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### 2443. `std::array` member functions should be `constexpr`

Section: 24.3.7 [array] Status: Resolved Submitter: Peter Sommerlad Opened: 2014-10-06 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

When experimenting with C++14 relaxed `constexpr` functions I made the observation that I couldn't use `std::array` to create a table of data at compile time directly using loops in a function. However, a simple substitute I could use instead:

>     template <typename T, size_t n>
>     struct ar {
>       T a[n];
>       constexpr ar() : a{{}}{}
>       constexpr auto data() const { return &a[0];}
>       constexpr T const & operator[](size_t i) const { return a[i]; }
>       constexpr T & operator[](size_t i) { return a[i]; }
>     };

>     template <size_t n>
>     using arr = ar<size_t, n>; // std::array<size_t, n>;

>     template <size_t n>
>     constexpr auto make_tab(){
>       arr<n> result;
>       for(size_t i=0; i < n; ++i)
>         result[i] = (i+1)*(i+1); // cannot define operator[] for mutable array...
>       return result;
>     }

>     template <size_t n>
>     constexpr auto squares=make_tab< n>();

>     int main() {
>       int dummy[squares<5>[3]];
>     }

Therefore, I suggest that all member functions of `std::array` should be made `constexpr` to make the type usable in `constexpr` functions.

Wording should be straight forward, may be with the exception of `fill`, which would require `fill_n` to be `constexpr` as well.

[2014-11 Urbana]

Move to LEWG

The extent to which `constexpr` becomes a part of the Library design is a policy matter best handled initially by LEWG.

[08-2016, Post-Chicago]

Move to Tentatively Resolved

Proposed resolution:

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