[Boost-users] Complete vs incomplete (fwd decl'd) types

Subject: [Boost-users] Complete vs incomplete (fwd decl'd) types
From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2010-02-12 11:41:22

In client code including the header for MyDerived, I'm using a
MyPtr<MyDerived>, but the static assert below was failing. It took me
a little while to figure out that in myderived.h, the
MyDerived::nested_type typedef is using a forward-declared
MyNestedDerived type declared in my_nested_derived.h, that the client
code was not including, which was the reason is_base_of was failing
with the error below (Boost 1.41, VC9). Adding a #include
<my_nested_derived.h> fixed it.

boost\type_traits\is_base_and_derived.hpp(228) : error C2139:
'MyNestedDerived' : an undefined class is not allowed as an argument
to compiler intrinsic type trait '__is_base_of'

But is there a way to get a more obvious error message stating that
MyNestedDerived is an incomplete type or something? In retrospect the
message is not that bad, but I'm hoping there's a better way.

Thanks, --DD

PS: Of course there's a reason the nested type is in a separate
header, and it's an idiom repeated often in our code base.

template <class E> class MyPtr {
public: ...
    typedef typename E::nested_type nested_type;

    explicit MyPtr(const shared_ptr<nested_type>& n) : ...  {
        BOOST_STATIC_ASSERT((is_base_of<MyNestedBase, nested_type>::value));
    }
...
};