C++11 std::generate and std::uniform_real_distribution called two times gives strange results

calling std::generate algorithm from the STL two times on different containers produces equivalent results.

Consider I want to fill up two float arrays with random numbers between -1. and 1. :

std::array<float, 1000> x;
std::array<float, 1000> y;

std::random_device rd;
std::mt19937_64 gen(rd());
std::uniform_real_distribution<float> dis(-1.f, 1.f);
auto rand = std::bind(dis, gen);
std::generate(x.begin(), x.end(), rand);
std::generate(y.begin(), y.end(), rand);

You can test it here : http://ideone.com/X712IU. Both arrays are filled up with the exact same values :

0:  -0.411968,  -0.411968
1:    0.55158,    0.55158
2:    0.69889,    0.69889
3:  -0.901328,  -0.901328
4:  -0.556142,  -0.556142
5:  -0.798431,  -0.798431
6:  -0.570874,  -0.570874
7:   0.928999,   0.928999
8:   0.118056,   0.118056
9:  -0.655123,  -0.655123

Now if I make a new generator between the generates it works ok :

std::array<float, 1000> x;
std::array<float, 1000> y;

// Generators in different scopes, OK
std::random_device rd;
{
  std::mt19937_64 gen(rd());
  std::uniform_real_distribution<float> dis(-1.f, 1.f);
  auto rand = std::bind(dis, gen);
  std::generate(x.begin(), x.end(), rand);
}
{
  std::mt19937_64 gen(rd());
  std::uniform_real_distribution<float> dis(-1.f, 1.f);
  auto rand = std::bind(dis, gen);
  std::generate(y.begin(), y.end(), rand);
}

Gives :

0:   0.391496,   -0.64993
1:   0.429592,   0.835015
2: 0.00735116,    0.77657
3:  -0.548355, -0.0794801
4:  -0.312095,  -0.119841
5:   0.931296,   0.997449
6:  -0.934924,  -0.832223
7:   0.432267,   0.181224
8:   0.942709,   0.165024
9:   0.315852,  -0.654576

And now with the same generator using for loops, it works as well :

// Both arrays assigned in the same loop, OK
for(size_t i = 0; i < x.size(); ++i)
{
  x[i] = rand();
  y[i] = rand();
}

// Arrays in separated loops, OK
for(size_t i = 0; i < x.size(); ++i)
  x[i] = rand();

for(size_t i = 0; i < y.size(); ++i)
  y[i] = rand();

It looks like something related to std::generate, but I can't find out what would cause this behavior.

Any explanation ? Cheers

EDIT:

As pointed out by dotcavader, the issue comes simply because the generator object has been copied (both by std::bind and std::generate). Hence the generator start with the exact same internal state for both generate.

Just to complete the answer, Praetorian and Casey gave two easy solutions for this :

Using std::reference_wrapper to store a reference in the bind instead of a copy :

auto rand = std::bind(dis, std::ref(gen));

Using a lambda that returns a reference :

auto rand = [&](){ return dis(gen); };