如何用隨機數據成員值創建對象？ C++

Question

下面的代碼生成的所有對象對於randomNumber1和randomNumber2具有相同的值。如何正確使用std :: default_random_engine？

#include <random> 
#include <iostream> 

class Food{ 

    public: 
     int randomNumber1; 
     int randomNumber2; 
     Food(){ 
      std::default_random_engine generator; 
      std::uniform_int_distribution<int> distribution(1,40); 
      std::uniform_int_distribution<int> distribution2(1,30); 
      randomNumber1 = distribution(generator) * 20; 
      randomNumber2 = distribution2(generator) * 20; 
     }; 
}; 

int main(){ 

    Food * food = new Food(); 
    std::cout << food->randomNumber1 << " " << food->randomNumber2 << std::endl; 
    Food * food1 = new Food(); 
    std::cout << food1->randomNumber1 << " " << food1->randomNumber2 << std::endl; 
    Food * food2 = new Food(); 
    std::cout << food2->randomNumber1 << " " << food2->randomNumber2 << std::endl; 

    return 0; 
}; 

Answer 1

那是因爲你重新播種的engine（使用相同的種子），每次創建一個Food對象。你可以做的是讓它static所以才播種一次：

static std::default_random_engine generator; 
std::uniform_int_distribution<int> distribution(1,40); 
std::uniform_int_distribution<int> distribution2(1,30); 
randomNumber1 = distribution(generator) * 20; 
randomNumber2 = distribution2(generator) * 20; 

或..讓來電者決定的種子。

或者，如@Unimportant在評論中說，使用std::random_device：

static std::random_device dev; 
static std::default_random_engine generator(dev());