std::discrete_distribution(3)

NAME
std::discrete_distribution - std::discrete_distribution

Synopsis
   Defined in header <random>
   template< class IntType = int > (since C++11)
   class discrete_distribution;
   std::discrete_distribution produces random integers on the interval [0, n), where
   the probability of each individual integer i is defined as w
   i/S, that is the weight of the ith integer divided by the sum of all n weights.
   std::discrete_distribution satisfies all requirements of RandomNumberDistribution.

Template parameters
             The result type generated by the generator. The effect is undefined if
   IntType - this is not one of short, int, long, long long, unsigned short, unsigned
             int, unsigned long, or unsigned long long.

Member types
   Member type Definition
   result_type (C++11) IntType
   param_type (C++11) the type of the parameter set, see RandomNumberDistribution.

Member functions
   constructor constructs new distribution
   (C++11) (public member function)
   reset resets the internal state of the distribution
   (C++11) (public member function)

Generation
   operator() generates the next random number in the distribution
   (C++11) (public member function)

Characteristics
   probabilities obtains the list of probabilities
                 (public member function)
   param gets or sets the distribution parameter object
   (C++11) (public member function)
   min returns the minimum potentially generated value
   (C++11) (public member function)
   max returns the maximum potentially generated value
   (C++11) (public member function)

Non-member functions
   operator==
   operator!= compares two distribution objects
   (C++11) (function)
   (C++11)(removed in C++20)
   operator<< performs stream input and output on pseudo-random number
   operator>> distribution
   (C++11) (function template)

Example
// Run this code
 #include <iomanip>
 #include <iostream>
 #include <map>
 #include <random>
 int main()
 {
     std::random_device rd;
     std::mt19937 gen(rd());
     std::discrete_distribution<> d({40, 10, 10, 40});
     std::map<int, int> map;
     for (int n = 0; n < 1e4; ++n)
         ++map[d(gen)];
     for (const auto& [num, count] : map)
         std::cout << num << " generated " << std::setw(4) << count << " times\n";
 }

Possible output:
 0 generated 4037 times
 1 generated 962 times
 2 generated 1030 times
 3 generated 3971 times