Do we have auto array in c++17?

Question

Consider the following code:

auto numbers = {1, 2, 3, 4};    // 'numbers' is an std::intializer_list<int>
auto num_array[] = {1, 2, 3, 4} // error -> declared as an array of 'auto'

Why is this limitation in place?

Answer 1

With C++17 and class template argument deduction, this works just fine:

#include <array>
std::array arr{1, 2, 3, 4};
int i = arr.size();  // 4

Answer 2

Asking why on language features is a notoriously difficult question to answer.

But the hopefully helpful version is: auto is a very simple language feature that just does full deduction, not constrained deduction. You can add some declarators, like auto& or auto*, but not all of them. As-is, it's incredibly useful. Just... not all powerful. It's often useful to start with a simple, restricted feature set and then expand later as we gain experience.

---

With Concepts, it is possible that further power may be added to declarations to do more kinds of things, like:

std::vector<int> foo();
std::vector<auto> v = foo();

Having an array of auto seems to fit into that mold. Definitely not in C++17, but possibly for C++20.

Answer 3

It's ugly, but it is possible to deduce the element type from an raw array list initializer and declare the array directly, as follows:

template<typename T>
struct array_trait
{
    using element_type = T;
    array_trait(T(&&)[]);
};

decltype(array_trait({4,5,7}))::element_type a[] = {4,5,7};