Size of std::array in class template depending on template parameter

Question

I have a the following class template

template<int N>
constexpr int arraySize() { return arraySize<N-1>() + N; }

template<>
constexpr int arraySize<0>() { return 0; }

template<int C>
class MyClass {
    public:
    std::array<int, arraySize<C>()> arr;
};

int main() {

    MyClass<3> cls;
    std::cout << cls.arr.size() << std::endl;    // Output: 6
}

Everything works but I would like to have calculateArraySize<N>() as a member function. I've tried the following:

template<int C>
class MyClass {
    public:

    static constexpr int arraySize();
    std::array<int, MyClass<C>::arraySize()> arr;
};

template<int C>
constexpr int MyClass<C>::arraySize(){ return MyClass<C-1>::arraySize() + C; }

template<>
constexpr int MyClass<0>::arraySize() { return 0; }

Results in the following error:

fatal error: recursive template instantiation exceeded maximum depth of 1024 std::array::arraySize()> arr;

template<int C>
class MyClass {
    public:

    template<int N>
    static constexpr int arraySize();
    std::array<int, MyClass::arraySize<C>()> arr;
};

template<int C>
template<int N>
constexpr int MyClass<C>::arraySize(){ return MyClass::arraySize<N-1>() + N-1; }

template<int C>
template<>
constexpr int MyClass<C>::arraySize<0>() { return 0; }

Gives the following error:

tmp.cc:19:27: error: cannot specialize (with 'template<>') a member of an unspecialized template constexpr int MyClass::arraySize<0>() { return 0; }

Is it possible to achieve the desired behaviour? Solutions using C++14/C++17 features (I guess it should be possible usinn if-constexpr) are welcomed but won't solve my particular problem since only C++11 is available.

Answer 1

You can move the function into the class and the specialize the entire class for the base case. That looks like:

template<int C>
class MyClass {
    public:

    static constexpr int arraySize(){ return MyClass<C-1>::arraySize() + C; }
    std::array<int, MyClass<C>::arraySize()> arr;
};

template<>
class MyClass<0> {
    public:
    static constexpr int arraySize(){ return 0; }
};

int main() {
    MyClass<3> cls;
    std::cout << cls.arr.size() << std::endl;    // Output: 6
}

Live Example

Answer 2

You can also use a member variable instead of a member function.

template <int C>
class MyClass {
   public:

      static constexpr int array_size = MyClass<C-1>::array_size + C;
      std::array<int, array_size> arr;
};

template <>
class MyClass<0> {
   public:
       static constexpr int array_size = 0;
};