Possible Duplicate:
How do I initialize a member array with an initializer_list?
You can construct an std::array just fine with an initializer list:
std::array<int, 3> a = {1, 2, 3}; // works fine
However, when I try to construct it from an std::initializer_list
as a data member or base object in a class, it doesn't work:
#include <array>
#include <initializer_list>
template <typename T, std::size_t size, typename EnumT>
struct enum_addressable_array : public std::array<T, size>
{
typedef std::array<T, size> base_t;
typedef typename base_t::reference reference;
typedef typename base_t::const_reference const_reference;
typedef typename base_t::size_type size_type;
enum_addressable_array(std::initializer_list<T> il) : base_t{il} {}
reference operator[](EnumT n)
{
return base_t::operator[](static_cast<size_type>(n));
}
const_reference operator[](EnumT n) const
{
return base_t::operator[](static_cast<size_type>(n));
}
};
enum class E {a, b, c};
enum_addressable_array<char, 3, E> ea = {'a', 'b', 'c'};
Errors with gcc 4.6:
test.cpp: In constructor 'enum_addressable_array<T, size, EnumT>::enum_addressable_array(std::initializer_list<T>) [with T = char, unsigned int size = 3u, EnumT = E]':
test.cpp:26:55: instantiated from here
test.cpp:12:68: error: no matching function for call to 'std::array<char, 3u>::array(<brace-enclosed initializer list>)'
test.cpp:12:68: note: candidates are:
include/c++/4.6.1/array:60:12: note: std::array<char, 3u>::array()
include/c++/4.6.1/array:60:12: note: candidate expects 0 arguments, 1 provided
include/c++/4.6.1/array:60:12: note: constexpr std::array<char, 3u>::array(const std::array<char, 3u>&)
include/c++/4.6.1/array:60:12: note: no known conversion for argument 1 from 'std::initializer_list<char>' to 'const std::array<char, 3u>&'
include/c++/4.6.1/array:60:12: note: constexpr std::array<char, 3u>::array(std::array<char, 3u>&&)
include/c++/4.6.1/array:60:12: note: no known conversion for argument 1 from 'std::initializer_list<char>' to 'std::array<char, 3u>&&'
How can I get it to work so that my wrapper class can be initialized with an initializer-list, as such:
enum_addressable_array<char, 3, E> ea = {'a', 'b', 'c'};
An std::array<>
has no constructor that takes an std::initializer_list<>
(initializer list constructor) and there is no special language support for what it may mean to pass a std::initializer_list<>
to a class' constructors such that that may work. So that fails.
For it to work, your derived class needs to catch all elements and then forward them, a constructor template:
template<typename ...E>
enum_addressable_array(E&&...e) : base_t{{std::forward<E>(e)...}} {}
Note that you need {{...}}
in this case because brace elision (omitting braces like in your case) does not work at that place. It's only allowed in declarations of the form T t = { ... }
. Because an std::array<>
consists of a struct embedding a raw array, that will need two level of braces. Unfortunately, I believe that the exact aggregate structure of std::array<>
is unspecified, so you will need to hope that it works on most implementations.