Generate random numbers in C++ at compile time

Question

I'm trying to precompute random values using C++11's random library at compile time. I'm mostly following examples. What am I doing wrong here?

using namespace std; #include <iostream> #include <vector> #include <random>  vector<double> rands; typedef std::mt19937_64 RNG; uint64_t seed_val; RNG rng;   void initialize() {      rng.seed(seed_val); }  constexpr vector<double> generate_random( )                 //size_t numbers) {        int numbers = 1000;     std::uniform_real_distribution<double> zero_one(0.0, 1.0);         for (unsigned int i = 0; i < numbers; i++) {               double rand_num = zero_one(rng);              rands.push_back( rand_num );     }     return rands; }  int main() {     cout << "TMP rands";     for_each( rands.begin(), rands.end(), [] (double value)     {         cout<<value<<endl;     }); } 

Here's an example compile-time random number generator shamelessly stolen from here, but thought it might be useful for anyone who looks this up:

template<u32 S, u32 A = 16807UL, u32 C = 0UL, u32 M = (1UL<<31)-1> struct LinearGenerator {     static const u32 state = ((u64)S * A + C) % M;     static const u32 value = state;     typedef LinearGenerator<state> next;     struct Split { // Leapfrog         typedef LinearGenerator< state, A*A, 0, M> Gen1;         typedef LinearGenerator<next::state, A*A, 0, M> Gen2;     }; }; 

Answer 1

Only constexpr functions and constant expressions may be evaluated at compile time. That rules out <chrono> and <random>.

What you can do is access the __TIME__ preprocessor macro and define your own PRNG composed of one-line, constexpr functions.

Answer 2

There is a research paper on the topic: Random number generator for C++ template metaprograms containing code snippet for the __TIME__ trick. It also talks about supporting different random number engines and distributions as orthogonal choices.