问题
Using enums for storing bitflags in C++ is a bit troublesome, since once the enum values are ORed they loose their enum-type, which causes errors without explicit casting.
The accepted answer for this question suggests overloading the | operator:
FlagsSet operator|(FlagsSet a, FlagsSet b)
{
return FlagsSet(int(a) | int(b));
}
I'd like to know if this method has any runtime implications?
回答1:
Runtime implications in terms of correctness? No - this should be exactly what you want.
Runtime implications in terms of speed? I would expect any decent compiler to optimize this away properly to the minimal number of instructions for a release build (although you might want to add inline just to be sure).
回答2:
It potentially does three copies and a function call, barring RVO, registers, and/or inlining optimizations.
Naked bitwise OR operations themselves usually decompose to a single processor instruction.
回答3:
The code is correct.
The code will be the same speed as without type casts.
But whether the code is fast is irrelevant, because a type named 'FlagSet' will most probably be used in a context of conditionals test (-> "if (Flag)"), which is more of a hit to speed than a bit wise 'or' of two values of the size of a register.
回答4:
use std::bitset for your bit flags... much simpler ;) or boost::dynamic_bitset.
来源：https://stackoverflow.com/questions/1693579/bitflag-enums-in-c