How can I make a3 compile?
int main()
{
int a1[] = { 1, 2, 3 };
std::array<int, 3> a2 = { 1, 2, 3 };
std::array<int> a3 = { 1, 2, 3 };
}
It's very inconvenient, and brittle, to hard-code the size of the array when using an initialization list, especially long ones. Is there any work around? I hope so otherwise I'm disappointed because I hate C arrays and std::array is supposed to be their replacement.
There is currently no way to do this without rolling your own make_array, there is a proposal for this N3824: make_array which has the following scope:
LWG 851 intended to provide a replacement syntax to
array<T, N> a = { E1, E2, ... };, so the following
auto a = make_array(42u, 3.14);is well-formed (with additional static_casts applied inside) because
array<double, 2> = { 42u, 3.14 };is well-formed.
This paper intends to provide a set of std::array creation interfaces which are comprehensive from both tuple’s point of view and array’s point of view, so narrowing is just naturally banned. See more details driven by this direction in Design Decisions.
It also includes a sample implementation, which is rather long so copying here is impractical but Konrad Rudolph has a simplified version here which is consistent with the sample implementation above:
template <typename... T>
constexpr auto make_array(T&&... values) ->
std::array<
typename std::decay<
typename std::common_type<T...>::type>::type,
sizeof...(T)> {
return std::array<
typename std::decay<
typename std::common_type<T...>::type>::type,
sizeof...(T)>{std::forward<T>(values)...};
}
std::vector<std::array<type, different_sizes>>? Making a vector of std::array would require all the sizes to be the same?
std::vector<std::array<type, different_sizes> > AFAIK is impossible.
return {std::forward<T>(values)...}?
make_array() above be better than the Intermediate version (in updated original question) posted here ? What are the pro's and cons of each ?You're being a little overdramatic when you say "such a horribly complex (to me) function is required". You can make a simplified version yourself, the proposal also includes a "to_array" function to convert C-arrays and deducing the type from the first parameter. If you leave that out it gets quite manageable.
template<typename T, typename... N>
auto my_make_array(N&&... args) -> std::array<T,sizeof...(args)>
{
return {std::forward<N>(args)...};
}
which you can then call like
auto arr = my_make_array<int>(1,2,3,4,5);
edit: I should mention that there actually is a version of that in the proposal that I overlooked, so this should be more correct than my version:
template <typename V, typename... T>
constexpr auto array_of(T&&... t)
-> std::array < V, sizeof...(T) >
{
return {{ std::forward<T>(t)... }};
}
{std::forward<N>(args)...}; works is the slight relaxation of the rules for eliding braces, described here.
make_array()function.auto a3 = std::to_array<int>({3, 2, 1});