can I declare a static array with constexpr function

Question

// since we can dedfine a static array like this
int a[5] = {0};
// decltype(a[5]) is `int [5]`

// Then how about this way?
constexpr int sum(int a, int b) { return a + b; }
int a, b;
std::cin >> a >> b;
int arr[sum(a, b)] = {0};

it can be compiled successfully, but is arr a static array? when I tried to print arr type with typeid().name() or boost::typeindex::type_id_with_cvr, I got below error:

error: cannot create type information for type 'int [(<anonymous> + 1)]' because it involves types of variable size
 std::cout << typeid(decltype(arr)).name() << std::endl;

Declaring an array with size produced by sum invocation requires it to be invoked at compile time, for function sum to be evaluated to compile time both arguments are required to be known at compile time. — user7860670
– user7860670, Commented Aug 8, 2018 at 8:31

Alan Birtles · Accepted Answer · 2018-08-08 08:33:05Z

6

As the value of a and b are not known at compile time the result of sum is not a constexpr.

The code compiles presumably because you are using GCC which has an extension which allows declaring arrays on the stack with variable size, standard c++ doesn't allow this.

answered Aug 8, 2018 at 8:33

Alan Birtles

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Daniel Langr Over a year ago

Just a note that these are called variable length arrays (VLA): gcc.gnu.org/onlinedocs/gcc/Variable-Length.html. You can disable them with GCC by using -Werror=vla: wandbox.org/permlink/iJtZ5nmyDTaxFW2E.

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