Cannot create a constexpr array from a parameter pack inside a template

Question

I wanted to do a static_assert inside my template on an array created from a parameter pack.

Consider the following code:

template<typename... Args>
auto constexpr CreateArrConst(Args&&... args)
{
  std::array arr
  {
      args...
  };
  return arr;
}

template<typename... Args>
auto constexpr CreateArrConst_NotWorking(Args&&... args)
{
  constexpr std::array arr
  {
      args...
  };
  static_assert(arr.back() == 4);
  return arr;
}

int main() 
{
  static_assert(CreateArrConst(4).back() == 4);
  // uncomment this to reproduce compile error
  // static_assert(CreateArrConst_NotWorking(4).back() == 4);

  return 0;
}

The first template can create a constexpr which I can use at compile time to check inside main. However, once I try to create the same array (this time as constexpr) inside the template in CreateArrConst_NotWorking I get:

error: ‘args#0’ is not a constant expression

Godbolt link to reproduce: https://godbolt.org/z/YjGq76e5j

Does anyone know why I cannot create the constexpr inside the template?

@StackDanny Did you try to uncomment the 2nd function call in main? — SupAl
– SupAl, Commented Jun 2, 2022 at 7:54

Richard · Accepted Answer · 2022-06-02 08:19:35Z

2

The problem is that even though the template parameter pack named Args is a constant expression the function parameter pack named args isn't.

And since args isn't a constant expression it(args...) cannot be used as an initializer for the constexpr array in your given example.

To solve this you can replace constexpr with const for the array, as then the initializer is not required to be a constant expression.

template<typename... Args>
auto constexpr CreateArrConst_NotWorking(Args... args)
{
//vvvvv------------------->constexpr changed to const
  const std::array arr
  {
      args...
  };
  return arr;
}

Demo

edited Jun 2, 2022 at 8:19

answered Jun 2, 2022 at 8:14

Richard

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9 Comments

SupAl Over a year ago

True, but then I cannot do the static_assert inside the template!

Richard Over a year ago

Agreed, but your question is: "Does anyone know why I cannot create the constexpr inside the template?" for which my answer provides the explanation.

SupAl Over a year ago

So, if args isn't constexpr, how can I create the constexpr arr in the first template (CreateArrConst)?

Richard Over a year ago

@SupAl Please ask a separate question for your follow-up question. I noticed that you edited your orignal question to include your follow up question. Please avoid doing that as it makes the existing answers obsolete. Feel free to ask as many separate questions as you want for your follow up questions. Like you can ask "how can I create the constexpr arr in the first template". Both of your questions are interesting and needs to be separately answered.

SupAl Over a year ago

makes sense. I'll follow that suggestion.

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