I've seen quite a few recommendations for not seeding pseudo-random number generators more than once per execution, but never accompanied by a thorough explanation. Of course, it is easy to see why the following (C/C++) example is not a good idea:
int get_rand() {
srand(time(NULL));
return rand();
}
since calling get_rand
several times per second produces repeated results.
But wouldn't the following example still be an acceptable solution?
MyRand.h
#ifndef MY_RAND_H
#define MY_RAND_H
class MyRand
{
public:
MyRand();
int get_rand() const;
private:
static unsigned int seed_base;
};
#endif
MyRand.cpp
#include <ctime>
#include <cstdlib>
#include "MyRand.h"
unsigned int MyRand::seed_base = static_cast<unsigned int>(time(NULL));
MyRand::MyRand()
{
srand(seed_base++);
}
int MyRand::get_rand() const
{
return rand();
}
main.cpp
#include <iostream>
#include "MyRand.h"
int main(int argc, char *argv[])
{
for (int i = 0; i < 100; i++)
{
MyRand r;
std::cout << r.get_rand() << " ";
}
}
i.e. even though MyRand
:s constructor is called several times in rapid succession, each call to srand
has a different parameter. Obviously, this is not thread-safe, but then again neither is rand
.