Why would the implicitly generated constructor (et al.) be more efficient than a user-defined (trivial) one?

Question

I read this article from D. Kalev this morning about the new c++11 feature \"defaulted and deleted functions\", and can\'t understand the part about performance, namely:

Answer 1

You ask, how is it that

class C { C() = default; };

can be more efficient than

class C { C(){} };

Well, both constructors do nothing, so it's meaningless to talk about efficiency for that example.

But more generally, in e.g. a copy constructor one can imagine that copying one POD item at a time will not be recognized as optimizable by simple optimizer, whereas with automatic generation it might just do a memcpy. Who knows. It's a Quality of Implementation issue, and I can easily imagine also the opposite.

So, measure, if it matters.

Cheers & hth.,