Friday, January 27, 2012

smart pointer casts - using shared_ptr for illustration

Smart pointers are the infrastructure of infrastructures for a quant library.

Minor performance issues become performance critical in this foundation component. One example is the allocation of the reference count. Another example is the concurrent update of the reference count. But here my focus is yet another performance issue -- the lowly pointer-cast. (I don't remember why performance is a problem ... but I believe it is.)

C++ support 4 specific casts but smart pointers need to -efficiently- implement const_cast and dynamic_cast, in addition to the implicit upcast.

Q1: why can't I assign a smartPtr-of-int instance to a variable declared as smartPtr of Dog?
A: compiler remembers the type of each pointer variable.

Q2: in that case, should c++ compiler allow me to assign a SP-of-Derived instance to a variable declared as SP of Base?
%%A: I don't think so. I don't think every smart pointer automatically supports upcast, but boost shared_ptr does, by design. See below.

For Boost, a Barcap expert said there's an implicit cast from shared_ptr of T (RHS) to (LHS) shared_ptr of const T. See the special casting methods in http://www.boost.org/doc/libs/1_48_0/libs/smart_ptr/shared_ptr.htm

Specifically, boost says shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*. This mimics the behavior of raw pointers. In particular, shared_ptr<T> is implicitly convertible to shared_ptr<T const> (i.e. add constness), to shared_ptr<U> where U is an public/protected base of T (i.e. upcast), and to shared_ptr<void>.I believe this is the same "member template" magic in the smart pointer chapter of [[more effC++]]

Allow me to explain the basics -- in basic C++, a raw-ptr-to-T instance can be assigned to these variables --
T const *  my_ptr_to_const_T;
U * my_ptr_to_a_T_parent_class_U;
void * my_ptr_to_void;

Therefore a raw ptr supports these 3 (among others) casts implicitly. So does shared_ptr.

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