Brace-enclosed initializer list of templated struct - c++

Question

#include <array>
#include <vector>
#include <cinttypes>
#include <iostream>
using namespace std;
template<size_t N>
struct item_t {
array<uint32_t, N> weight = {0};
};
int main(void) {
vector<item_t<3>> items;
items.emplace_back({{9,2,3}});
cout << items[0].weight[0] << endl;
return 0;
};
I'm at a bit of a loss here. Error is on the emplace_back line and no idea how to resolve it. Any help or hints would be appreciated, thanks.
EDIT
gcc version 4.8.2
$ g++ -std=c++11 test.cpp
test.cpp: In function ‘int main()’:
test.cpp:16:30: error: no matching function for call to ‘std::vector<item_t<3ul> >::emplace_back(<brace-enclosed initializer list>)’
items.emplace_back({{9,2,3}});
^
test.cpp:16:30: note: candidate is:
In file included from /usr/include/c++/4.8/vector:69:0,
from test.cpp:2:
/usr/include/c++/4.8/bits/vector.tcc:91:7: note: void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {}; _Tp = item_t<3ul>; _Alloc = std::allocator<item_t<3ul> >]
vector<_Tp, _Alloc>::
^
/usr/include/c++/4.8/bits/vector.tcc:91:7: note: candidate expects 0 arguments, 1 provided
The problem is with the struct initialization = {0} and with emplace_back.

Answer 1

emplace_back() uses template argument deduction to determine the types of the elements passed to the function. A brace enclosed initializer list is not an expression and doesn't have type and therefore cannot be deduced by the template. You have to explicitly call the constructor here:
items.emplace_back(item_t<3>{{1,2,3}});

Answer 2

There are two issues here :
Trying to init a object of type T like this T{...} is referred to as aggregate initialization. Under some conditions, there is a default behaviour specified for it, even if you don't have a constructor which accepts a initializer_list. In C++11, you are not allowed to provide non-default constructors or in-class initializers. So, given this definition
template<size_t N>
struct item_t {
array<uint32_t, N> weight = {0};
};
you cannot write item_t<3> t{1,2,3};.
That, however, isn't your problem. The reason your code fails is that emplace_back tries to forward the arguments to a constructor of the vectors underlying type. In your case, there isn't a match. Note that nice a braced-init list isn't equivalent to an initializer_list in this context, you cannot solve this problem by adding an initializer_list constructor and will have to help the compiler out some other way.