Use and utility of std::shuffle?

Question

If you look to the specifications of random shuffle in C++11, there are 3 functions. My question is what is the typical use and advantage of :

template< class RandomIt, class URNG >
void shuffle( RandomIt first, RandomIt last, URNG&& g );

compared to:

template< class RandomIt >
void random_shuffle( RandomIt first, RandomIt last );

I mean, it seems that whatever URNG is (a uniform distribution), the result will be the same (from a statistical point of view). The only point I see, is that std::shuffle is thead-safe, whereas this overload ofstd::random_shuffle is not. Could you confirm that ?

EDIT: I thought that URNG should be a uniform distribution but that does not seem to compile. So can someone provide a little example of use of std::shuffle?

Answer 1

As mentioned in the comments, std::shuffle takes a random number generator (or engine in standard speak), not a random number distribution. Different random number generators have different characteristics even if they have a theoretically uniform distribution.

• Random or pseudo-random - True random number generators use some sort of an external entropy source. Pseudo-random generators (PRNGs) are strictly deterministic.
• Performance - some generators are faster than others.
• Memory usage - some PRNGs need more memory to store their state than others.
• Period length - all PRNGs have a finite period after which they start repeating the same sequence from the beginning. Some have much longer periods than others.
• Randomness quality - there are numerous tests for measuring whether there are subtle (or not-so-subtle!) patterns in a pseurorandom stream. See, for example, the Diehard tests.
• Whether the stream is cryptographically secure or not. AFAIK, none of the standard PRNGs are.

For an overview of the different generators offered by the standard, please refer to http://en.cppreference.com/w/cpp/numeric/random.