Difficulties getting a constexpr property from a constexpr array

Question

I'm having this issue where I can't seem to, at compile time, check if all elements in an std::array are equal. It seems so simple and I'm not new to C++ by any means, but I can't figure it out! (I would just use <algorithm> but sadly those aren't marked constexpr in C++17, and I'm stuck with C++17 because CUDA.)

Here's an example (that doesn't compile).

#include <array>

int main()
{
    constexpr std::array<int, 3> a {0, 0, 0};

    constexpr bool equal = [=](){
        for (int i = 1; i < 3; i++)
        {   
            if constexpr (a[0] != a[i])
                return false;
        }
        return true;
    }();
}

Why does a[0] != a[i] not qualify as constexpr? (This is the error GCC and Clang give me.) How do I get the result I need?

The thing is, for loops aren't the issue. My program has way more complicated constexpr evaluations using for loops that work just fine. It might solve it, but not well. — Olaf Willocx
– Olaf Willocx, Commented Nov 9, 2021 at 1:33

康桓瑋 · Accepted Answer · 2021-11-09 01:44:22Z

2

Since your i is not a compile-time constant, you cannot use if constexpr. A simple if is enough which still can check your array at compile-time.

#include <array>

int main()
{
    constexpr std::array<int, 3> a {0, 0, 0};

    constexpr bool equal = [=](){
        for (int i = 1; i < 3; i++)
        {   
            if (a[0] != a[i])
          //^^
                return false;
        }
        return true;
    }();
}

edited Nov 9, 2021 at 1:44

answered Nov 9, 2021 at 1:31

康桓瑋

46.5k5 gold badges69 silver badges135 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Olaf Willocx Over a year ago

Thanks, that works! Makes sense too. Unhelpful compiler error was pointing to the array instead of the index.

康桓瑋 Over a year ago

Which compiler are you using? The error message of GCC is relatively clear: error: the value of 'i' is not usable in a constant expression.

Olaf Willocx Over a year ago

I was using Clang. Tried to compile it once in GCC, didn't pay close enough attention. Looking more closely now, Clang does say "read of non-const variable 'i' is not allowed in a constant expression" a few lines down.

Collectives™ on Stack Overflow

Difficulties getting a constexpr property from a constexpr array

1 Answer 1

3 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Linked

Related