c++ - constexpr array and std::initializer_list

Question

I was trying to write an compile-time valarray that could be used like this:

constexpr array<double> a = { 1.0, 2.1, 3.2, 4.3, 5.4, 6.5 };

static_assert(a[0] == 1.0, "");
static_assert(a[3] == 4.3, "");

static_assert(a.size() == 6, "");

I managed to do it with the following implementation and it works fine (with GCC 4.7):

#include <initializer_list>

template<typename T>
struct array
{
    private:

        const std::size_t _size;
        const T* _data;

    public:

        constexpr array(std::initializer_list<T> values):
            _size(values.size()),
            _data(values.begin())
        {}

        constexpr auto operator[](std::size_t n)
            -> T
        {
            return _data[n]
        }

        constexpr auto size() const
            -> std::size_t;
        {
            return _size;
        }
};

Even though it works fine for me, I am not sure about the behaviour of std::initializer_list and may use some that are undefined behaviour.

constexpr for std::initializer_list constructor, begin and size is fine even though it is not strictly speaking C++11 since N3471 recently got adopted and made it to the standard.

Concerning the undefined behaviour, I am not sure whether the underlying array of the std::initializer_list will live or if not, whether there is a mean to have it live longer than only array's constructor. What do you think?

EDIT: I may not have been clear, but I do not really care about the actual array. What really interests me is the behaviour of std::initializer_list and its underlying array at compile-time.

Answer 1

Your current code should not compile according to current C++11 rules. When compiled with clang 3.2 I get the following error:

source.cpp:33:28: error: constexpr variable 'a' must be initialized by a constant
expression 
constexpr array<double> a = { 1.0, 2.1, 3.2, 4.3, 5.4, 6.5 };
                        ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is because std::initializer_lists ctors and member functions begin and end are not labeled constexpr. However, there already is a proposal to change this. BTW, libstdc++ already marks these as constexpr.

Now the next problem is the lifetime of the underlying array of std::initializer_list. This is explained in 8.5.4p6:

The array has the same lifetime as any other temporary object (12.2), except that initializing an initializer_list object from the array extends the lifetime of the array exactly like binding a reference to a temporary.

This means that the underlying array has the same lifetime as values object, and expires at the end of your array constructor when it exits. Therefore, _data is pointing to expired memory and _data[n] is undefined behavior.