Inline constexpr and anonymous namespace variables?

Question

I have the following problem. In a.hpp file I have a function Space::foo() that must use a namespace global variable, I thought of hiding it in an anonymous namespace. But now I wonder, do I risk having values defined in each translation unit with multiple copies or there will only be one? Thanks for helping. This comes from the fact that I needed a bunch of functions in the Space namespace with a sort of private data part (the anonymous namespace stuff) so that Space is sort of like a class with only static members, but I don't want to have multiple copies of these variables if I include the.hpp file in different units.

// .hpp file
#include <array>

namespace Space {
namespace {
  constexpr std::array<int, 10000> fill() { /* ... */ };
  inline constexpr std::array<int, 10000> values = fill();
}
  inline int foo(int i) {
    return values[i];
  }
}

Answer 1

do I risk having values defined in each translation unit with multiple copies

Yes. You'll have one definition of values in each TU. You can't declare it extern constexpr either because[dcl.constexpr] /1 says " The constexpr specifier shall be applied only to the definition of a variable ".

Since the purpose seems to be to be able to pick values that are constant evaluated you could possibly make both fill and foo consteval and skip <anonymous>::values .

namespace Space {
    namespace {
        consteval std::array<int, 10000> fill() {
            return {}; // replace with your filling algorithm
        };
    }

    inline consteval int foo(size_t i) {
        return fill()[i];
    }

    inline consteval int bar(size_t i, size_t j) {
        constexpr auto values = fill();
        return values[i] + values[j];
    }
}

This would make it possible for foo and any other consteval function to use the algorithm in fill() and to use the result in constexpr / consteval situations.

constexpr auto x = Space::bar(10, 20);