Using std::shuffle with custom Random Number Generator?

Question

I see that std::shuffle takes an URNG argument. I am using a custom version of a generator (WELL512a) and would like to use it with std::shuffle.

My question is : Is it possible to use it with std::shuffle? For example maybe by using URNG as a base class?

Answer 1

We can see from the cppreference documentation on std::shuffle that:

URNG must meet the requirements of UniformRandomNumberGenerator.

unfortunately the site does not document what those requirements are, so we need to go to the draft C++11 standard section 26.5.1.3 Uniform random number generator requirements which says the following:

1. A uniform random number generator g of type G is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned

2. A class G satisfies the requirements of a uniform random number generator if the expressions shown in Table 116 are valid and have the indicated semantics, and if G also satisfies all other requirements of this section 26.5.1.3. In that Table and throughout this section:

   • T is the type named by G’s associated result_type, and
   • g is a value of

Table 116 is summarized as follows:

• G::result_type
  • return type is T
  • T is an unsigned integer type
• g()
  • return type T
  • Returns a value in the closed interval [G::min(), G::max()].
• G::min()
  • return type T
  • Denotes the least value potentially returned by operator().
• G::max()
  • return type T
  • Denotes the greatest value potentially returned by operator().

The following relation shall hold: G::min() < G::max().

Update

To clarify when it says T is an unsigned integer type it means one of the following types covered in section 3.9.1 Fundamental types which include:

• unsigned char
• unsigned short int
• unsigned int
• unsigned long int
• unsigned long long int

This also include extended unsigned integer types such as uinit64_t, if your platform supports them.