Alex Alex - 3 months ago 21
C++ Question

Can I initialize an array using the std::initializer_list instead of brace-enclosed initializer?

Can I initialize an array using the

std::initializer_list
object instead of brace-enclosed initializer?

As known, we can do this: http://en.cppreference.com/w/cpp/language/aggregate_initialization

unsigned char b[5]{"abc"};
// equivalent to unsigned char b[5] = {'a', 'b', 'c', '\0', '\0'};

int ar[] = {1,2,3};
std::array<int, 3> std_ar2{ {1,2,3} }; // std::array is an aggregate
std::array<int, 3> std_ar1 = {1, 2, 3};


But I can't initialize an array by
std::initializer_list il;
:

http://ideone.com/f6aflX

#include <iostream>
#include <initializer_list>
#include <array>

int main() {

int arr1[] = { 1, 2, 3 }; // OK
std::array<int, 3> arr2 = { 1, 2, 3 }; // OK

std::initializer_list<int> il = { 1, 2, 3 };
constexpr std::initializer_list<int> il_constexpr = { 1, 2, 3 };

//int arr3[] = il; // error
//int arr4[] = il_constexpr; // error

//std::array<int, 3> arr5 = il; // error
//std::array<int, 3> arr6 = il_constexpr; // error

return 0;
}


But how can I use
std::initializer_list il;
to initialize an array?

Answer

Other answered correctly said this is not possible upfront. But with little helpers, you can get pretty close

template<typename T, std::size_T N, std::size_t ...Ns>
std::array<T, N> make_array_impl(
    std::initializer_list<T> t,
    std::index_sequence<Ns...>) 
{
    return std::array<T, N>{ *(t.begin() + Ns) ... };
}

template<typename T, std::size_t N>
std::array<T, N> make_array(std::initializer_list<T> t) {
    if(N > t.size())
       throw std::out_of_range("that's crazy!");
    return make_array_impl<T, N>(t, std::make_index_sequence<N>());
}

If you are open to more work arounds, you can put this into a class to catch statically-known length violations for the cases where you pass a braced init list. But be warned that most people who read this code will head-desk

template<typename T, std::size_t N>
struct ArrayInitializer {
    template<typename U> struct id { using type = U; };
    std::array<T, N> t;

    template<typename U = std::initializer_list<T>>
    ArrayInitializer(typename id<U>::type z) 
        :ArrayInitializer(z, std::make_index_sequence<N>())
    { 
        if(N > z.size())
            throw std::out_of_range("that's crazy!");
    }

    template<typename ...U>
    ArrayInitializer(U &&... u)
       :t{ std::forward<U>(u)... }
    { }

private:
    template<std::size_t ...Ns>
    ArrayInitializer(std::initializer_list<T>& t,
                     std::index_sequence<Ns...>)
       :t{ *(t.begin() + Ns) ... }
    { }
};

template<typename T, std::size_t N>
std::array<T, N> f(ArrayInitializer<T, N> ai) {
    return std::move(ai.t);
}

int main() {
   f<int, 5>({1, 2, 3, 4, 5});  // OK 
   f<int, 5>({1, 2, 3, 4, 5, 6});  // "too many initializers for array<int, 5>"

   std::initializer_list<int> il{1, 2, 3, 4, 5};
   f<int, 5>(il); // ok
}

Note that both the non-static case at the top of the answer and the "head-desk" case do only check whether you provide too few initializing elements, and errors out then, for the initializer_list case. If you provide too many for the initializer_list case, the trailing elements are just ignored.

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