foo.cc

#include "foo.h"

constexpr int Foo::kValue;

foo.h

#pragma once

class Foo {
 public:
  static constexpr int kValue = 10;
};

main.cc

#include <array>
#include <iostream>

#include "foo.h"

constexpr int addOne(int x) { return x + 1; }

int main() {
  // Foo::kValue is a constant value.
  std::cout << "result: " << Foo::kValue << std::endl;
  // That's a constant, so you can use it as a template parameter.
  std::array<int, Foo::kValue> arr = {0};
  for (const auto& x : arr)
    std::cout << x << " ";
  std::cout << std::endl;
  for (auto& x : arr)
    ++x;
  for (const auto& x : arr)
    std::cout << x << " ";
  std::cout << std::endl;

  // You can calculate values on compilation time if all values and functions
  // are declared as constexpr.
  std::array<int, addOne(Foo::kValue)> arr2;
  std::cout << "arr2 size: " << arr2.size() << std::endl;

  // You cannot retrieve the address if Foo::kValue doesn't have the
  // storage. You need to have it at somewhere (like foo.cc).
  std::cout << &Foo::kValue << std::endl;
  return 0;
}