C++ Variable declarable in function body, but not class member?

Question

I want to create a C++ class with the following type:

1. It can be declared inside of a function.
2. It can be declared inside of a member function.
3. It can not be declared as a class member.

The use of this: think "Root" objects for a GC.

Is this possible in C++? In particular, I'm using g++. Willing to switch to clang. Either templates or macro solution fine.

Thanks!

Answer 1

You could do it with a macro, perhaps:

#define MY_TYPE \
    do { } while(0); \
    RealType

void foo() {
    MY_TYPE myvar;
    myvar.Whatever();
}

This would only compile inside a function (because of the "do ... while" bit - though you'd get a really weird error message). It seems like one of those "evil" uses of macros that you would want to avoid, however...

Answer 2

Even though I gotta love codeka's answer, I cannot but imagine what the problem is with a declaration as a member attribute.

For something like a GC's root I would probably use the Monoid Pattern. All instances of the class are in fact proxies to a Singleton (in essence), ie they all share the same state. This way it doesn't matter how many are instantiated, they all point to the same resource.

If you do so to avoid cyclic references, I am afraid it's not nearly enough.

struct A { boost::shared_ptr<B> mB; };

struct B { boost::shared_ptr<A> mA; };