Saturday, December 31, 2011

clever use of enum(char?) in demanding c++ app

class Der: public Base {.....}

Requirement -- we need to down cast a Base pointer. dynamic_cast has overhead.

Der* returnVal = static_cast< Der* > (new Base) // will compile but is potentially disastrous, because the returnVal can be a partially wild pointer.

Q: without incurring the overhead of vtbl lookup, how do I decide if a pointer to Base given to me actually points to a Base or Der instance?

%%A: type_info still uses vtbl.
%%A: use a Base field to indicate the type. Both ctor will set the field, so the Der ctor will overwrite.
A: enum. If sizeof(aBase) is a tiny 2 bytes, then adding an enum field adds just 1 byte. Adding vptr would add 4 bytes. Big difference if we instantiate billions of this class. I guess enum can be configured to take 1 byte, but you can also use a char, which is always 1 byte.

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