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;
};
};
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.