Initializing an array with a initializer_list? [duplicate]

Question

Is it at all possible?

#include <array>
#include <initializer_list>

struct A
{
    A ( std::initializer_list< int > l )
        : m_a ( l )
    {
    }

    std::array<int,2> m_a;
};

int main()
{
    A a{ 1,2 };
}

But this results in this error:

t.cpp: In constructor ‘A::A(std::initializer_list<int>)’:
t.cpp:7:19: error: no matching function for call to ‘std::array<int, 2ul>::array(std::initializer_list<int>&)’
         : m_a ( l )
                   ^
t.cpp:7:19: note: candidates are:
In file included from t.cpp:1:0:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note: std::array<int, 2ul>::array()
     struct array
            ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note:   candidate expects 0 arguments, 1 provided
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note: constexpr std::array<int, 2ul>::array(const std::array<int, 2ul>&)
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note:   no known conversion for argument 1 from ‘std::initializer_list<int>’ to ‘const std::array<int, 2ul>&’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note: constexpr std::array<int, 2ul>::array(std::array<int, 2ul>&&)
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/array:81:12: note:   no known conversion for argument 1 from ‘std::initializer_list<int>’ to ‘std::array<int, 2ul>&&’

This isn't possible, as a workaround, manually copy the elements of the initializer_list: A (std::initializer_list<int> l) /*Don't initialize m_a*/ { std::copy(l.begin(),l.end(),m_a.begin());} — Mankarse
– Mankarse, Commented Mar 19, 2015 at 11:38
note that this is not special to constructors; the same issue arises with std::initializer_list<int> x = { 1, 2 }; std::array<int, 2> m { ?????? }; . You can only put things here that are a part of aggregate initialization, i.e. initializers for each element one by one. — M.M
– M.M, Commented Mar 19, 2015 at 11:49
See stackoverflow.com/q/5549524/1436796 which also provides a possible solution, even if the member array should be qualified const (which disqualifies leaving it empty and then copying the list contents later). — Marc van Leeuwen
– Marc van Leeuwen, Commented Mar 19, 2015 at 12:37

Angew is no longer proud of SO · Accepted Answer · 2015-03-19 11:58:00Z

15

Not in this case. You can initialize array with a list-initializer

std::array<int, 2> a{1,2};

but you cannot initialize array with initializer_list, since array is just an aggregate type with only the default and copy constructor.

You can just leave the array empty and then copy the contents of the initializer_list into it.

edited Mar 19, 2015 at 11:58

Angew is no longer proud of SO

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answered Mar 19, 2015 at 11:35

ForEveR

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