C++11 random numbers and std::bind interact in une

问题:

I am using GCC 4.6.3 and was trying to generate random numbers with the following code:

#include <random>
#include <functional>

int main()
{
    std::mt19937 rng_engine;

    printf("With bind\n");
    for(int i = 0; i < 5; ++i) {
        std::uniform_real_distribution<double> dist(0.0, 1.0);
        auto rng = std::bind(dist, rng_engine);
        printf("%g\n", rng());
    }

    printf("Without bind\n");
    for(int i = 0; i < 5; ++i) {
        std::uniform_real_distribution<double> dist(0.0, 1.0);
        printf("%g\n", dist(rng_engine));
    }

    return 0;
}

I expected both methods to generate a sequence of 5 random numbers. Instead, this is what I actually get:

With bind
0.135477
0.135477
0.135477
0.135477
0.135477
Without bind
0.135477
0.835009
0.968868
0.221034
0.308167

Is this a GCC bug? Or is it some subtle issue having to do with std::bind? If so, can you make any sense of the result?

Thanks.

回答1:

When binding, a copy of rng_engine is made. If you want to pass a reference, this is what you have to do :

auto rng = std::bind(dist, std::ref(rng_engine));

回答2:

The std::uniform_real_distribution::operator() takes a Generator & so you will have to bind using std::ref

#include <random>
#include <functional>

int main()
{
    std::mt19937 rng_engine;

    printf("With bind\n");
    for(int i = 0; i < 5; ++i) {
        std::uniform_real_distribution<double> dist(0.0, 1.0);
        auto rng = std::bind(dist, std::ref(rng_engine));
        printf("%g\n", rng());
    }

    printf("Without bind\n");
    for(int i = 0; i < 5; ++i) {
        std::uniform_real_distribution<double> dist(0.0, 1.0);
        printf("%g\n", dist(rng_engine));
    }
}

回答3:

bind() is for repeated uses.

Putting it outside of the loop...

std::mt19937 rng_engine;
std::uniform_real_distribution<double> dist(0.0, 1.0);
auto rng = std::bind(dist, rng_engine);

for(int i = 0; i < 5; ++i) {
    printf("%g\n", rng());
}

... gives me the expected result:

0.135477
0.835009
0.968868
0.221034
0.308167