std::ranges::uninitialized_default_construct - cppreference.com (original) (raw)

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Exceptions
• 5 Notes
• 6 Possible implementation
• 7 Example
• 8 Defect reports
• 9 See also

[edit] Parameters

[edit] Return value

As described above.

[edit] Complexity

Linear in the distance between first and last.

[edit] Exceptions

Any exception thrown on construction of the elements in the destination range.

[edit] Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible.

[edit] Possible implementation

[edit] Example

#include #include #include #include   int main() { struct S { std::string m{"▄▀▄▀▄▀▄▀"}; };   constexpr int n{4}; alignas(alignof(S)) char out[n * sizeof(S)];   try { auto first{reinterpret_cast<S*>(out)}; auto last{first + n};   std::ranges::uninitialized_default_construct(first, last);   auto count{1}; for (auto it{first}; it != last; ++it) std::cout << count++ << ' ' << it->m << '\n';   std::ranges::destroy(first, last); } catch (...) { std::cout << "Exception!\n"; }   // Notice that for "trivial types" the uninitialized_default_construct // generally does not zero-fill the given uninitialized memory area. constexpr char sample[]{'A', 'B', 'C', 'D', '\n'}; char v[]{'A', 'B', 'C', 'D', '\n'}; std::ranges::uninitialized_default_construct(std::begin(v), std::end(v)); if (std::memcmp(v, sample, sizeof(v)) == 0) { std::cout << " "; // Maybe undefined behavior, pending CWG 1997 to be resolved: // for (const char c : v) { std::cout << c << ' '; } for (const char c : sample) std::cout << c << ' '; } else std::cout << "Unspecified\n"; }

Possible output:

1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also