How do i forward declare a class that has been typedef'd?

Question

I have a string class that, unsurprisingly, uses a different implementation depending on whether or not UNICODE is enabled.

#ifdef UNICODE
typedef StringUTF16 StringT;
#else
typedef StringUTF8 StringT;
#endif

This works nicely but I currently have a problem where I need to forward declare the StringT typedef. How can I do this?

I can't do typedef StringT; so it makes forward declaration tricky. Is it possible to do a forward declare of this typedef'd type without having to past the code above into the top of the header file?

Answer 1

Follow the example set by the iosfwd standard header. Write a header file that contains this, and call it StringTFwd.h

class StringUTF16;
class StringUTF8;

#ifdef UNICODE
typedef StringUTF16 StringT;
#else
typedef StringUTF8 StringT;
#endif

At least this is reusable and doesn't ugly up the headers that refer to it.

Answer 2

You can't. (Rational: you can use typedef to define an alias for a basic type and those may use different ABI conventions depending on the precise type).

Can't you forward declare both StringUTF16 and StringUTF8 and then use your #idef?