How can CString be passed to format string %s?

Question

class MyString
{
public:
    MyString(const std::wstring& s2)
    {
        s = s2;
    }

    operator LPCWSTR() const
    {
        return s.c_str();
    }
pri         


        

Answer 1

    wprintf(L"%s\n", (LPCWSTR)cstring); // Okay. It's been cast to a const wchar_t*.
    wprintf(L"%s\n", cstring); // UNDEFINED BEHAVIOUR
    wprintf(L"%s\n", (LPCWSTR)s); // Okay, it's a const wchar_t*.
    wprintf(L"%s\n", s); // UNDEFINED BEHAVIOUR

The only thing you can pass to this function for %s is a const wchar_t*. Anything else is undefined behaviour. Passing the CString just happens to work.

There's a reason that iostream was developed in C++, and it's because these variable-argument functions are horrifically unsafe, and shoud never be used. Oh, and CString is pretty much a sin too for plenty of reasons, stick to std::wstring and cout/wcout wherever you can.