Return pointer to array virtual template function - c++

Question

I would like to return an array to a pointer, in a virtual function that is a member of a derived class of a template class. In details, my classes definition is:
Sampler.h
#ifndef SAMPLER_H
#define SAMPLER_H
template <class T>
class Sampler
{
public:
virtual T getnumber()=0;
virtual T* simulation(int n)=0;
};
class UniformSampler:public Sampler<double>
{
public:
virtual double getnumber();
virtual double* simulation(int n);
UniformSampler(double a=0.0, double b=1.0);
private:
double low_bound;
double up_bound;
};
#endif
The class Sampler is a template class in order to be able to derive an other sampler with vectors later. The implementation is:
Sampler.cpp
#include "Sampler.h"
#include<iostream>
#include<cstdlib>
#include<cmath>
using namespace std;
//Uniform
UniformSampler::UniformSampler(double a, double b)
{
low_bound=a;
up_bound=b;
}
double UniformSampler::getnumber()
{
int myrand=rand();
while((myrand==0)||(myrand==RAND_MAX)){myrand = rand(); } //We want a number in (0, RAND_MAX).
double myuni = myrand/static_cast<double>(RAND_MAX); //Create a number in (0,1).
return low_bound + myuni*(up_bound-low_bound);
}
double* UniformSampler::simulation(int n){
double simulations[n];
for(int i=0; i<n; i++){
simulations[i] = this->getnumber();
}
return simulations;
}
My problem is that, when I try to call this program in the main(), it looks like the assignment of the pointer doesn't work. Here is my main.cpp:
#include <iostream>
#include <math.h>
#include <cstdlib>
#include <time.h>
using namespace std;
#include "Sampler.h"
int main(){
srand(time(0));
int n=10;
double *unif = new double[n];
UniformSampler uni;
unif = uni.simulation(n);
for ( int i = 0; i < n; i++ ) {
cout << "*(p + " << i << ") : ";
cout << *(unif + i) << endl;
}
delete[] unif;
return 0;
}
When I run it, it doesn't print any of the elements that unif points to. I don't understand what is wrong there.

Answer 1

UniformSampler::simulation is twice wrong:
double simulations[n]; uses VLA extension, so not C++ standard compliant.
you return pointer on local variable, so dangling pointer.
Solution: use std::vector instead.
#include <vector>
template <class T>
class Sampler
{
public:
virtual ~Sampler() = default;
virtual T getnumber() = 0;
virtual std::vector<T> simulation(int n) = 0;
};
class UniformSampler:public Sampler<double>
{
public:
explicit UniformSampler(double a=0.0, double b=1.0);
double getnumber() overrid;
std::vector<double> simulation(int n) override
{
std::vector<double> res(n);
for (auto& val : res){
res = getnumber();
}
return res;
}
private:
double low_bound;
double up_bound;
};
int main(){
srand(time(0));
constexpr int n = 10;
UniformSampler uni;
auto unif = uni.simulation(n);
for (int i = 0; i < n; i++ ) {
std::cout << "p[" << i << "]: " << unif[i] << endl;
}
}