Is it modern C++ to use srand to set random seed?

Question

For code that uses std::random_shuffle, I need to set a random seed so that the pseudorandom sequences produced vary in each program run.

The code example here makes a call to

srand ( unsigned ( time (NULL) ) );

which needs to

#include <ctime>
#include <cstdlib>

I wonder: Since C++11 includes major updates to pseudorandom number generation, is this still up to date? What should I use to set the random seed for std::random_shuffle?

Answer 1

random_shuffle uses an implementation-defined random number generator unless you provide one. So, no, using srand is not necessarily correct.

Otherwise it uses the generator you provide. You can use rand if you want to be sure that is what gets used.

srand(seed);
std::random_shuffle(first, last, [](int n) { return rand() % n; });
// this is a biased generator
// see <http://eternallyconfuzzled.com/arts/jsw_art_rand.aspx>

However, I recommend using the new <random> facilities instead of rand(). Example follows.

std::default_random_engine gen(seed);

std::shuffle(first, last, gen);

Answer 2

If you are using C++11, think about using std::shuffle instead of std::random_shuffle, and passing a random-number generator, as in the last example here