CODEWITHSUNDEEP
c++c++23
Mr.Barbo
Mr.Barbo

Reputation: 119

Compile time evaluation of static arrays

I am experimenting a little bit with the const... specifiers. I have written the following code, in which I use constexpr for two arrays:

class MyClass{
  private:

    static constexpr std::array<uint64_t, 5> arr = {0,1,2,3,4};
    
    static constexpr std::array<uint64_t, 5> arr_sum {[](const std::array<uint64_t, 8> & arr) {
      std::array<uint64_t, 5> arr_sum;
      arr_sum[0] = arr[0];
      for (int i = 1; i < 5; i++) { 
        arr_sum[i] = arr[i] + arr_sum[i-1]; 
      }
      return arr_sum;
    }(arr)};


  public:
    ...
};

Where the array arr_sum stores the sum of the first i elements in the i position.

As far as I know, with constexpr the compiler tries to make arr and arr_sum at compilation time. But, there is also consteval and constinit. The first one ensures that a function is evaluated at compilation time, and the second one ensures constant initialization of a static or thread-local variables.

I have two questions here:

  1. is there any chance that the arrays arr or arr_sum are not created at compilation time despite using constexpr?

  2. can I ensure compilation time creation of both arrays using the consteval or constinit specifiers?

Upvotes: 5

Views: 146

Answers (0)

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