On Monday 26 October 2015 08:48:55 John Weeks wrote: > > Both are correct, since one is implemented in terms of the other. That > > means they MUST expand to exactly the same assembly (and they do, I've > > just tested). > > > > If you're seeing something different, the problem is probably your code. > > And that's why Marc is recommending qSwap: because people don't know how > > to use std::swap. There's a big gotcha: you MUST NOT write the "std::" > > part of std::swap. > > At the risk of making a fool of myself, what is the difference? I thought I > had learned that namespaces were just a scoping mechanism for names to > avoid name collisions. Why does this make such a difference in the > generated code?
Like Konstantin answered, this is due to Argument Dependent Lookup. You may not have known it happens with non-templates, but it does. A call f(x) where x is of type N::X will first search for N::f(N::X), then walk through a very complex list of associated namespaces. > I'm old enough now that the risk of making a fool of myself is a small price > to pay in order to learn something. Don't worry. ADL is one of the most complex and obscure corners of C++. It's several pages long in the standard, all started from an innocent-looking idea, "wouldn't it be nice if f(x) called N::f(N::X)?". I think it's only less complex than overload resolution, but overloads are better understood (you can't live without them). I mean, overloads without templates... that gets complex again. -- Thiago Macieira - thiago.macieira (AT) intel.com Software Architect - Intel Open Source Technology Center _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
