
                   nullptr) };

    if (!handle) return;

    if (condition1) return;

    std::vector<char> buffer(1024);
    unsigned long bytesRead = 0;
    ReadFile(handle.get(),
        buffer.data(),
        buffer.size(),
        &bytesRead,
        nullptr);

    if (condition2) return;

    function_that_throws();
}

int main()
{
    try
    {
        bad_handle_example();
    }
    catch (...) {}

    try
    {
        good_handle_example();
    }
    catch (...) {}

    return 0;
}