std::to_array - cppreference.com (original) (raw)

Creates a std::array from the one dimensional built-in array a. Copying or moving multidimensional built-in array is not supported.

1. For every i in 0, ..., N - 1, copy-initializes result's correspond element with a[i]. This overload is ill-formed when std::is_constructible_v<T, T&> is false.

2. For every i in 0, ..., N - 1, move-initializes result's correspond element with std::move(a[i]). This overload is ill-formed when std::is_move_constructible_v<T> is false.

Both overloads are ill-formed when std::is_array_v<T> is true.

Contents

• 1 Parameters
• 2 Return value
• 3 Notes
• 4 Possible implementation
• 5 Example
• 6 See also

[edit] Parameters

[edit] Return value

[edit] Notes

There are some occasions where class template argument deduction of std::array cannot be used while to_array is available:

• to_array can be used when the element type of the std::array is manually specified and the length is deduced, which is preferable when implicit conversion is wanted.
• to_array can copy a string literal, while class template argument deduction constructs a std::array of a single pointer to its first character.

std::to_array({3, 4}); // OK: implicit conversion // std::array{3, 4}; // error: too few template arguments std::to_array("foo"); // creates std::array<char, 4>{'f', 'o', 'o', '\0'} std::array{"foo"}; // creates std::array<const char*, 1>{"foo"}

[edit] Possible implementation

[edit] Example

#include #include #include #include #include   // creates a constexpr array of string_view's
constexpr auto w1n = std::to_array<std::string_view>({ "Mary", "Patricia", "Linda", "Barbara", "Elizabeth", "Jennifer" }); static_assert(std::is_same_v<decltype(w1n), const std::array<std::string_view, 6>>); static_assert(w1n.size() == 6 and w1n[5] == "Jennifer");   int main() { // copies a string literal auto a1 = std::to_array("foo"); static_assert(a1.size() == 4);   // deduces both element type and length auto a2 = std::to_array({0, 2, 1, 3}); static_assert(std::is_same_v<decltype(a2), std::array<int, 4>>);   // deduces length with element type specified // implicit conversion happens auto a3 = std::to_array({0, 1, 3}); static_assert(std::is_same_v<decltype(a3), std::array<long, 3>>);   auto a4 = std::to_array<std::pair<int, float>>( {{3, 0.0f}, {4, 0.1f}, {4, 0.1e23f}}); static_assert(a4.size() == 3);   // creates a non-copyable std::array auto a5 = std::to_array({std::make_unique(3)}); static_assert(a5.size() == 1);   // error: copying multidimensional arrays is not supported // char s[2][6] = {"nice", "thing"}; // auto a6 = std::to_array(s); }

[edit] See also