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The snippet of code you provided implements the is_base_of trait in C , which checks if one class inherits from another. It uses a technique called "Curiously Recurring Template Pattern" (CRTP).
Host is a template class that wraps the derived class and provides a way to access its operator B* function. When we want to check if B is the base class of D, we call is_base_of::value.
The check function overloads two functions:
The operator B* must be const because otherwise, it would be ambiguous when selecting which check function to call. With the const keyword, the compiler knows to use the check function that takes B* const&.
The first check function is better because it uses a user-defined conversion sequence that converts from D* to B*. This is more specific than the second check function, which uses a user-defined conversion sequence that converts from B* to int.
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