Undefined reference while calling a static constexpr array of variadic types

Question

I have the following class

template<int ... Args>
struct foo {
    constexpr static int arr[sizeof...(Args)]={Args...};
    constexpr int dimension(int i) {return arr[i];}
};

But I get undefined reference to arr, while calling dimension. If I move arr inside the function dimension then the function cannot be a constexpr anymore, because it requires two semicolons within the body of the function. For instance, I cannot do

constexpr int a = foo_obj.dimension(2);

My goal is to metaprogrammatically iterate over all the dimensions of a varidic template and compare it to another integral number? Ideally if I have two objects of foo I want to determine if they are equal in every dimension.

Please provide a minimal reproducible example. Can't repro (once I add the missing int in arr and ; for foo) — Barry
– Barry, Commented May 24, 2016 at 20:34
@Barry Fairly sure that you will run into ODR issues once you do a dimension call with a run-time value. — T.C.
– T.C., Commented May 24, 2016 at 20:41
Not sure I understand the question; two specializations of foo have the same type iff their Args... are the same, so why do you need to do anything beyond std::is_same? — T.C.
– T.C., Commented May 24, 2016 at 20:42

Barry · Accepted Answer · 2016-05-24 20:51:14Z

6

Every variable that is odr-used needs a definition. This:

constexpr static int arr[sizeof...(Args)]={Args...};

is a declaration that also initializes arr, but it isn't a definition. So you just have to provide a definition, which must be both (1) external to the class and (2) still constexpr. That is:

template<int ... Args>
struct foo {
    constexpr static int arr[sizeof...(Args)]={Args...};
    constexpr int dimension(int i) const {return arr[i];}
};

template <int... Args>
constexpr int foo<Args...>::arr[sizeof...(Args)];

And now foo<Args...>::arr is defined.

answered May 24, 2016 at 20:51

Barry

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1 Comment

Tasam Farkie Over a year ago

Fantastic. Thanks a ton for the fix and explanation.

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