I am curious why std::lower_bound() asks for passing by value for the compare function rather than by reference?
Passing by value means a copy is needed, slowing things down; especially, if someone passes in a "big" compare function object.
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- It's often a good candidate for a move. Lambdas, temporary functor objects, function pointers.Qaz– Qaz2013-06-20 02:14:21 +00:00Commented Jun 20, 2013 at 2:14
- Right, probably calling move constructor in the copy constructor? But I think move exists in C11 recently but this STL algo exists for a long time. Temporary function object can probably be identified and then optimized by the compiler, but seems like a constructor call per invocation will still happen. Passing a function pointer is still not as optimal as passing a function object by reference because it can't be possibly inlined. That's why I raised this question :(Hei– Hei2013-06-20 02:36:52 +00:00Commented Jun 20, 2013 at 2:36
- The move constructor is separate from the copy constructor. I'm not sure about functors, though they could have figured they would probably have zero data members in C++03 or something. I really don't see passing a function pointer as a problem.Qaz– Qaz2013-06-20 02:40:31 +00:00Commented Jun 20, 2013 at 2:40
- A function pointer requires deference and so the compiler can't do static binding (of course, you can argue that a good compiler might be able to do so but I doubt it is required by the standard).Hei– Hei2013-06-20 02:49:19 +00:00Commented Jun 20, 2013 at 2:49
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Many if not most comparison objects are stateless and take almost no size (they can't have zero size though). Passing a reference could actually be more expensive than passing a stateless predicate by value (especially if the compiler is able to totally elide the copy).
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Hei
This is actually an interesting answer. I will try to benchmark a bit. Thanks!