C++ constexpr operations on each element of a constexpr array

Question

Would be possible to iterate over a constexpr C-array/std::array/vector and execute constexpr operations on each element and everything to be done at compile time?

https://godbolt.org/z/5a4v3Eerh

#include <string_view>
using namespace std::string_view_literals;

constexpr std::string_view arr[] = {"ab"sv, "xyz"sv,};

int main(){
    static_assert(arr[0] == "ab");  //OK
    static_assert(arr[1] == "xyz");

    #if 0 //would something like this be possible?
    for(const auto i: arr){
        static_assert(i == "xyz"); //???
    }
    #endif
    return 0;
}

I used static_assert() but there could be any constexpr function ...

Any suggestion would be appreciated! any C++ standard (preferably the latest), any compiler (preferably Microsoft VisualStudio 2022)

Answer 1

After reading all your suggestions, my conclusions are:

1. the simplest solution would be to use std::array<std:string_view> because it is constexpr-friendly and the operator==() can be used to compare 2 objects of this type:

constexpr std::array<std::string_view,2> inp = {/*...*/};
constexpr std::array<std::string_view,2> out = someConstexprFunc(inp);
constexpr std::array<std::string_view,2> ref = {/*...*/};

static_assert(out == ref);

It is simpler but doesn't provide information about the index causing an eventual mismatch

2. if the index causing the mismatch is needed then replace operator==() with a constexpr function returning the index and use that index as template parameter for an error class with a meaningful name:

#include <algorithm>
#include <array>
#include <ranges>
#include <string_view>
using namespace std::string_view_literals;

consteval auto run(std::ranges::range auto const& range){
    std::remove_cvref_t<decltype(range)> res;
    std::transform(range.cbegin(), range.cend(), res.begin(), [](const std::string_view s){
        return s.substr(0, 1);
    });
    return res;
}

template<size_t N>
constexpr size_t cmp(const std::array<std::string_view,N>&a, const std::array<std::string_view,N>&b){
    for(size_t i=0; i<N; ++i){
        if(a[i] != b[i]){
            return i+1;
        }
    }
    return 0;
}

template<size_t index> struct CompareErrorAtIndex{
    static_assert(index == 0, "compare `out` with `ref`");
};

constexpr std::array<std::string_view,2> inp = {"ab"sv, "xyz"sv};
constexpr std::array<std::string_view,2> out = run(inp);
constexpr std::array<std::string_view,2> ref = {"a"sv, "x"sv,};

int main(){
    //static_assert(out == ref); //OK but no info about index generating the error

    constexpr auto index = cmp(out, ref);
    if constexpr(index){
        CompareErrorAtIndex<index>();
    }

    return index;
}

The error class together with its template argument (the index) will appear in the build output for all 3 main compilers (msvc, clang, gcc) like, for example: ... CompareErrorAtIndex<1> ...

You can see it in action at https://godbolt.org/z/z41TqhvPE

Thank you very much for your input! (I'm still open for comments, suggestions, improvements, constructive critics)