Use const int by reference to create static array

Question

I am trying to cretate an array of doubles in pi_sequence and then return a pointer to it. Then I want to print out its values. I tried this but I get the error storage size of ests is not constant:

#include <iostream>
#include <stdlib.h>


double* pi_sequence(const int& len)
{
  static double ests[len];
  ests[0] = 1.11;
  ests[1] = 2.22;
  return ests; // address of?
}

int main() {

  double* ests = pi_sequence();
  std::cout << "will write to file: " << ests[0]  << std::endl;

}

That's because the size of a static array needs to be a known compile time constant. But your implementation seems to assume len is always 2 anyway, since you assign to ests[0] and ests[1] so I have a hard time understanding what you actually want to do. — François Andrieux
– François Andrieux, Commented Jan 16, 2019 at 20:21
@FrançoisAndrieux its a very simplified case. My case is bigger. Is it possible to create static array without compile time constant? Or is it possible to return array another way from that fucntion? — Noitidart
– Noitidart, Commented Jan 16, 2019 at 20:22
You will necessarily need to know at compile time the size of the array, or at least deduce it at compile time. It cannot depend on user input. constexpr and templates will let you do sophisticated calculations are compile time. But if you need it to depend on user input you will need to use another approach, maybe just returning an std::vector<double>. — François Andrieux
– François Andrieux, Commented Jan 16, 2019 at 20:24
"Is it possible to create static array without compile time constant?" - NO. In order to create a fixed-length array, the compiler needs to know the exact array size to use. Hence a compile-time constant is required. Otherwise, you have to allocate the array dynamically at runtime, using new[] or std::vector — Remy Lebeau
– Remy Lebeau, Commented Jan 16, 2019 at 20:51

eerorika · Accepted Answer · 2019-01-16 20:28:06Z

2

Use const int by reference to create static array

Is not possible. The length of all arrays which have non-dynamic storage must be a compile time constant.

You can have a static vector:

assert(len >= 2);
static std::vector<double> ests(len);
// ...
return ests.data();

edited Jan 16, 2019 at 20:28

answered Jan 16, 2019 at 20:26

eerorika

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3 Comments

Noitidart Over a year ago

May you please show how to use that in a function like above.

eerorika Over a year ago

@Noitidart just replace the contents of your function with what I show. You can replace the // ... with whatever you have between declaration of ests and return.

eerorika Over a year ago

@Noitidart given the implementation that I show, it should be double*.

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