C++: Can virtual inheritance be detected at compile time?

Question

I would like to determine at compile time if a pointer to Derived can be cast from a pointer to Base without dynamic_cast<>. Is this possible using templates and metaprogramm

Answer 1

First, your code is doing sizeof of a pointer instead of a dereferenced pointer so it wouldn't work even if gcc wasn't complaining.

Second, sizeof trick have to work with casts of 0 and not actual pointers or object - that guarantees zero overhead and also that it won't complie till you do it right.

3rd, you need to declare 2 templated classes or structs one deriving just from D, the other deriving from D and virtual B, and then cast 0 to their pointers, dereference them and then sizeof.

4th - Do you have any big reason for trying to be politically correct with static_cast instead of straight cast here? Compiler will always infer from that that you are looking for more whining and in this case you are definitelly not.

BTW you don't need to grab full code from Alexandrescu - just grab the core technique which basically just:

sizeof(*((T*)0))

Alexandrescu is really good at cleaning up after a trick.

Oh and remember that compiler is not supposed to evaluate sizeof args or instantiate unused templated classes and structs - so if it does then it's a compiler bug and if you force it to do that then it's your bug :-)

Once you have that, you need to define precisely and in positive terms wha your statement "if a pointer to Derived can be cast from a pointer to Base without dynamic_cast<>" actually means in terms of class relationships - just saying "without operator/function Q" doesn't makea problem well defined and you can't solve what you can't define - honest :-)

So just take the first clean step that compiles and then try to define in which way would the two cases you mentioned be different in reality - what would one have or do that the other one wouldn't.