C++ ::  How To Pass A String Variable To Message Box

I am having trouble finding the solution to printing the contents of a variable. Here is my code:

void OnSize(HWND hwnd, UINT flag, int width, int height) {
std::wstringstream ss (std::wstringstream::in | std::wstringstream::out);
ss << flag;
std::wstring myStr = ss.str();
MessageBoxW(hwnd,myStr, L"Window Resized",NULL);
}

Note, I have #defined UNICODE at the beginning. The compiler tells me this for myStr.

std::wstring myStr

Error: No suitible conversion function from "std::wstring" to "LPCWSTR" exits