I'm looking for a solution using only native C++ language features (up to C++17) for accomplishing the following:
std::array<Type, unsigned int Elem> array_{Type(), // 1 - Call constructor on Type()
Type(), // 2 - ...
... , // 3 - ...
Type()} // Elems - Call the Elem:th Type() constructor
In addition, what I'd also like is that each constructor call should be able to take an arbitrary number of arguments.
A concrete example would be to automate the writing of the following:
std::array<std::shared_ptr<int>, 4> array_{std::make_shared<int>(),
std::make_shared<int>(),
std::make_shared<int>(),
std::make_shared<int>()}
I.e., provided that I know Type and Elem, I'd like to automate the process of creating the brace-enclosed initializer list and in the process call Type:s constructor.
Any ideas?
Update, the real problem I'd like to solve is the following:
template <typename Type, unsigned int Size>
class Storage {
public:
Storage(std::initializer_list<Type> initializer) : array_{initializer} {}
private:
std::array<Type, Size> array_;
};
void foo(){
Storage<std::shared_ptr<int>, 100> storage(...);
// or perhaps
auto storage = std::make_shared<Storage<std::shared_ptr<int>, 100>>(here be an initializer list containing calls to 100 std::make_shared<int>());
}
Like this:
#include <array>
#include <memory>
#include <utility>
template <std::size_t ...I>
std::array<std::shared_ptr<int>, sizeof...(I)> foo(std::index_sequence<I...>)
{
return {(void(I), std::make_shared<int>())...};
}
std::array<std::shared_ptr<int>, 4> array_ = foo(std::make_index_sequence<4>());
Guaranteed copy elision from C++17 ensures that the array is constructed in place, and no extra moves happen.
The return statement expands to {(void(0), std::make_shared<int>()), (void(1), std::make_shared<int>())...}. void(...) is not strictly necessary, but Clang emits a warning otherwise.
But if it was my code, I'd write a more generic helper instead:
#include <utility>
template <typename R, typename N, typename F, N ...I>
[[nodiscard]] R GenerateForEach(std::integer_sequence<N, I...>, F &&func)
{
return {(void(I), func(std::integral_constant<N, I>{}))...};
}
template <typename R, auto N, typename F>
[[nodiscard]] R Generate(F &&func)
{
return (GenerateForEach<R, decltype(N)>)(std::make_integer_sequence<decltype(N), N>{}, std::forward<F>(func));
}
Then:
auto array_ = Generate<std::array<std::shared_ptr<int>, 4>, 4>([](auto){return std::make_shared<int>();});
While the lambda discards the index in this case, it often ends up being useful, especially given that in [](auto index){...}, index.value is constexpr.