Question

First is the code

#include <stdio.h>

typedef wchar_t* BSTR;

wchar_t hello[] = L"Hello";

class _bstr_t {
public:
    operator const wchar_t*() const throw() { return hello; }
    operator wchar_t*() const throw() { return hello; }
};

class container {
public:
    operator _bstr_t() { return _bstr_t(); }
};

int main()
{
    // This gives error (with gcc 4.5.2 at least):
    // test.cpp:20:27: error: cannot convert "container" to "wchar_t*" in initialization
    wchar_t *str = container();
    printf("%S\n", str);
    return 0;
}

The problem here is that container() can be casted to _bstr_t and then to wchar_t*, but, gcc does not.

The problem can be solved using manual cast:

wchar_t *str = (_bstr_t)container();

But what I need is to avoid manual cast, I would like gcc to figure this out automatically.

Why I need this is because the returned container type objects will be used in calls like

void Func(wchar_t* str);
Func(myObject->Container);

where I don’t want to do manual casting.

I verified Visual Studio and it does not seem to support such scenario, too. Too bad, but I would be glad if someone can provide a workaround, even if for this specific case.

UPDATE: for those who suggests operator wchar_t* on the container, that was the problem in first place. This will either leak or crash when destroyed before Func() has a chance to accept the pointer.

Answer 1

When doing implicit conversions, there’s at most one user-defined conversion that can happen. MSVC behaviour is not standard compliant in this matter.

C++11 (12.3 Conversions):

At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.

For the implicit conversion to work, container has to convert directly to wchar_t*.