How to write test doubles for Windows API calls that pass arguments by address

I have a number of working mock classes that replace actual calls to the Windows API. However, I am struggling to put together something for a function that utilises FormatMessageW. In fact the problem is more to do with how a wchar_t buffer is passed into the function.

When using the FormatMessageW function I am first declaring my buffer as follows:

wchar_t * buffer = nullptr;

I then pass the buffer by address as the lpBuffer argument (expected type is LPWSTR):

reinterpret_cast<::LPWSTR>(&buffer)

The Windows API function will automatically create a buffer of the correct size.

I go on further with the buffer by stripping out line breaks, converting from wide characters to multibyte characters etc.

In order to fully unit test the clean up of the output buffer I am attempting to mock the FormatMessageW call by having the function simply return a predefined string (which will be a member of the mock object).

To simplify the problem, the following code attempts to replicate my problem:

// represents my mock class
class mocker
{

public:
    // takes a wchar_t pointer and attempts to reassign it
    int mockFunction(wchar_t * buffer)
    {
        // assigns local copy of wchar_t pointer...
        buffer = &message[0];

        return message.length();
    }

protected:

    std::wstring message = L"test";
};

// test code
mocker mocking;
wchar_t * buffer = nullptr;
auto size = mocking.mockFunction(reinterpret_cast<wchar_t *>(&buffer));

// at this point buffer is still null
// but I want the buffer to point to L"test"

Is there a way to achieve my aim of redirecting the pointer to an existing std::wstring without changing anything but the implementation of int mockFunction(wchar_t * buffer)?