Why does std::array not have an constructor that takes a value for the array to be filled with?
C++C++11StdarrayC++ Problem Overview
Is the absence of
std::array<T,size>::array(const T& value);
an oversight? It seems mighty useful to me, and dynamic containers (like std::vector
) do have a similar constructor.
I am fully aware of
std::array<T,size>::fill(const T& value);
but that is not a constructor, and the memory will be zeroed out first. What if I want all -1
's like this guy?
C++ Solutions
Solution 1 - C++
std::array
is, by design, an aggregate, so has no user-declared constructors.
As you say, you could use fill
after default constructing. Since it's an aggregate, default construction won't zero the memory, but will leave it uninitialised (if the contained type is trivially initialisable).
Solution 2 - C++
Note that you can efficiently simulate this type of constructor by taking advantage of the fact that array is not zero-initialized, and has a copy constructor and do.
template <size_t N, class T>
array<T,N> make_array(const T &v) {
array<T,N> ret;
ret.fill(v);
return ret;
}
auto a = make_array<20>('z');
Solution 3 - C++
You may use std::index sequence
for that:
namespace detail
{
template <typename T, std::size_t...Is>
constexpr std::array<T, sizeof...(Is)>
make_array(const T& value, std::index_sequence<Is...>)
{
return {{(static_cast<void>(Is), value)...}};
}
}
template <std::size_t N, typename T>
constexpr std::array<T, N> make_array(const T& value)
{
return detail::make_array(value, std::make_index_sequence<N>());
}
std::make_index_sequence
is C++14, but can be implemented in C++11.
static_cast<void>(Is)
is to handle evil operator,
that T
might provide.
Solution 4 - C++
First of all, it is not std::array<T>
, it is std::array<T,N>
where N
is compile time constant integral expression.
Second, std::array
is made aggregate by design. So it doesn't have anything which makes it non-aggregate, which is why it doesn't have constructor... and destructor, virtual functions, etc.
Solution 5 - C++
I took answer from Jarod42 and made an extension to be able to use variable amount of Constructor arguments and also added automatic indexer as a first arg:
namespace detail {
template <typename T, std::size_t... Seq, typename... Args>
constexpr std::array<T, sizeof...(Seq)> make_array(std::index_sequence<Seq...>, Args &... args)
{
return {{(static_cast<void>(Seq), T(Seq, args...))...}};
}
} // namespace detail
template <typename T, std::size_t N, typename... Args>
constexpr std::array<T, N> make_array(Args &... args)
{
return detail::make_array<T>(std::make_index_sequence<N>(), args...);
}
class myClass {
myClass(unsigned int i, float a, std::string b, int c):... {};
}
Usage:
auto myArray = make_array<myClass, 64>(a, b, c);