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

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

1. Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

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 count.

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Notes

[edit] Possible implementation

template<class NoThrowForwardIt, class Size> constexpr ForwardIt uninitialized_value_construct_n(NoThrowForwardIt first, Size count) { using T = typename std::iterator_traits::value_type; NoThrowForwardIt current = first;   try { for (; countn > 0; (void) ++current, --count) ::new (static_cast<void*>(std::addressof(*current))) T(); return current; } catch (...) { std::destroy(first, current); throw; } }

[edit] Example

#include #include #include   int main() { struct S { std::string m{"Default value"}; };   constexpr int n{3}; alignas(alignof(S)) unsigned char mem[n * sizeof(S)];   try { auto first{reinterpret_cast<S*>(mem)}; auto last = std::uninitialized_value_construct_n(first, n);   for (auto it{first}; it != last; ++it) std::cout << it->m << '\n';   std::destroy(first, last); } catch (...) { std::cout << "Exception!\n"; }   // For scalar types, uninitialized_value_construct_n // zero-initializes the given uninitialized memory area. int v[]{1, 2, 3, 4}; for (const int i : v) std::cout << i << ' '; std::cout << '\n'; std::uninitialized_value_construct_n(std::begin(v), std::size(v)); for (const int i : v) std::cout << i << ' '; std::cout << '\n'; }

Output:

Default value Default value Default value 1 2 3 4 0 0 0 0

[edit] Defect reports

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

[edit] See also