The proper way to use wcsrtombs to convert a const wchar_t* to an std::string (so far as I can tell)

WCharPtrToStdString.cpp

#include <cstring>
#include <cwchar>

inline void ResetMbState(std::mbstate_t &state) {
    // this is really the recommended way to zero-initialize an mbstate_t
    std::memset(&state, 0, sizeof(std::mbstate_t));
}

inline std::string WCharPtrToString(const wchar_t *data) {
    std::mbstate_t state;
    ResetMbState(state);
    const wchar_t *src = data;
    size_t length = std::wcsrtombs(nullptr, &src, 0, &state);
    // replace by `if` if you want to actually do something on error
    ASSERT(length != static_cast<size_t>(-1));
    std::string result(length + 1, '\0);
    ResetMbState(state);
    src = data;
    std::wcsrtombs(&result[0], &src, length + 1, &state);
    return result;
}