How do I initialize std::array<T, n> if T is not default constructible?
I know it's possible to initialize it like that:
T t{args};
std::array<T, 5> a{t, t, t, t, t};
But n for me is template parameter:
template<typename T, int N>
void f(T value)
{
std::array<T, N> items = ???
}
And even if it wasn't template, it's quite ugly to repeat value by hand if n is too large.
Given N, you could generate a sequence-type calledseq<0,1,2,3,...N-1> using a generator called genseq_t<>, then do this:
template<typename T, int N>
void f(T value)
{
//genseq_t<N> is seq<0,1,...N-1>
std::array<T, N> items = repeat(value, genseq_t<N>{});
}
where repeat is defined as:
template<typename T, int...N>
auto repeat(T value, seq<N...>) -> std::array<T, sizeof...(N)>
{
//unpack N, repeating `value` sizeof...(N) times
//note that (X, value) evaluates to value
return {(N, value)...};
}
And the rest is defined as:
template<int ... N>
struct seq
{
using type = seq<N...>;
static const std::size_t size = sizeof ... (N);
template<int I>
struct push_back : seq<N..., I> {};
};
template<int N>
struct genseq : genseq<N-1>::type::template push_back<N-1> {};
template<>
struct genseq<0> : seq<> {};
template<int N>
using genseq_t = typename genseq<N>::type;
Hope that helps.
Sadly the existing answers here don't work for non-copyable types. So I took @Nawaz answer and modified it:
#include <utility>
#include <array>
template<typename T, size_t...Ix, typename... Args>
std::array<T, sizeof...(Ix)>
repeat(std::index_sequence<Ix...>, Args &&... args) {
return {{((void)Ix, T(args...))...}};
}
template<typename T, size_t N>
class initialized_array: public std::array<T, N> {
public:
template<typename... Args>
initialized_array(Args &&... args) :
std::array<T, N>(
repeat<T>(std::make_index_sequence<N>(), std::forward<Args>(args)...)
)
{}
};
Note that this is an std::array subclass so that one can easily write
class A {
A(int, char) {}
}
...
class C {
initialized_array<A, 5> data;
...
C(): data(1, 'a') {}
}
Without repeating the type and size. Of course, this way can also be implemented as a function initialize_array.
Following will solve your issue:
#if 1 // Not in C++11, but in C++1y (with a non linear better version)
template <std::size_t ...> struct index_sequence {};
template <std::size_t I, std::size_t ...Is>
struct make_index_sequence : make_index_sequence<I - 1, I - 1, Is...> {};
template <std::size_t ... Is>
struct make_index_sequence<0, Is...> : index_sequence<Is...> {};
#endif
namespace detail
{
template <typename T, std::size_t ... Is>
constexpr std::array<T, sizeof...(Is)>
create_array(T value, index_sequence<Is...>)
{
// cast Is to void to remove the warning: unused value
return {{(static_cast<void>(Is), value)...}};
}
}
template <std::size_t N, typename T>
constexpr std::array<T, N> create_array(const T& value)
{
return detail::create_array(value, make_index_sequence<N>());
}
So test it:
struct NoDefaultConstructible {
constexpr NoDefaultConstructible(int i) : m_i(i) { }
int m_i;
};
int main()
{
constexpr auto ar1 = create_array<10>(NoDefaultConstructible(42));
constexpr std::array<NoDefaultConstructible, 10> ar2 = create_array<10>(NoDefaultConstructible(42));
return 0;
}
Inspired by @Nawaz, this is more concise and makes better use of the standard: https://godbolt.org/z/d6a7eq8T5
#include <array>
#include <type_traits>
#include <fmt/format.h>
#include <fmt/ranges.h>
// Inspired by https://stackoverflow.com/a/18497366/874660
//! Return a std::array<T, Sz> filled with
//! calls to fn(i) for i in the integer sequence:
template <typename Int, Int...N, typename Fn>
[[nodiscard]] constexpr auto
transform_to_array(std::integer_sequence<Int, N...>, Fn fn) {
return std::array{(static_cast<void>(N), fn(N))...};
}
//! Repeated application of nullary fn:
template <std::size_t N, typename Fn>
[[nodiscard]] constexpr auto
generate_n_to_array(Fn fn) -> std::array<decltype(fn()), N> {
return transform_to_array(
std::make_integer_sequence<std::size_t, N>(),
[&](std::size_t) { return fn(); }
);
}
int main() {
static constexpr std::array<int, 3> a = generate_n_to_array<3>(
[i = 0]() mutable { return 2 * (i++); }
);
fmt::print("{}\n", a);
}
There are already many great answers, so a single remark:
most of them use N twice: once in the array<T,N> and once in the repeat/genseq/generate/whatever you call it...
Since we are using a single value anyway, it seems we can avoid repeating the size twice, here's how:
#include <iostream>
#include <array>
#include <type_traits>
// Fill with a value -- won't work if apart from non-default-constructible, the type is also non-copyable...
template <typename T>
struct fill_with {
T fill;
constexpr fill_with(T value) : fill{value} {}
template <typename... Args>
constexpr fill_with(std::in_place_type_t<T>, Args&&... args) : fill{ T{std::forward<Args>(args)...} } {}
template <typename U, size_t N>
constexpr operator std::array<U, N> () {
return [&]<size_t... Is>(std::index_sequence<Is...>) {
return std::array{ ((void)Is, fill)... };
}(std::make_index_sequence<N>{});
}
};
// A little more generic, but requires C++17 deduction guides, otherwise using it with lambdas is a bit tedious
template <typename Generator>
struct construct_with {
Generator gen;
template <typename F> constexpr construct_with(F && f) : gen{std::forward<F>(f)} {}
template <typename U, size_t N>
constexpr operator std::array<U, N> () {
return [&]<size_t... Is>(std::index_sequence<Is...>) {
return std::array{ ((void)Is, gen())... };
}(std::make_index_sequence<N>{});
}
};
template <typename F>
construct_with(F&&) -> construct_with<F>;
struct A {
A(int){}
A(A const&) = delete;
};
int main() {
std::array<int, 7> arr = fill_with<int>{ 7 };
std::array<int, 7> arr = fill_with{ 7 }; // Ok since C++17
// or you can create a maker function to wrap it in pre-c++17 code, as usual.
std::array<A, 7> as = construct_with{[]{ return A{7}; }};
for (auto & elem : arr) std::cout << elem << '\n';
}
Note: Here I used a few C++20 constructs, which may or may not be an issue in your case, but changing from one to the other is pretty straight-forward
I'd also recommend using <type_traits> and std::index_sequence and all the other sequences from the std if possible to avoid reinventing the wheel)
