Thanks. Well have to wait until dev is passing again. Meanwhile, there are some style issues with this PR. For example if urbg is an input it should be const.

Technically, the random number generator URBG should be passed as a rvalue reference URBG &&, similar to the signature of std::shuffle.

@alecjacobson Thanks for reviewing. Yes. I guess I'll have to wait until the issue with CGAL is resolved. About the style, I forgot to make urbg const reference and put it before output parameters. I'll try to change this.

@jdumas True... If std::shuffle takes rvalue reference, may be a good idea to follow it. However, I have a feelling that const URBG urbg can be a valid option, too. If a user wants to pass a URBG using move semantics, he or she can just call randperm(n, std::move(urbg), I)

EDIT : I guess it depends on what kind of interaction we want to get. If we want the urbg to produce different values after the call to randperm, URBG &&urbg will be a good choice. Otherwise, I think const URBG urbg is a way to go.

EDIT 2: standard rng engines does not have rvalue constructors (i.e. std::linear_congruential_engine). So std::move() will be equivalent to copy construction.
(http://www.cplusplus.com/reference/random/linear_congruential_engine/linear_congruential_engine/)

I guess URBG &&urbg is better because it is intended to be used that way, as shown in the example of std::shuffle signature. I'm more inclined towards this approach.

Yes, let's not try to be smart and follow the signature of the stdlib function :D